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“Origin and Evolution of Organics in Planetary Systems”
A proposal submitted in response to NASA CAN 02-OSS-02
Principal Investigator:
Dr. Michael J. Mumma
Laboratory for Extraterrestrial Physics, Code 690
NASA’s Goddard Space Flight Center, Greenbelt, MD 20771
Voice: 301-286-6994
Fax: 301-286-0212
e-mail: [email protected]
Co-Investigators: Dr. Michael F. A’Hearna
Dr. William Brinckerhoffb
Dr. Steven B. Charnleyc*
Dr. L. Drake Demingd
Dr. Michael A. DiSantid
Dr. Jason P. Dworkine
Dr. Bruce Fegley, Jr.f
Dr. Sara R. Heapg
Dr. J. Michael Hollish
Dr. Reggie L. Hudsone,i
Dr. Monika E. Kressj*
Dr. Paul R. Mahaffyk
Dr. Marla H. Mooree
Dr. Lee G. Mundya
Dr. Joseph A. Nuth, IIIe
Dr. Jeffrey A. Pedeltyl
Dr. Rob Petrem
Dr. Derek C. Richardsona
Dr. Richard J. Walkern
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
*To relocate to NASA’s Goddard Space Flight Center upon funding of this proposal
a
Dept. of Astronomy, University of Maryland, College Park, MD 20742-2421
Applied Physics Lab, 11100 Johns Hopkins Road, Laurel, MD 20723-6099
c
SETI Institute and Planetary Systems Branch, MS 245-3, NASA’s Ames Research Center
d
Planetary Systems Branch, code 693, NASA’s Goddard Space Flight Center
e
Astrochemistry Branch, code 691, NASA’s Goddard Space Flight Center
f
Dept. Earth and Planetary Science, Washington Univ., St. Louis, MO 63130-4899
g
UV/Optical Astronomy Branch, code 681, NASA’s Goddard Space Flight Center
h
Earth and Space Data Computing Division, code 930, NASA’s Goddard Space Flight Center
i
Dept. of Chemistry, Eckerd College, 4200 54th Ave. S., St. Petersburg, FL 33711
j
Dept. of Astronomy, Box 351580, University of Washington, Seattle, Wash. 98195-1580
k
Atmospheric Experiments Branch, code 915, NASA’s Goddard Space Flight Center
l
Biospheric Sciences Branch, code 923, NASA’s Goddard Space Flight Center
m
X-ray Astrophysics Branch, code 662, NASA’s Goddard Space Flight Center
n
Dept. of Geology, University of Maryland, College Park, MD 20742
b
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Extra-National Scientific Collaborators:
Dr. François Raulin, Directora
Dr. Patrice Collb
Dr. Herve Cottinc
Dr. Pascale Ehrenfreund, Directord
Dr. Oliver Bottad
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
Domestic Scientific Collaborators:
Dr. Conel Alexandere
Dr. Louis J. Allamandolaf
Dr. Max P. Bernsteing
Dr. George Codyh
Dr. David Deameri
Dr. Neil Dello Russoj
Dr. Erika Gibbk
Dr. Carol Gradyl
Dr. Kenji Hamaguchik
Dr. Douglas M. Hudginsf
Dr. Timothy Kallmank
Dr. Raj Khannam
Dr. Katharina Loddersn
Dr. Frank Lovaso
Dr. Antonio Manninop
Dr. Larry Nittlere
Dr. Scott A. Sandfordf
Dr. Lewis Snyderq
Dr. Bruce Woodgater
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
a
Laboratoire Interuniversitaire des Systèmes Atmosphériques, Université Paris 7 and 12, Paris, France
Assistant Professor in Chemistry and Astronomy, Université Paris 7, Paris, France
c
Assistant Professor in Chemistry and Astronomy, Université Paris 12, Paris, France
d
Leiden Observatory, P O Box 9513, 2300 RA Leiden, The Netherlands
e
Dept. Terr. Magnetism, Carnegie Institute of Washington, 5451 Broad Branch Rd., NW, Washington DC 20015
f
Astrophysics Branch, MS 245-6, NASA’s Ames Research Center
g
SETI Institute/Planetary Systems Branch and Astrophysics Branch, MS 245-6, NASA Ames Research Center
h
Geophysical Laboratory, Carnegie Institution of Washington 5251 Broad Branch Rd., NW, Washington DC 20015
i
Dept. of Chemistry and Biochemistry, University of California at Santa Cruz, Santa Cruz, CA 95064
j
Dept. of Physics, The Catholic University of America and Lab for Extraterrestrial Physics, code 690.2, GSFC
k
NAS-NRC and Laboratory for Extraterrestrial Physics, code 690.2, NASA’s Goddard Space Flight Center
l
Laboratory for Astronomy and Solar Physics, code 680, NASA’s Goddard Space Flight Center
m
Chemical Physics Program, University of Maryland, College Park, MD 20742
n
Dept. of Earth & Planetary Sciences, Washington University, St. Louis, MO 63130-4899
o
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899
p
Oceans and Ice Branch, Code 971.1, NASA’s Goddard Space Flight Center
q
Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801
r
UV/Optical Astronomy Branch, code 681, NASA’s Goddard Space Flight Center
b
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Table of Contents
(To be completed)
Volume 1
Page
Frontispiece
4
Executive Summary
5
Summary of Personnel, Commitments, and Costs
8
Research and Management Plan
Overview of the Proposed Research
Section 1: Organics in Icy Planetesimals: A Key Window on the Early Solar System
Section 2: From Molecular Cores to Planets: Our Interstellar Heritage
Section 3: Organic Material from Laboratory Simulations of Astrophysical Environments
Section 4: Advanced Analysis of Primitive Materials
Section 5: Management Plan
References
9
10
35
46
55
65
67
Plan for Strengthening the Astrobiology Community
Education and Public Outreach
15 pages max
5 pages max
Volume 2
Facilities and Equipment
5 pages max
Curriculum Vitae
Principal Investigator
Co-Investigators
E/PO Lead
3 pages
one page each
one page
Current and Pending Support
As required
Letters of Commitment from Investigators
one page each
Budget
Summary (NASA format)
Detailed Breakdown
Explanation of Budget details
As required
As required
As required
Reprints/Preprints
(probably none)
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Origin and Evolution of Organics in Planetary Systems
Executive Summary
Planetary Systems form by collapse of dense
interstellar cloud cores (Frontispiece). Some
stages in this evolution can be directly observed when stellar nurseries are imaged (Figure ES.1), while other stages remain cloaked
behind an impenetrable veil of dust and gas.
Yet to understand the origin of life on Earth,
we must first develop a comprehensive understanding of the formation of our own planetary
system.
evolution of organic material in the proto-planetary
disk. The location within the disk is important since
the nature and effectiveness of such processing depends strongly on distance from the young star, on
distance above the nebular mid-plane, and on time.
The ultimate delivery of these primitive organics to
young planets and their moons also evolves with
time, as the bodies grow in size and as the nebula
clears.
We propose to investigate the origin
and evolution of organic compounds in
planetary systems and their possible
delivery to young planets.
The proposed research addresses the heart of
Goal 3 of the Astrobiology Roadmap:
Understand how life emerges from cosmic and
planetary precursors.
The central question is this:
Did delivery of exogenous organics and water
enable the emergence and evolution of life?
To this end, we divide the investigation into
four Themes:
Theme 1: Establish the taxonomy of icy planetesimals and their potential for delivering prebiotic organics and water to the young Earth
and other planets.
Figure ES.1 HST image of NGC 3603 showing the life cycle
of carbon in a star-forming region. A cycle of stellar birth and
death leads to the synthesis and evolution of organic compounds. Carbonaceous material ejected from dying stars enters
the diffuse medium and then is cycled into dense clouds. The
collapse of a dense cloud forms an evolved stellar system (see
the frontispiece for more detail) where these organic compounds
can be delivered intact to planetary surfaces and mixed with
those produced endogenously. As the life time of the evolved
system comes to a close, stellar mass loss recycles material to
begin the process anew.
Theme 2: Investigate processes affecting the origin
and evolution of organics in planetary systems
Theme 3: Conduct laboratory simulations of processes that likely affected the chemistry of material in natal interstellar cloud cores and in proto-planetary disks.
Dense cloud cores are very cold (10-50 K), and their
dust grains are coated with ices comprised of water
and organic compounds. Many of these organics
have potential relevance to the origin or early evolution of life, if delivered to planets.
Theme 4: Develop advanced methods for the insitu analysis of complex organics in small bodies in the Solar System.
The survival of these organics through the violent
birth-phase of a star is less certain. Properties of the
young star (its mass, multiplicity, spectral energy
distribution, etc.) play a key role in controlling the
We seek to better understand the organic compounds generated and destroyed in the interstellar
and proto-planetary environments, through observational, theoretical, and laboratory work. We will
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Origin and Evolution of Organics in Planetary Systems
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• Students and post-docs will be encouraged to ex-
examine the potential for and limitations to delivery
of exogenous pre-biotic organics to planets, examining factors that enhance or restrict this potential. We
will, for the first time, investigate the flux and effect
of astrophysical X-rays on organics and study the
entire history of exogenous organics . We will follow these factors over time, from the natal cloud
core through the end of the late heavy bombardment
(~ 4.1 Ga). We will evaluate the possible role of
exogenous organic material in terrestrial biogenesis.
plore other aspects of Astrobiology in fortnightly luncheons.
Education and Public Outreach.
To be completed once I have a plan from Stephanie
The proposed research will significantly improve
our understanding of the nature of organics in other
planetary systems, the processes affecting them, and
the potential for delivering pre-biotic organic compounds to exo-planets.
Lead Institution Commitment
NASA’s Goddard Space Flight Center has
long-established scientific expertise in all four
Theme areas. The proposed research draws
upon large and highly productive ongoing
programs in areas of Stellar, Planetary Systems, and Cometary Astronomy, Laboratory
Astrochemistry, High Energy Astronomy, and
Flight Instrument Development.
The Center has made an advance commitment to
the Astrobiology Program by hiring Drs. Jason
Dworkin and Michael DiSanti as civil servants, and
providing infrastructure support to them. Goddard
also plans to fill additional civil service positions in
the areas of nebular and cometary chemistry.
The Management plan: An Integrated Research
Approach
The proposed research is interdisciplinary and it
involves researchers at multiple institutions. This is
both an intellectual asset and an organizational challenge. The effectiveness of a Team is demonstrated
when its total output exceeds the sum of its individual parts. We have developed a management strategy that we believe will enable this objective.
• Internal collaboration will be enhanced by bridg•
•
•
•
ing of post-doctoral associates and students
across sub-projects within a Theme.
Theme-Based “Expeditions” will be mounted in
which all students and post-docs will be expected to participate.
An Executive Scientist will ensure smooth operations of the Node, and timely reporting to NAI
Central and to NASA Headquarters.
An Executive Committee will review the scientific
progress and activities, monthly. An independent Board of Visitors will assess progress on an
annual basis.
An Education and Public Outreach Lead will ensure that our E/PO plan is smoothly and efficiently executed.
The Center has recently devoted an NAS-NRC Resident Research Associateship to Astrobiology and is
actively seeking a candidate to fill this position.
If our Node is selected, the Executive Scientist will
re-locate to Goddard.
Visiting faculty, post-doctoral associates, and graduate students will be supported to augment the already significant scientific complement.
Leverage
This proposal heavily leverages existing research
programs of the individual Investigators. Our Investigators have access to the most advanced laboratories and observatories through their existing institutional arrangements and partnerships.
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Origin and Evolution of Organics in Planetary Systems
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Summary of Personnel, Commitments, and Costs.
Insert table here.
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Origin and Evolution of Organics in Planetary Systems
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Overview of the Proposed Research
Exogenous organic material and water were delivered to Earth in great amounts during the late heavy
bombardment, and small amounts arrive even today. Intact examples of exogenous organic moieties
abound in meteorite collections and their analysis
provides a key window on delivery from source
regions within 5 AU of the young Sun.
nebular chemistry, using measured parameters as
end points (e.g., isotopic abundances, nuclear spin
temperatures, and chemical abundance ratios) (Section 1.4).
We will compare cometary organic composition
with organic volatiles in dense interstellar cloud
cores and in disks around young stars, to clarify the
origin and evolution of such material (Section 2.1).
Energetic radiation from the young star can process
pre-cometary grains and ices in the proto-planetary
disk, but the effectiveness depends on many factors
such as flux and spectral-energy-distribution, distance from the young star, height above the midplane, time since the star formed, etc. Spectra of
young stars will be explored at X-ray through ultraviolet wavelengths (Section 2.2) and used to guide
models of organic evolution in the proto-planetary
disk (see Section 1.4). We will extend our
knowledge to other evolved planetary systems by
searching for organics in the atmospheres of transiting planets (Section 2.3).
However, the dominant mass flux likely arrived
from the giant planets’ region where conditions differed greatly, and for which even the most unfractionated meteorites (CI chondrites) provide an uncertain analog. This source, and its potential astrobiological significance, can be evaluated by measuring the organic composition of comets. The proposed research directly addresses Goal 3 of the Astrobiology Roadmap.
We propose to investigate the sources available for
delivery of pre-biotic organics. We will investigate
volatile and refractory organics in comets, and will
establish their taxonomic classes based on organic
composition (Section 1.1). The size distribution of
bodies scattered from each giant-planet domain and
their rate of delivery to the terrestrial planets region,
are key issues that will be evaluated through dynamical models (Section 1.2). The timing of such arrivals will be evaluated through geochronology of
Lunar samples (Section 1.3). For each taxonomic
class, the place of origin in the proto-planetary disk
will be constrained by comparison with models of
Extensive laboratory investigations will be conducted to test aspects of organic production and processing by energetic particles and radiation (Section
3). In addition, certain enabling methods will be
developed to permit more comprehensive measurement of organic composition on future cometary
flight missions and on returned samples (Section 4).
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1.0 Organics in Icy Planetesimals: A Key Window on the Early Solar System
In recent decades it has become increasingly evident
that certain types of asteroids and the even more
primitive comets are rich in diverse classes of organic species. The rapid increase in our knowledge
of comets has come from flyby missions to Comet
Halley1 in the mid-1980’s and improved remote
sensing observations of several recent comets2-4.
An improved understanding of the organic composition of and chemistry in certain asteroid types is
derived from a more detailed characterization of the
chondritic meteorites5 that are believed to originate
in the 2-4 AU region of the asteroid belt. However,
our understanding of organics in small bodies is still
very limited. A much more detailed chemical and
organic characterization of these primitive bodies is
required.
ments.
Organics in Comets
Comets may contain some interstellar dust and ice
that pre-dated Solar System formation, along with
material processed in the Solar Nebula.6 Aspects of
such processing are illustrated in Figure 1.0. Detailed study of interstellar ices has advanced greatly
in recent years7 with enhanced theoretical models
and laboratory analogs, as well as with remote sensing. If comets in fact participated in the earliest
formation of life by contributing pre-biotic material
to early Earth, it is likely that similar processes
would occur in other young stellar systems with
Earth-sized planets.
The recent discovery of chemical diversity among
comets formed in the giant-planets’ region demonstrates that a chemical record of early organics survives today. The discovery of an Oort cloud comet
with drastically different organic composition argues that icy planetesimals formed in the JupiterSaturn region of the proto-planetary disk form a distinct population. This is important because that region contained far more initial than did the UranusNeptune region, and potentially most of the icy
planetesimals that impacted Earth were formed
there. Thus, the issue of delivery of water and prebiotic organic compounds depends on a detailed
understanding of the mass delivered from distinct
In Section 1, we propose to establish the taxonomic
classes of comets, based on their organic chemistry,
and to tie each class to a place of origin in the protoplanetary disk. We propose to evaluate the size distribution of icy planetesimals scattered towards
Earth, and to assess the delivery of organics from
each giant-planets’ zone. Organics contained in
comets and asteroids may have played a key role in
pre-biotic chemistry after their delivery to Earth.
Along with remote observations, in situ analyses
and studies on returned samples are also required to
assess this potential role. In Section 4, we propose
to establish Analytic Protocols for such measure-
Figure 1.0 Processes affecting material in the proto-planetary nebula during Planetary System formation.
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Table 1.1 Carbon-containing molecules in comets. Only ethane (italicized) has not been seen in the ISM.
nebular zones. It is critically important to establish
the taxonomy of comets based on their chemical
composition.
CH4
Remote sensing can establish the gross taxonomic
classes of icy planetesimals, by measuring the volatile organic chemistry at abundances down to 100
parts per million (relative to H2O). For at least a few
comets, more detailed compositional information is
needed, at the parts-per-billion level, especially for
the less-volatile organics. For example, at this level,
more than 500 distinct organic compounds have
been isolated from the Murchison carbonaceous
chondrite.
C2H2
C2H6
CO
CO2
H2CO
CH3OH
HCO2H
HCO2CH3
HCN
CN
HNCO
NH2CHO
CS OCS
H2CS
CH3CH
HC3N
Other carbon-containing molecules or ions identified in the ISM include: C3 c-C3H
C5 C5H C6H CH3C3N CH3C4H HC9N HC11N C2H l-C3H C4H l-H2C4 CH2CHCN
l-C3H2 HC5N C7H CH3CH2OH CH CH2 CH2OHCHO C3S c-C3H2 CH3N
HCOCH3 H2C6 HC7N CH+ CH2CN NH2CH3 C8H HCO CH3SH c-C2H4O
HCCN HC3NH+ HCS+ HCO+ HCNH+ HC2NC HC3CHO CH3CHOH CP HOC+
CSi HNC H2CHN C5N HOCO+ H2C2O H2NCN H2CN HNC3 MgCN H2CS
MgNC H2COH+ SiC3 NaCN c-SiC2 CH3CH2CN (CH3)2CO C3O C3N c-SiC2 C4Si
C2H4 CH3C2H (CH3)2O C2S C3O HNC H2NCN SiCN AlNC.
Remote sensing of many comets provides the taxonomic context for extrapolating the in-depth analytical information obtained for the few comets visited
by spacecraft. Tying those few comets to the general population of icy planetesimals will establish
the significance of each taxonomic class for exogenous delivery of organics to Earth. In this way, the
significance of exogenous organics for astrobiology
can be assessed.
may be diagnostic of their origin8,9. Chemically
controlled organic synthesis in the proto-Solar Nebula produces a thermodynamically driven abundance distribution in any such series. Alternately,
kinetically controlled processes in the ISM would
produce a greater abundance of high-mass members
of these series. Synthesis of organic chains in the
Solar Nebula has been postulated to follow the
Fischer-Tropsch mechanism, catalyzed by silicate
dust. This mechanism10 favors straight chain over
highly branched alkanes.
Low Molecular Weight Organic Molecules in Comets and Carbon Containing Compounds in the ISM
Members of our team and their colleagues11 have
demonstrated that complex organic molecules are
also produced from the simple starting materials CO
and H2 on silica smokes that may be more representative of proto-Solar dust catalysts than other
Fischer-Tropsch catalysts that have previously been
examined. Key in situ measurements at comets
therefore should include analysis of higher molecular weight molecules, including those chains with
two and three carbons and the higher mass members
of these series, plus their isomeric distributions.
Such measurements may reveal the detailed chemical complexity and formation processes of the carbonaceous material delivered to Earth and other
planetary bodies.
Table 1.1 lists many of the carbon-containing molecules in recent comets as detected by remote sensing, sorted by the number of carbon atoms. With
the exception of CO2, CO, H2CO, and CH3OH, all
these species are present in the observed comets at
mixing ratios of 0.1% to 1% relative to water. The
measurement errors for detection of the heavier
molecules in this list are often substantial, and it is
nearly certain that even more complex species are
present but are simply not detectable with measurements to date.
Excepting ethane, all molecules in Table 1.1 are also
seen in the ISM, along with a rich diversity of other
carbon-containing species (listed below the Table).
A detailed measurement of mixing ratios in homologous carbon chain series, such as CH4, C2H6,
C3H8, … CnH2n+2 or the similar series for the more
oxidized alcohols, aldehydes, or carboxylic acids,
Chemical processing in the ISM leaves distinctive
chemical and isotopic signatures that can help reveal
the nature of the link between the proto-Solar material preserved in comets and the parent molecular
cloud. Formation mechanisms for various molecu-
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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and H2O in Comet Hale-Bopp.12
lar species in the ISM produce enrichments in the
heavy isotope of light elements, especially H but
also C and N. These isotopic enrichments vary
from species to species for molecules formed by
different mechanisms in the ISM so a characterization of the variation of the isotopic ratios in different
molecules will allow the degree of chemical equilibration in the nebula to be studied. There is for example, a great difference in the D/H ratio in HCN
These studies have been initiated for the simple volatiles that can be observed spectroscopically in
bright comets. A much more comprehensive study
can be realized with the detailed analysis that can be
carried out with rendezvous missions or on samples
returned to terrestrial laboratories.
1.1 Organic Composition of Comets: Establishing the Taxonomic Classes
Investigators: Michael A’Hearn, Michael DiSanti, and Michael Mumma
The structure and composition of cometary nuclei
are key to understanding the formation and evolution of matter within the early Solar System.1,2 The
ices are most sensitive to temperature and to radiation processing, so their identities and abundances
are seen as central to cometary science.
example, the ratio CO/H2CO/CH3OH in comets
can test the efficiency of hydrogenation of CO
on ice mantles (e.g. Fig. 3.2).
• Establishing the taxonomy of comets to form a
context within which analysis of more complex
species in comets can be compared. These latter measurementscan only be made in situ or
from samples returned to Earth (Section 4).
We propose to characterize the volatile chemistry of all suitable comets that become available
over the 5-year period of performance of this
proposal. This involves measuring the abundance
of sublimed native ices (parent volatiles), through
gas-phase spectroscopy . For the proposed study, we
will use state-of-the-art long-slit echelle grating
spectrometers at ground-based observatories to conduct this study. Observations from airborne and
space-based platforms will be performed as these
facilities become available. This characterization
will entail:
The work proposed in this Section is an extension of
our ongoing program of cometary studies, which
has been funded through the NASA Planetary Astronomy Program for many years. Under this proposal, we will emphasize problems of particular
importance to Astrobiology, for example those that
test the delivery of pre-biotic molecules to the early
Earth by comets.
Results from our proposed remote sensing observations will be compared with predictions obtained
from models of nebular chemistry (Section 1.4), and
with detailed results obtained in laboratory simulations of chemical processing (Section 3). The proposed observations complement work to develop
analytical tools for characterizing the organic component of comets and asteroids observed in situ, and
also through samples returned to Earth (Section 4).
Our ability to detect cometary species remotely is
limited by their abundance – typically to those present at about one or more parts per 104 relative to
H2O. The Protocols proposed in Section 4, however, can characterize trace species to the parts per billion level. Nonetheless, remote sensing is funda-
• Comparing measured abundances of parent volatiles to those predicted by chemical models
(Section 1.4). This will provide information on
the degree of processing experienced by the
pre-cometary ices.
• Measuring the ratio HDO/H2O to provide a test
for exogenous delivery of water and other molecules to Earth by comets. Comets formed in
the Jupiter-Saturn region are expected to be
relatively deficient in HDO/H2O.
• Comparing the abundances of chemically-related
molecules with results of irradiation experiments on cometary ice analogs (Section 3). For
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mental in that it is the only means of establishing the
overall taxonomy of comets.
comets from the OC, and this has indeed been the
case.
Based on recent counts, we expect several long period comets per year to become available for observation. This is due largely to serendipitous discovery of such comets by current NEO search programs using large-format, highly sensitive CCD’s.
Chemical differences among OC comets are revealed directly through observations of sublimed
parent ices (i.e., those that are native to the nucleus)
(Figure 1.1). The detection of a comet that likely
formed in the Jupiter-Saturn region5 demonstrates
that OC comets today provide key insights into organic chemistry in the giant-planets’ region of the
proto-planetary disk.
Comets likely formed at diverse distances from the
young Sun, ranging from just outside the nebular
“frost line” (Rh ~ 5 AU) to distances beyond 100
AU. In the current paradigm, Oort cloud (OC)
comets were ejected from the giant-planets’ region
of the Solar Nebula (~5 – 40 AU), and these comets
The direct study of cometary parent volatiles is accomplished from the ground through use of long-slit
echelle grating spectrometers operating at nearinfrared wavelengths ( ~ 1 - 5.5 µm). These instruments incorporate large-format two-dimensional
detector arrays having sub-arc-second pixel sizes.
They combine the high spectral resolving power
4
(
, or higher) required for isolation of
individual cometary emission lines in the coma with
high (PSF-limited) angular resolution.
Our group at GSFC has pioneered the field of longslit spectroscopy of comets in the near-infrared (IR).
Our goal has been and continues to be to extract the
maximum science return from the combined high
spectral and high angular resolution (typically ~ one
arc-second or better) provided by these instruments.
We measure emission intensities as a function of
line-of-sight distance from the nucleus, accounting
for optical depth in the Solar pump when appropriate. This permits us to construct maps of excitation
(i.e., rotational temperature) in the coma, which in
turn establishes accurate molecular column densities
along each line-of-sight.
Figure 1.1. Organic diversity among Oort cloud comets.
Comets 1 – 6 exhibit similar organic composition, excepting
CO. Comet 1 is 1P/Halley. Comet 7 (C/1999 S4) is depleted
in all organics.
probably contributed most of the mass impacting
Earth during the late heavy bombardment. TransNeptunian comets likely contributed only a tiny
fraction of the incoming mass, instead remaining
largely in place where they comprise today’s Kuiper-Edgeworth belt.
We use our measured spatial profiles of column
density to obtain absolute production rates of emitting species. This avoids problems associated with
slit losses introduced by seeing, guiding errors, and
other observing factors. These invariably cause the
measured production rate to be too low along linesof-sight passing near the nucleus. Our methodology
also permits us to separately quantify native and
distributed source contributions in the coma, and so
provides unambiguous information on the abun-
During the formation period, nebular temperatures
ranged from about 200 K near 5 AU to about 30 K
near 40 AU, and other conditions also varied strongly with location in the proto-planetary disk1,3. Each
of the four giant planets scattered about the same
number of comets into the OC. Dynamical models
for its subsequent evolution4 suggest that roughly
half of comets remaining in the OC today originated
in the 5 - 30 AU region, and the remainder formed
beyond 30 AU. We might therefore expect to find
chemical differences among even a small sample of
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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dance of each constituent ice present in the cometary nucleus.
There are currently three instruments available to
pursue these studies, and additional ones are scheduled to become available during the 5-year period of
performance of this proposal. The available instruments are (1) CSHELL6,7 at the NASA Infrared
Telescope Facility, (2) NIRSPEC8 at Keck 2, and
(3) Phoenix9 at Gemini South. To date our group at
GSFC has used CSHELL and NIRSPEC to measure the abundance of native ices in eight OC comets, and we are approved to use Phoenix in early
2003.
CSHELL features a 256x256-pixel array and produces spectra in a single order. Our group at GSFC
used CSHELL to measure the abundances of several key parent molecules in an OC comet (Figure
1.2). In C/1996 B2 (Hyakutake), cometary CH4 and
C2H610 as well as C2H211 were definitively seen for
the first time, and other key species (H2O, HCN,
CH3OH, CO) were also measured12-14. The Goddard group subsequently conducted serial measurements of these and other parent volatiles over a
range in heliocentric distance, in C/1995 O1 (HaleBopp)15-19.
Figure 1.2

~20,000), long slit spectra (0.2 arc-sec/pixel) of comet
C/1996 B2 Hyakutake, showing emission from parent
volatiles and dust. Telluric lines are seen in absorption
against the cometary continuum, and their cometary
counterparts are Doppler-shifted to higher frequencies
by the comet’s relative geocentric velocity (~ -15 km
s-1). Top. The P2 and P3 lines of CO 1-0 vibrational
band near 4.69 µm. Middle. The R0 line of methane 3
band near 3.30 µm. Bottom. The rQ0 and pQ1 branches
7 band near 3.351 µm. (Right Panels).
Corresponding spectral extracts, representing the signal
summed over 1.4 arc-sec (7 rows), centered on the row
containing the peak continuum signal. The synthetic
transmittance model, convolved to the resolution of
each comet spectrum is also shown (dotted curves)10.
While these represent significant findings, CSHELL
provides limited free spectral range, therefore many
separate settings are required to adequately sample
volatile abundances in cometary nuclei. NIRSPEC
has a 1024x1024-pixel array and is cross-dispersed,
so that multiple echelle orders are observed simultaneously. This makes NIRSPEC the ideal instrument for time-resolved measurements of molecular
abundances in comets. For example, in C/1999 H1
(Lee), the first comet observed with NIRSPEC, seven parent volatiles (H2O, CO, CH3OH, CH4, C2H2,
C2H6, and HCN) were measured in the space of two
hours (Figure 1.3)20.
chemical heterogeneity within the nucleus, by making time-resolved measurements of their relative
abundances. If seen, this could indicate that the
constituent cometesimals were formed over a range
in heliocentric distance.
The availability of long-period comets has increased substantially in the last few years. This
is due largely to dedicated NEO search programs such as LINEAR, and more recently
NEAT. Since 1996, our group at GSFC has
characterized the volatile chemistry of eight
OC comets, and six of these were observed
between 1999 and 2002. Of the eight comets
in our database, five revealed similar abun-
NIRSPEC therefore permits a near-simultaneous
study of chemically-linked pre-biotic molecules.
For example, C2H2, CH4, and C2H6 can be characterized with only two settings, and CO, H2CO, and
CH3OH are adequately sampled with one additional
setting. This also permits investigation of possible
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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dances (relative to H2O) of HCN, C2H2, C2H6,
and CH3OH (Figure 1.1).However, one comet in
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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Figure 1.3 Extracted spectra of comet C/1999 H1 (Lee) obtained with NIRSPEC. The six orders shown were
obtained within a space of two hours of clock time. Molecular emissions from seven different parent volatiles,
as well as OH and NH2 are indicated. The top trace in each panel represents observed sky line emission (arbitrarily scaled in intensity).
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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was accomplished for 3 from the KAO for comet
1P/Halley23.
our sample (C/1999 S4 LINEAR) showed significant depletion of these key molecules, and this may
indicate its formation in the Jupiter-Saturn region.
Due to the density gradient in the proto-planetary
disk, this region would have contained the majority
of icy planetesimals between 5 and 40 AU, and
these likely delivered much of the water and prebiotic organics to the early Earth.
A key indicator of cometary origin having implications for the emergence of life on Earth is the D/H
ratio in comets. Its measurement provides unique
insights into the thermal history of the pre-cometary
ices. We are continually evaluating the prospects
for detecting deuterated isotopes. These depend on
molecular production rates, isotopic fractions, and
the specific observing geometry (geocentric and
heliocentric distances). The isotopic fraction is enhanced for molecules having multiple sites available
for deuterium substitution. Examples of these are
CH3D and C2H5D.
We detect routinely seven parent volatile species
(H2O, CO, CH3OH, CH4, C2H2, C2H6, and HCN)
from the ground. We also have measured NH3 in
two comets (Hale-Bopp and Ikeya-Zhang), H2CO
in three comets (Lee, LINEAR A2, and IkeyaZhang), and OCS in three comets (Hyakutake,
Hale-Bopp, and Ikeya-Zhang). HCN, NH3 and
H2CO are thought to play key roles in the synthesis
of amino acids (see below). OCS provides a link to
the sulfur chemistry through comparison with other
sulfur-bearing species (e.g., H2S, CS, CS2).
Because water is the dominant ice in cometary nuclei, we might expect HDO to be the most readily
detectable such isotope. The D/H ratio in water
measured in comets Halley24, Hyakutake25 and
Hale-Bopp26 was enriched by a factor of two relative to Standard Mean Ocean Water (SMOW). The
enrichment is characteristic of ion-molecule chemistry below 30 K, and could be the signature of interstellar ice27, or perhaps of X-ray driven ionmolecule chemistry in the nebula. Other indicators
suggest these three comets originated beyond 30
AU. This observed enrichment has been used to
argue against replenishment of Earth’s oceans by
comets of this type.
In certain instances, the capabilities of ground-based
observations are severely limited by poor transmittance through the terrestrial atmosphere. The molecule most affected by this is CO2 – its detection requires observations from airborne altitudes or. preferably, from space. In other cases, a targeted molecule may not be optically active at wavelengths accessible by the aforementioned instrumentation.
One such molecule is CS2; its 3 fundamental band
is centered near 6.5 µm. Our group intends to use
the high-resolution spectrometer (EXES21) on
SOFIA to conduct observations of future comets.
However, the ratio HDO/H2O should be depleted in
comets formed near Jupiter and Saturn, and contributing impacts by such bodies may provide a means
of explaining the value measured in SMOW. We
plan to measure the D/H ratio in future bright comets to test this hypothesis. At this writing, comet
C/2001 Q4 (NEAT), which comes to perihelion in
May 2004, appears to be a good candidate for conducting this key observation. Simultaneous coverage can be achieved with NIRSPEC by targeting the
1 band of HDO near 3.67 µm and several hot
bands of H2O near 2.9 µm.
Because it is both fairly abundant and uniformly
mixed in the terrestrial atmosphere, CO2 presents a
special challenge, even from stratospheric altitudes.
It is best observed from space-based platforms. We
anticipate observing cometary CO2 using SIRTF,
and later JWST when it becomes available.
We measure H2O from the ground by targeting
bands whose lower states are not significantly populated in Earth’s atmosphere.17,22 However, these
“hot” bands are considerably weaker than fundamental bands (which are extinguished from the
ground). Observations with EXES will permit targeting of fundamental bands of water in comets, as
Another ratio of interest is DCN/HCN. The value
found in Hale-Bopp28 further supports an interstellar
origin. Because of their different temperature dependences, measuring both DCN/HCN and
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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HDO/H2O can better constrain formation temperatures. The mixing ratio of HCN is typically less
than one percent relative to H2O, however some
models of proto-planetary environments27 predict
values as high as 0.1 for DCN/HCN.
An additional key measurement is the ortho-para
ratio (OPR) in cometary water. The water molecule
consists of two distinct nuclear spin species, orthoand para-H2O. Their abundance ratio (the OPR)
can be related to a nuclear spin temperature, which
may reflect the formation temperature of cometary
water. The OPR is thought to be a cosmogonic invariant33.
We will similarly conduct searches for other deuterated molecules, such as CH3D, C2H5D. Their respective isotopic fractions may also be higher compared with HDO/H2O27. Also under evaluation is
the measurement of cometary 13C/12C, particularly
in C2H6 and CO. This can be compared with the
Solar abundance ratio (~ 1/90 per carbon atom substitution site). If measured, it can provide additional
information on the processing history of the precometary ices. Some or all of these key isotopic
searches may be attepted in C/2001 Q4 (NEAT),
depending on its gas production near perihelion.
Spin temperatures have been measured for H2O in
several comets in our existing database. They generally fall in the range 25-30 K32. Similar values
were obtained for H2O in Halley33 and Hale-Bopp34
from airborne and spaceborne observatories, respectively, and for NH3 in comet C/1999 S4 LINEAR
assuming ammonia is the parent of NH235. These
results stand in contrast to the higher value found for
water in comet Wilson (1986l)36, a dynamicallynew comet. The ability to measure this important
parameter in many comets may test the nature and
diversity of conditions under which cometary water
was formed, provided the OPR is indicative of for-
The abundance ratio of the isomers HNC and HCN
may also provide information on cometary origins.
The discovery of abundant HNC from radio observations of comet Hyakutake29 was viewed as evidence for the survival of interstellar ices in comets.
However, the dependence of HNC/HCN on heliocentric distance observed subsequently in HaleBopp30 suggested that most if not all of the HNC
was produced in the coma rather than being housed
in the nucleus. Yet HNC/HCN ratios in Hale-Bopp
as well as in more recent (and less productive) comets31 are not consistent with current models of coma
chemistry. Based on these findings, it has been proposed that cometary HNC may be produced from
the breakup of organic grains or large organic polymers contained in the nucleus31.
Under this proposal, we will attempt measurement
of HNC/HCN in future bright comets. We will target the 3 band of HNC near 4.9 µm in addition to
our standard measurements of HCN 3 emission
(near 3.0 µm). Establishing the mechanism(s) for
production of HNC could have important implications for the delivery of pre-biotic nitrogen-bearing
species to Earth. For example, if substantial fractions of simple molecules (such as HNC) are stored Figure 1.4 The oxidation sequence of carbon, and the status of
detections in comets and the interstellar medium. Members in a
within more complex species (or grains), this pre- given homologous series (vertical sequences) have similar
sumably would enhance their ability to reach Earth chemical properties.
intact.
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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mation temperature.
Sounding H2O through its 2 fundamental band will
permit the OPR to be measured in lower-production
rate comets. For brighter comets, D/H in water can
be studied through the corresponding band of HDO.
Both measurements will be feasible using EXES on
SOFIA.
Measuring the relative amounts of individual species present in the nucleus of a comet can provide
information on their formation mechanism, and thus
on cometary origins.37 The abundances of homologous hydrocarbons (Figure 1.4) provide one such
test.38
For example, CH4 is the simplest member of the
saturated hydrocarbons (CnH2n+2). The abundance
ratios of successive members can reveal whether
their formation was controlled by kinetics (e.g., in
gas-phase ion-molecule reactions), by thermodynamic processes, or by processing of icy grain mantles (e.g., by photolysis or by H-atom addition reactions). The presence of C2H6 with abundances rivaling those of CH4 in our sample of comets39 is not
consistent with its production by thermochemical
equilibrium processes in the Solar Nebula or in a
giant planet subnebula40 (e.g., Figure 1.5)
Figure 1.5 Relative abundances of saturated hydrocarbons in
primitive bodies, normalized to methane. The ratio CH4/C2H6
in Hyakutake agrees closely with that found in the Murchison
meteorite. However, this is not true for all comets in our sample.
icant in its polymerization to sugar46, but it has been
proposed as the only one-carbon compound capable
of generating the multi-carbon compounds needed
for the origin of life47,38.
However, C2H6 can be produced by hydrogen addition to acetylene ice, for example. The intermediate
species C2H4 is present at very low abundance in
laboratory simulations, because the rate for conversion of C2H4 to C2H6 is much faster than that for
converting C2H2to C2H4.41
HCN is often cited as a significant feedstock for the
synthesis of an array of potentially pre-biotic compounds on Earth or elsewhere — the presence of
both monomer48 and polymer49 in liquid water is a
robust way of generating amino acids50 (among
other interesting compounds). Along with HCN,
NH3 and H2CO are thought to play key roles in the
Strecker-cyanohydrin synthesis of amino acids in
primitive bodies such as the parent of the Murchison
meteorite51.
CO should also undergo surface chemistry. Its
abundance relative to H2CO and CH3OH can test
the efficiency for hydrogenation of CO on icemantled grains. H-atom addition to CO produces
the highly-reactive formyl radical (HCO). This subsequently converts to monomeric formaldehyde
(H2CO), to formaldehyde polymers such as polyoxymethylene (POM42,43), or alternatively to formic
acid (HCOOH). Such reactions have been confirmed in laboratory simulations44,45. The presence
of these and related compounds in comets can potentially provide key information pertaining to the
origin of life. For example, H2CO not only is signif-
However, recent studies have shown that nonStrecker syntheses in pre-cometary ices generate
amino acids without liquid water52,53. These studies
either employ HCN, NH3 and H2CO or may generate them in situ.54 In addition to the formation of
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amino acids, such relatively simple molecules
which may be ubiquitous in comets may also have
contributed to the formation of nucleobases55, sugars47, possible pre-RNA backbones56, and a host of
biochemical intermediates57 either on the icy body
or after accretion to the early Earth. Such syntheses
are all are likely necessary requirements for the
origin of life.
individual chemical classes within the overall
population.
Recently, Goddard has made a long-term
commitment to our work on cometary science by hiring Co-Investigator Dr. Michael
DiSanti. He is now a permanent member of
the Planetary Systems Branch. He is the chief
architect of algorithms specifically tailored to
treat long-slit spectra acquired with CSHELL
and NIRSPEC. Our group at GSFC has used
these algorithms for analysis of the suite of
cometary parent molecules described in this
section, and also for analysis of observations
of the Martian atmosphere58. Dr. DiSanti is
lead scientist on studies of the volatile oxygen
chemistry of comets, specifically CO15,19,
H2CO, and CH3OH59 and how this integrates
with the overall chemistry of cometary parent
volatiles.. Scientific Collaborator Dello Russo
will emphasize cometary H2O and C2H6. Scientific Collaborator Gibb will emphasize cometary CH4 and HDO. Scientific Collaborator
Magee-Sauer will emphasize emissions in the
3.0 µm region, specifically those due to HCN,
NH3, and C2H2.
The role of comets in the emergence of life on
Earth can be fully understood only after establishing the presence and abundance of biologically-important species in these primitive bodies. If present, many of these are likely trace
species, present in amounts at the level of parts
per billion. This is well beyond detection by
remote sensing – in situ measurements and,
ultimately, analysis of samples returned to
Earth will be required. Discussion of analytical techniques and protocols to address such
work is the subject of Section 4. Even then,
however, extrapolation of those results to the
grand picture will depend on the taxonomy
being developed through remote observing
(from the ground, from airborne altitudes, and
from space). This is necessary to relate the
chemistry of the few comets visited to that of
1.2 Dynamical Transport of Icy Planetesimals in the Early Solar System
Investigator: Derek Richardson
Current models of Solar System formation attribute
the compositional difference between terrestrial and
giant planets to the falloff in temperature with distance from the proto-Sun.1 Volatile-rich chemical
species are depleted from bodies inside ~5 AU but
dominate outside this limit. Nevertheless, Earth
somehow acquired volatile organics after its formation was largely complete.
an open question that we propose to address by dynamically modeling the delivery mechanism.
If comets formed in a region at least commensurate
with the current distribution of outer planets, i.e., in
a zone from ~5 to ~40 AU, it is reasonable to suppose that a chemical gradient would develop among
comets, much as the giant planets show a tendency
toward more methane-rich constituents with distance. This then provides a clue to a comet’s origin,
if its chemical makeup can be established upon apparition.
It is thought that most comets, being volatile-rich
bodies as evidenced by their outgassing on close
approach to the Sun, were formed in the outer region of the nebula and could therefore be a significant source of these organics. The relative importance of a cometary origin for Earth organics is
As an example, comet C/1999 S4 showed unusual
ice chemistry that is consistent with formation near
Jupiter (e.g., Figure 1.1).2 The comet chemical record is complicated however by dynamical processes
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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Table 1.2. Timescales in the Solar System.
that caused radial mixing of material in the outer
Solar System. Oort recognized that the orbital inclination distribution of long-period comets implies
that these bodies come from a spherical distribution
of material at large distance (> 5000 AU).3 Current
theories cannot explain how these bodies could
form in such a configuration, since the mass density
of nebular material at these large distances would
have been negligible.
Event
Jupiter/Saturn formed
Nebular gas cleared
Uranus/Neptune formed
Earth/Moon formed
Instead, it is thought that Oort cloud comets originated in the Uranus-Neptune region but were scattered outward by close approaches with these giant
planets.4 Jupiter and Saturn also scattered icy planetesimals, but a large fraction of these bodies would
have been ejected entirely from the Solar System
owing to the larger mass of Jupiter and Saturn.
However, some entered the Oort cloud, and C/1999
S4 may be one such comet.
Timescale
1-10 Ma
10 Ma
20+ Ma?
10-100 Ma
Further, in this model the giants acquire their gaseous envelopes in the final stage, so a large mass
component of gas must persist throughout the early
life history of planetesimals/comets. This means
many comets would have experienced significant
size-dependent radial migration due to gas drag that
would have affected their delivery rate both into the
giant-planet scattering zones and into the inner Solar
System. Moreover, a large gas density may have
led to significant mobility of the giant-planet cores
themselves.
At the same time, a large population of potential
comets remains in the Edgeworth-Kuiper Belt, a
region postulated by its namesakes as a logical extension of the proto-planetary disk that also may
give rise to short-period comets with low inclinations.5 Pluto is the largest member of this group.
Therefore, in order to determine the delivery rate of
organics to the early Earth, we must study the dynamical history of comets from their place of birth
to their insertion into stable reservoirs such as the
Oort cloud or Kuiper belt or to their demise through
collisions with other planets, moons, or the Sun.
Support has been increasing recently for the directcollapse model of giant-planet formation,7,8 in
which gas giants formed through gravitational instability of the gas disk. In principle a planet could
form in just a few hundred to a thousand years by
this method. In that case the dynamical problem
becomes simpler because the planetesimals in this
model would form after the major perturbers were
already essentially in place.
Both models however have trouble explaining the
formation of Uranus and Neptune. In the coreaccretion model the time to build up the cores at
their respective distances would exceed the likely
lifetime of gas in the disk. In the direct-collapse
model the required density and temperature of the
outer nebula would imply a much more massive
disk than is currently envisioned. If on the other
hand the cores of Uranus and Neptune formed between proto-Jupiter and proto-Saturn, perturbations
from the larger planets could have kicked out the
smaller ones to larger radii.9,10 This clearly has implications for the scattering history of planetesimals
in the core-accretion model since the outer planets
The most crucial unknowns in this problem are the
timescales for various important events that occurred throughout the early history of the Solar System (Table 1.2). In the core-accretion model of giant-planet formation,6 Jupiter and Saturn formed in
about 1-10 Ma. The cores of these planets developed through accretion of icy planetesimals, meaning that the precursors to comets existed before the
giants were fully grown. In this scenario, the dynamical history of comets is intimately tied to the
formation rate of the giant planets, since the scattering rate is a sensitive function of the perturber mass.
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Figure 1.7 Snapshot of a planetary ring simulation
with ~2  105 small ice spheres in a central patch of
dimensions 120  290 m. Surrounding patches are
ghost replicas that provide realistic boundary conditions and a smoother gravitational potential.
Clumping is caused by a shearing instability.
Figure 1.6 Simulation of a disk of 106 planetesimals
between 0.8 and 3.8 AU afte 103 yr of evolution. An
already-formed Jupiter-sized planet (near top right) at
5.2 AU caused the gaps in the disk.
forces to order (N log N).12 The code shows excellent near-linear inverse scaling with number of processors on a variety of common platforms. Since
Richardson is a developer of this code, the proposed
modifications described below will be straightforward to implement.
would have traversed virtually the entire disk, scattering everything in their paths.
Current estimates for Earth/Moon formation range
from 10 to 100 Ma.1,11 If the latter estimate is correct, then the precise details of what was happening
in the giant-planet region may be less important for
understanding the delivery rate of organics to the
primitive Earth. Nonetheless it remains important to
understand the relative distribution of cometary material in the major reservoirs (Kuiper belt/Oort
cloud) and the size distribution of the bodies (since
the smallest bodies would vaporize long before they
reached Earth) by the time Earth finished forming.
For this we still need to model the early history.
We will start with Weidenschilling’s model13 of
comet formation, in which comets are built up from
10-100 m cometesimals on timescales of 105 yr.
These initial conditions are valid only up to the point
when gravity becomes important, so to assess the
true evolved size distribution it will be necessary to
model the dynamical growth of these bodies. The
preferred method under these circumstances is to
use a “patch” model in which only a relatively small
region with periodic boundary conditions is considered (Figure 1.7). This model has been used successfully with pkdgrav in the past.14
Our approach will be to combine analytical and
semi-analytical theories regarding the formation of
comets, the buildup of giant planets and the Earth,
and the effect of nebular gas, with a state-of-the-art
planetesimal dynamics code (pkdgrav). The code,
developed in part by Co-I Richardson, is capable of
accurately following the orbits of millions of bodies
(Figure 1.6) by using a parallel tree method to distribute work among an arbitrary number of processors and reduce the cost of computing interparticle
The goal will be to evolve the Weidenschilling population (using 105 particles in a small periodic patch
with initial dispersion set by the model) until the
system reaches at least a quasi-steady state and before the onset of runaway growth, i.e. before particle
sizes reach ~1 km. Essentially we are interested in
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Figure 1.8 Simulation of two rubble piles colliding. This particular impact resulted in a remnant containing ~70% of the total
mass of the system, a net accretion event.
deriving the slope of the power-law size distribution
on the basis of true gravitational accretion but with
the possibility of some fragmentation events. For
this purpose we will use a modification of pkdgrav
currently under development by Richardson’s graduate student Zoë Leinhardt for her thesis. The new
algorithm determines the outcome of planetesimal
collisions on the basis of earlier experiments15,16
that simulated collisions between so-called “rubble
piles” or gravitational aggregates under conditions
similar to those proposed here (Figure 1.8).
stage simulations will be used to select the initial
comet masses and radii (otherwise this information
is unimportant since the bodies will be treated as test
particles; nonetheless it is still needed to determine
the actual mass flux into the inner Solar System).
The simulations will proceed very much along the
lines of earlier dynamical mapping work on this
topic.17-20 As with these earlier studies, pkdgrav
uses a symplectic second-order leapfrog integrator,
meaning that in the absence of non-conservative external forces, the energy and angular momentum of
all particle orbits remain bounded. In particular,
there is no spurious radial drift as can gradually develop with non-symplectic methods even at high
order. The non-conservative nature of the tree does
introduce some error, but this will be negligible for
the current problem given that most bodies are test
particles.
By a combination of interpolation from these experiments and direct modeling for (rare) cases that result in significant fragmentation, a realistic treatment
of the gravitational and collisional evolution of these
early comets will be obtained. Note that these simulations assume that external perturbations, apart
from Solar tides, are negligible. In the coreaccretion model this is reasonable given that the
timescale for formation of bodies up to several hundred km in diameter is by definition less than the
time-scale for core formation. However, gas drag
may play an important role at this stage. This can
be modeled in the patch so long as the radial migration rate is small compared to the orbital frequency.
Experiments with gas drag will be run concurrently
with the second-stage simulations (described below)
and will be tuned based on results obtained from
studies of X-ray luminosity vs. time that will reveal
the true lifetime of disk gas (Section 2.2).
The advantage of using pkdgrav over other codes is
that it is designed for handling large numbers of particles and is optimized for parallel execution. A hierarchical timestep method will be used so that unperturbed bodies will be drifted along their Kepler
orbits with comparatively few updates while the
relatively small number of particles undergoing
strong perturbations will be advanced at much
smaller time intervals. Switching timesteps does
introduce minor discontinuities in the symplectic
mapping, but since these interactions will be few in
number and in any case the sheer number of particles will provide a statistical smoothing of any
glitches, these errors can be ignored.
The dynamical evolution of comets in the giantplanet region will be modeled by treating the comets
as test particles that do not exert forces on other bodies and do not collide, which effectively reduces the
computation cost to O(N). For models that include
gas drag, the size distribution derived from the first-
Modifications needed to the code include a simple
flag to not compute forces between test particles
(currently pkdgrav has no provision for this), monitoring of particle trajectories to determine whether
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any particle gets ejected from the system (such particles will be removed from the simulation), and
logging to keep track of collisions with the Sun or a
planet. Optional features that are straightforward to
implement include an analytical expression for mass
growth of each planetesimal as a function of position (to reflect that planetesimal growth is still taking
place; this expression could be extrapolated from
the first-stage simulations), and a way to account for
possible giant planet migration21,22. Strictly speaking, once the nebular gas density has dropped sufficiently, migration is actually driven by planetesimal
scattering. To model this directly would require
allowing the test particles to exert forces on the
planets (but still not each other), which is straightforward to implement.
correspond to gravitational equilibrium among the
population and with the planetary perturbers. The
simulation will be run for at least 10 Ma.
The result will be the most complete map ever obtained showing, as a function of source region, the
flux of organic material into the Oort cloud, into the
terrestrial region, and out of the Solar System altogether. Subsequently, a run with giant planet migration based on Malhotra’s work21,22 will be performed. Finally, the Thommes model9,10 of Uranus
and Neptune formation will be explored. The radial
movement (and eccentricity distribution) of the
planets in these latter runs will be imposed analytically. It will also be necessary to explore a range of
formation timescales.
Despite the fact that few interparticle forces will be
computed in these models, they nonetheless represent a computational challenge due to the long timescales involved. Supercomputing facilities at
NASA’s GSFC will be employed to make the
simulations tractable. As a benchmark, a planetesimal simulation with one million particles evolved
for one thousand years with full interparticle gravity
and collisions required 200 wall-clock hours on an
earlier version on the Goddard Cray T3E.12 With
gravity and collisions turned off, the same simulation would take 20 wall-clock hours or less. Therefore the first proposed second-stage run, with onetenth the number of particles, would require
~20,000 wall-clock hours on the T3E if only 128
processors were used (less than 10% of the machine). However, use of hierarchical time-stepping
will provide roughly another factor of 10 speedup
(since most particles will be far from the Sun), making the proposed simulation practical within the
proposed period.
To match the actual extent of migration, the quantity
of mass interacting with the planets must be realistic. Either many orders of magnitude more particles
are needed or the mass of each particle must be artifically high. This could introduce an undesirable
element of stochasticity. For the present purposes it
will be sufficient to simply impose the migration
analytically, as was done in the earlier studies. It
would even be possible to implement the Thommes
model9,10 in which Uranus and Neptune originate as
proto-planets between Jupiter and Saturn, using a
similar analytical method. It has been proposed that
this event triggered the Late Heavy Bombardment.23
We propose to conduct a number of simulations that
gradually increase in complexity as time allows.
We will start with a disk of one hundred thousand
test particles spread out between 4 AU and 40 AU
with the Earth and the four giant planets in their current orbital configuration. The choice of particle
number is a compromise between high resolution
and computational expediency. The rationale for
starting the planets in their current configurations is
to reflect the fact that by the time Earth began forming the giant planets were likely mostly finished
forming, owing to the disk temperature gradient.
The planetesimals will be tagged so that their starting locations (and therefore compositions) are
known. Initial eccentricities and inclinations will
For testing on smaller problems, and for the firststage simulations, Co-I Richardson’s Beowulf cluster of PCs at theUniversity of Maryland will be
used.
Finally, we hope to use the capabilities of pkdgrav
to carry out direct simulations of the first stage of
planetesimal formation: growth of grains followed
by settling to the mid-plane. A new feature of the
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code, based on earlier work by Co-I Richardson,24
allows particles to be “frozen” into aggregates that
obey Euler’s laws of rigid-body rotation and that
respond to torques delivered through collisions
and/or gravity (Figure 1.9). By replacing the rigid
bonds with a configurable realistic electrostatic
binding force, and adding (analytical) external forces due to gas drag and turbulence, we will be able to
model the stresses that act on early grains in order to
determine their survival time during settling.25 This
work will tie in directly with the disk observations
discussed in Section 2.1 that will provide constraints
on grain sizes and lifetimes, and will also provide a
test for the Weidenshilling starting model.13
Unlike previous studies, thousands of particles will
be modeled simultaneously, providing the largest
dynamic range of planetesimal dust simulations to
date. However, due to the complexity of the required code modifications, this task will be deferred
until significant progress has been made with the
simpler calculations.
Apart from early detailed numerical studies of grain
growth using a handful of particles,26 this will be the
first series of simulations to directly test analytical
and semi-analytical theories of grain formation.
Figure 1.9 Fractal aggregate of dimension ~1.8 produced via
ballistic accretion with a rigid-body model.
1.3 Timing of Additions of Organic Matter to Earth
Investigator: Richard Walker
Core formation likely stripped the silicate Earth of
highly siderophile elements (HSE; Ir, Rh, Ru, Pd,
Pt, Os, Re, Au), given that metal-silicate distribution
coefficients for these elements are >106 at both low
and high pressures1,2. Despite this putative, nearly
quantitative removal, HSE in the upper mantle are
only approximately 200 times less abundant than in
chondritic meteorites, and occur in generally chondritic proportions. A variety of models have been
proposed to explain these observations.
tion of organic matter and the origin of life on the
Earth, and perhaps other planets.
We propose to study the nature of late accreted materials to the Earth-Moon system as a function of
time via the examination of the HSE contained in
Lunar breccias. The HSE contained in the breccias
were added to the Moon during the period of time
from the origin of the Lunar highlands crust (4.4-4.5
Ga) to the end of the late bombardment period (3.9
Ga), and to even younger events. These materials
provide the only direct chemical link to the late accretionary period. The chemical fingerprints of the
HSE in late accreted materials may enable us to ascertain where in the Solar System the late accreted
materials formed, and possibly link the genetic type
of material with time of addition.
Some authors have argued that these characteristics
are best explained via a late influx or “veneer” of
large planetesimals after formation of the Earth’s
core and the Moon35. The envisioned late influx
would have been a major event in Earth history,
adding a veneer comprising as much as 0.3% of the
total mass of the Earth (and a somewhat lower proportion of mass to the Moon). It has been argued
that this late veneer added both volatiles and organic matter to the Earth. Consequently, constraints on
the timing of late veneer additions and their compositions may provide new insights to the accumula-
Chondritic meteorites are among the most primitive
materials remaining from the early Solar System.
Chondrites are chemically divided into three major
groups: ordinary, enstatite and carbonaceous. The
carbonaceous chondrites most closely match the
composition of the Sun’s photosphere. Of note
here, there are significant differences between the
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modern 187Os/188Os of carbonaceous compared with
ordinary and enstatite chondrites. These differences
indicate long-term differences in the Re/Os, given
that 187Re decays to 187Os via the negatron transition
(t1/2 = ~ 42 Ga). The isotopic data reveal that ordinary chondrites define a narrow range of present
day 187Os/188Os of 0.1283 ± 0.0017 (2).
drites is generally higher than for ordinary and carbonaceous chondrites. It is important to note that
the differences in long-term elemental ratios most
likely reflect chemical fractionation within the Solar
Nebula6 or soon thereafter7.
Lunar Studies
Enstatite chondrites define a very similar average,
present day 187Os/188Os of 0.1281 ± 0.0004. In contrast, carbonaceous chondrites, the group that contains the most organic-rich chondrites, define a 23% lower 187Os/188Os of 0.1262 ± 0.0006. There is
minimal overlap between the ordinary and enstatite
chondrites versus carbonaceous chondrites (Figure
1.10). These results require that the time integrated
Re/Os of carbonaceous chondrites has been on average approximately 7-8% lower than that of ordinary and enstatite. Thus, Os isotopes may be useful
for detecting the addition of possible organicbearing materials to the Lunar surface (and by inference, Earth).
The presence of numerous large impact basins and
craters attests to the fact that the Moon underwent a
severe bombardment by very large objects, subsequent to the 4.3-4.5 Ga formation of the Lunar highlands. Moreover, in comparison to the Earth, the
Moon has been geologically inactive for most of its
history; there has been no plate-tectonic recycling of
surface material back into the mantle. Aside from
continued bombardment, the Lunar highlands have
experienced only minor volcanism since they
formed. Consequently, Lunar rocks that contain
meteoritic components can be used to constrain the
characteristics of the planetesimals that impacted the
Moon, and by inference, the Earth8. Because radiometric ages have been determined for many Lunar
samples, studying these samples can yield information on variation of the characteristics of the impactors with time.
Figure 1.10 Histogram of data for the three major groups of
chondrites6. Note the difference between organic-rich carbonaceous chondrites and enstatite and ordinary chondrites.
In addition to the Re-Os isotope system, the relative
abundances of other HSE can be used to fingerprint
the provenance of the late veneer. Our work has Figure 1.11 Bulk chondrite data6,9. The estimate for the primishown that with Os isotopic and elemental HSE tive upper mantle of the Earth (PUM) is from Meisel et al.10 and
data combined, the different types of chondritic Morgan et al.11.
groups can be discriminated (Figure 1.11). For example, the Pd/Ir ratio of metal-rich enstatite chon———————————————————————————————————————
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Two types of Lunar rocks that contain abundant
meteorite-derived HSE are especially important:
granulitic breccias and impact-melt breccias. Granulitic breccias are metamorphosed, recrystallized
impact melts, cataclasites and fragmental polymict
breccias. They are one of the most abundant types
of clast in Lunar breccias; most were metamorphosed prior to 3.9 Ga, so their HSE record the
compositions of Lunar impactors in the first halfbillion years of Lunar history. Impact-melt breccias
consist of Lunar materials melted by impacts. Their
melt-derived matrices contain abundant HSE,
which, in many cases, appear to be mostly derived
from the impacting bodies12,13.
from the breccias should provide a record of the
relative abundances of these elements contributed
by the impactors. When the Lunar data are normalized to CI and Ir, the siderophile elements in the two
lithologies in breccias 73215 and 73255 fall into two
groups: refractory siderophiles Re, Os and Ir; and
“normal” siderophiles Pd, Ni and Au. The pattern
in the granulitic breccia clasts is almost unfractionated relative to CI, except for Au enrichment,
whereas in the aphanites the Re, Os and Ir are depleted (Figure 1.12).
The similarity between enstatite chondrites and
these Apollo 17 aphanites is quite close, although
the analytical uncertainty for Pd in the aphanites is
rather large. Recent HSE measurements on other
Lunar impact-melt breccias from the Apollo 17 site
also indicate a Pd enhancement21.
Most of the melt breccias in the Apollo sample collection appear to be the product of impacts that
formed basins and large craters at about 3.9 Ga, but
some of the Apollo melt breccias and some meltbreccia clasts in Lunar meteorites formed more recently than 3.9 Ga14. Thus, impact-melt breccias
record the characteristics of Lunar impactors from
about 3.9 Ga to the present.
Because the Moon was formed in situ15,16, the
Moon and Earth almost certainly sampled the same
population of impactors throughout their histories17,
with the Earth getting the lion’s share18,19. Morgan
et al.11 estimated that the proportion of materials
added to the Moon was 20-50 times less than for the
Earth. Thus, information on the changes of Lunar
impactor composition with time applies equally to
the Earth.
Figure 1.12 C1 chondrite normalized abundances of some HSE
for Lunar aphanites20 and two enstatite chondrites, Kota Kota
and Indarch9.
Apollo 17 impact-melt breccias 73215 and 73255
were studied by Morgan and Petrie20 using the same
techniques as for the peridotites analyzed by Morgan4. These breccias from the South Massif at Taurus-Littrow are thought to represent melt generated
in the Serenitatis basin-forming impact and have
39
Ar/40Ar ages of about 3.87 Ga. Two lithologies in
these breccias contain significant siderophile elements; aphanites (representing the impact-melt matrices) and granulitic breccia clasts (representing
older, unmelted rock).
In summary, the prior work characterizing the relative abundances of HSE in the upper mantle and
Lunar breccias has produced promising results and
set the stage for a campaign to more accurately and
precisely constrain the HSE characteristics of the
impactors that dominated the late influx.
Proposed Work
Overview.
Despite recent progress, there are many uncertainties and important questions that remain with regard
to the nature and timing of the late influx. The ob-
The indigenous Lunar component of HSE abundances is very low, so the HSE content of materials
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ject of this aspect of the study will be to continue to
precisely and accurately characterize the HSE characteristics of the late influx to the Moon (and by
inference the Earth) via analysis of Lunar breccias.
Questions we hope to answer are:
shows that there is very little variation in the Os/Re
of carbonaceous chondrites.
Second, each sample aliquot measured for HSE will
also be characterized with respect to Re-Os isotopic
systematics. Thus, we will team two powerful
methods for characterizing HSE in the primitive
upper mantle of the Earth (PUM) and Lunar breccias. Previous studies have utilized either HSE abundances or Os isotopes to characterize terrestrial or
Lunar materials, no study has as yet applied both.
• What is the dominant signature – or signatures –
of HSE in Lunar breccias, and can this signature(s) be tied to precursor materials that experienced particular nebular or other early Solar
System processes?
• Did the composition of the late influx change with We can achieve sample/blank for all targeted ele-
To accomplish these goals, we will determine Os
isotopic compositions and HSE abundances in Lunar impact-melt breccias and granulitic breccias.
These data will enable us to directly constrain the
HSE characteristics of materials involved in the late
influx to the Moon, which were the same as the materials involved in the late influx to the Earth.
ments of greater than 50:1 using standard Carius
tube digestion via miniaturization of digestion vessel and columns, and reduction of reagent volumes.
In addition, the mass spectrometry of 100’s of pg of
these elements yield sufficiently large signals that
ratios can be accurately and precisely measured to
better than +0.5%. Although the Lunar data will
result in abundance data of about +3% for Re, Pt
and Pd (Ir, Ru and Os will be significantly better,
and 187Os/188Os can be easily measured) the data
will be more than sufficient to distinguish among
the major types of chondrites.
Analytical Plan.
Lunar Sample Selection.
All concentration measurements will be conducted
via isotope dilution and either thermal ionization
mass spectrometry (Os concentrations and isotopic
compositions) or multi-collector inductivelycoupled plasma mass spectrometry (Re, Ir, Ru, Pt,
Pd concentrations). Both types of instruments are
available at the University of Maryland. We will
use dilutions of the same spikes applied to chondritic meteorites6,9, and the same digestion techniques
to make most of the Lunar measurements. This
means that we will be able to compare our terrestrial
and Lunar results directly with results for chondritic
meteorites that have been measured using identical
techniques.
Two Apollo 17 breccias have been previously studied via neutron activation analysis20. These rocks
are breccias 73215 and 73255. Morgan also reported enrichments of the HSE in microbreccia clasts
within the Apollo 14 breccia 14321. No Ru, Pt or
Pd elemental, or Os isotopic data, however, exist for
these rocks.
time?
• Can any of the additions be attributed to impactors with HSE relative abundances similar to
those of organic-rich chondrites.
We have already begun to re-examine HSE abundances and Os isotopic compositions of lithologies
from these three well-studied breccias as part of an
NSF sponsored study. In addition to the pristine
materials, we have also acquired from NASA >50
clast and matrix samples of the three breccias that
were previously analyzed for some elements
by instrumental neutron activation analysis
(INAA). Most of these samples are suitable for
HSE and Os isotopic analysis. For these studies, we
have recruited Dr. Odette B. James (Emeritus,
USGS) to help us in separation and selection of appropriate materials; Dr. James is the petrologist
This aspect is critical to our proposed comparisons.
To emphasize the importance of this commonality
of technique, we note the extent of variability in
Os/Re that has been reported in the literature for
supposedly well-characterized carbonaceous chondrites. In comparison, our work on chondrites
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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who directed the initial consortium studies of breccias 73215 and 73255.
broad knowledge of the Apollo collection, will assist us with sample selection. As part of this work,
we will also analyze several impact-melt breccia
samples provided by Dr. David Kring (University
of Arizona). These samples are clasts from Lunar
meteorites and Kring and his associates are currently determining their radiometric ages by 40Ar-39Ar
dating; preliminary results14 suggest that most of
these melt breccias formed more recently than 3.9
Ga ago.
After we have completed the initial NSF-sponsored
work on breccias 73215, 73255 and 14321, we propose here to undertake a broader study of Lunar
impact-melt breccias and granulitic breccias; the
aim of this study will be to outline how the characteristics of Lunar impactors varied with time during
the first billion years of Lunar history. We will analyze samples for which radiometric ages have previously been determined; Dr. James, who has a
1.4 Chemical Models of Nebular Processes
Investigators: Steven Charnley, Bruce Fegley, Jr., and Monika Kress
Dark molecular clouds are regions where low-mass
stars and their associated planetary systems form.
Their fundamental importance for Astrobiology lies
in the fact that all the starting materials available for
the assembly of planetary systems, including many
biomolecules, must be obtained from this reservoir1.
observations then permit the trace products of solid
state chemistry to be probed.6 For example, the recently-discovered organics vinyl alcohol, glycolaldehyde and ethylene glycol are only present in trace
amounts in ices.7 Many simple organic molecules,
also found in the interstellar medium and in comets,
have been detected in the disks around several proto-stars8,9: HNC, CS, CCH, CO, HCN, CN, HCO+,
DCO+, H2CO, CH3OH. This indicates the possibility of directly studying the chemistry of protoplanetary accretion disks. Thus, it is now possible to
observe in detail the organic composition of each
phase prior to formation of proto-stellar disks, a
chemical sequence analogous to that which led to
the proto-Solar Nebula. Theoretical models of the
chemistry occurring in each phase can be constrained and allow us to predict with confidence
which large biomolecules should be present.
In molecular clouds, dust grains shield the inner regions of the cloud from external ultraviolet radiation. Cosmic rays penetrate into the deepest cloud
interiors and drive a rich gas phase chemistry, involving ion-molecule and neutral-neutral reactions,
which leads to the formation of many organic molecules.1
Atoms and molecules also stick to dust grain surfaces and undergo catalytic surface reactions. At temperatures of around 10K, both these chemistries lead
to very strong isotopic fractionation (D, 15N and
13
C).2,3,4 The identity of the most abundant molecules present in interstellar ices has now been determined: H2O, CO, CO2, CH3OH, H2CO, NH3,
CH4, OCS and HCOOH. However, many larger
complex organic molecules are also expected to be
present.5
Another suite of chemical constraints lies within the
measured composition of primitive Solar System
material. The meteoritic record contained in carbonaceous chondrites indicate that interstellar organic
material underwent a very significant degree of processing. For comets, the key issue is elucidating
how much of their organic composition and chemical heterogeneity can be attributed to being pristine
ISM, or partially-processed ISM, or purely nebular,
or a complex mix of the three.10 The goal of the
dynamical-chemical modeling proposed in this
module is to understand the incorporation of inter-
The physics and chemistry of low-mass star formation provides the boundary conditions for the
chemical evolution of proto-planetary systems.
Warm proto-stellar cores, where ice mantles have
been evaporated, contain the richest inventory of
interstellar organic molecules, many of which are
important in biochemistry1. Radio astronomical
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stellar matter in the proto-Solar Nebula, and to calculate the subsequent spatial and temporal evolution
of the organic chemistry in the comet-forming region (5-100 AU). In conjunction with dynamical
orbit calculations, the results will elucidate the likely
organic inventory of the comets that gave the early
Earth its initial budget of volatile material.
Objectives and Motivation
We will model the processing of organic interstellar
matter as it is first incorporated in the disk of a lowmass protostar. We will model in detail the gasgrain chemistry of parcels of gas and dust as they
undergo collapse towards the proto-stellar disk and
pass through the accretion shock. We will pay particular attention to the deuterium fractionation
chemistry. Molecular line observations of protostellar accretion shocks can now be made. This will
be the first study to compute in detail how ISM gas
and ices are first processed as they collapse down
onto the nebular disk and through the accretion
shock, and to relate this to the organic volatiles in
comets.
1.4.1 Chemistry During Accretion of Interstellar
Matter
Background
As cold interstellar gas and ice-mantled dust grains
collapse onto the luminous protostar/disk system
they are heated by thermal radiation and the ice
molecules evaporate. Frictional heating through
gas-grain drag can also erode grain mantles as dust
grains approach the disk.11 As material approaches
the proto-stellar disk, many more endothermic reactions come into play, driven by the increasing temperature. Radiation chemistry involving X-rays and
ultraviolet (UV) light from the accretion shock may
also play a role. Eventually the accretion shock is
encountered where it and other processes associated
with disk accretion begin to dominate the chemistry
of the material first entering the nebula. Neufeld
and Hollenbach12 modeled the disk accretion shock
as a hydrodynamic ‘J-shock’ and calculated the regions of the disk where various interstellar materials
(refractory metals, refractory and volatile organics,
and ice) would be vaporized. The smallest shock
speeds and pre-shock densities favor the survival of
the most volatile material and occur in the outermost
regions.
Scientific Approach
For the in-fall calculations we will adapt a previously developed model of the spatial and temporal
chemistry occurring in gravitationally collapsing
proto-stellar cores.14 This model is based on analytic treatments of the dynamics the temperature distribution in a spherical envelope1516, and we will modify it in order to take into account the angular momentum of the system. Hence, rather than falling
directly into the central protostar, fluid elements are
swept along ballistic trajectories until they reach the
surface of the proto-planetary disk. At this point,
the material encounters the accretion shock. The
dynamical scenarios we will employ are those described by Cassen & Moosman17 and by Terebey et
al.18. These allow determination of analytic density
and velocity solutions for rotating collapse of a gas
cloud to form a proto-stellar disk.
Lunine et al.13 studied the gas-grain physics of material falling into the proto-Solar Nebula and concluded that there may be two populations of dust in
the disk: unaltered interstellar grains and grains
which possess re-condensed water and other nebular molecules. Compositional and isotopic evidence
from analysis of meteorites, interplanetary dust particles (IDPs) and comets suggests that some volatile
interstellar molecules may have entered the nebula
relatively unscathed10. However, until now there
have been no detailed studies of how the accretion
process affects the volatile organic material.
Following Ceccarelli et al.19 and our previous
work20, we will neglect the effect of the thermal
evolution on the hydrodynamics. For given evolving envelope/disk structures we will simply use parameterized dust and gas temperature profiles derived from radiative transfer calculations, assuming
gas and dust in thermal equilibrium. Non-rotating
collapse calculations indicate that, soon after the
collapse has started in Shu’s dynamics, in-fall timescales become shorter than most chemical timescales. This results in material from the cool enve-
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• How many trace cometary organics, such as the
lope collapsing onto the central protostar without
significant chemical alteration20.
simple hydrocarbons (CH4, C2H2, and C2H6),
can also be formed at the accretion shock?
As the motivation for these calculations was to
study the large-scale chemical evolution of the core,
we will add additional processes, which may be important in modifying the chemistry of the in-falling
pre-shock gas near the disk surface. These will include: 3-body reactions, gas-grain drag heating (for
various grain sizes to quantify the effects of dust
coagulation), and UV photons from the disk surface
and protostar. This model will then allow us to accurately calculate the initial chemical composition
of the gas entering the accretion shock, as a function
of both time and distance from the central star.
• Which specific interstellar deuterium fractionation
signatures are most robust, as a function of entry
point, as ISM material is incorporated into the
nebula?
1.4.2 Chemistry of Proto-stellar Disks and the
Proto-Solar Nebula
A theoretical understanding of the organic chemistry occurring in proto-stellar disks will be a major
component of the proposed work. In what follows
we use ‘nebula’ to be specific to the proto-Solar
Nebula, and ‘disk’ for the general case.
A rotating collapse model allows the composition of
accreted interstellar material to be studied as a function of entry position on the nebular disk. For the
shock calculation will use a steady J-shock chemical
code.21 This is based on the model of Hollenbach
& McKee22 and will be modified to treat the more
extreme conditions of the accretion shock problem
(e.g., 3-body reactions). For these calculations we
will abstract shock chemistry reactions from the
UMIST kinetic database23, as well as from published astrochemical shock models and the NIST
database. The post-shock chemistry of volatile organic material will then be followed as it is heated
and allowed to cool and re-condense onto dust
grains. We will initially not consider shock chemistry of aromatic compounds.24 Although an important component of carbonaceous meteorites, and
probably also of comet dust, polycyclic aromatic
hydrocarbons (PAHs) have not yet been identified
in cometary ices. However, the proposed modeling
will place us in an excellent position to eventually
address this issue. Several comprehensive PAH
chemistry networks are available which can be
adapted for this purpose.25
Background
Nebular Structure & Physics
The evolution of the proto-Solar Nebula and other
proto-stellar disks can be roughly divided into four
phases26: (1) accretion of molecular cloud material
(lasting a few hundred thousand years) most of
which is consumed by the proto-Sun; (2) disk dissipation (lasting about 50,000 years) where there is
large scale mass and angular momentum transport;
(3) final Solar accumulation (1-2 Ma) where giant
planet formation is almost complete but less so for
the terrestrial planets; (4) removal of nebular gas by
winds and stellar photo-evaporation (3-30 Ma).
Comets started to be assembled towards the end of
stage (2) when viscous effects dominated the nebular evolution.
The proto-stellar disk undergoes differential rotation
and shearing motions lead to frictional (or viscous)
forces, which heat the disk and cause radial motions.
Possible sources of this dissipation are turbulence
induced by thermal convection27; magnetorotational instabilities28; and various gravitational
instabilities27. For the case of a turbulent viscosity,
the disk temperature is determined by this viscous
dissipation and by convection in the vertical plane.
In all cases, radial motions involve the inward
transport of most of the gas and dust and the outward transport of most of the angular momentum.
All previous and current disk chemistry models
We will initially address three specific chemical
questions:
• How does the physics of in-fall cause specific
chemical changes in the major volatile components?
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have assumed a turbulent viscosity and, as this is the
primary focus of this research, we shall also (see
Balbus & Hawley28 for an alternative view).
This decoupling also allows the vertical chemical
structure to be computed in a straightforward manner34. These models consistently show that ionization is greater in the upper regions of the disk and
can explain the existence of the molecules observed35. Depletion on to dust becomes increasingly more important as one approaches the mid-plane.
The CO depletions observed in disks have been attributed to this. In the dense disk, the gas-grain interaction is a key process and either X-ray desorption or photodesorption36 could be responsible for
those CO molecules that are detected.
Nebular Chemistry
Dynamical models predict the existence of a hot,
high-pressure inner region where dust can be vaporized. In this region (1 AU or less), chemical timescales are so rapid, relative to dynamical timescales, that chemical equilibrium is an excellent approximation. A radial temperature profile that decreases monotonically to around 10 K in the farthest
outer disk (i.e., 100 AU or so) is also predicted by
most dynamical models. Beyond 5 AU, the timescales of chemical processes eventually become
much longer than all dynamical time-scales with
increasing distance; at the farthest radii they are
longer than the age of the disk. Intermediate to
these regions is one where some dynamical and
chemical time-scales can be comparable. This is the
giant-planet formation zone and where the comets,
which bombarded the early Earth, were produced.
The chemistry of mass accretion involving inward
radial transport onto the central protostar was treated
by Bauer et al.37 and by Aikawa et al.38. In this
case, they solved the continuity equation for species
being advected into the inner disk. However, as one
moves inwards, more physical and chemical processes come into play.
A significant difference between the model of Bauer
et al.37 and all those outlined above, is that they also
considered the destruction of dust and volatiles as
they are transported into the hot inner disk regions.
Chemical modeling of the inner nebula is not new39.
Much of this effort has been based on chemistry in
thermodynamic equilibrium, with the introduction
of kinetic inhibition and quenching of chemical
abundances in the low-temperature, comet-forming,
regions where equilibrium is not valid. One qualitative difference concerns the gas-grain chemistry. In
the inner nebula, catalysis can involve dissociative
chemi-sorption onto metallic particles.40 This is
fundamentally different from the cold surface catalysis, driven by physisorption, that occurs in interstellar clouds and, presumably, in the outer nebula.41
In the regions of the disk greater than about 10 AU,
the surface density is low enough that cosmic rays
could penetrate with minimal attenuation. Other
important sources of ionization for driving the ionmolecule and neutral-neutral chemistry in this region are X-rays29, UV photons30, and radioactive
decay (e.g., 26Al and 60Fe). The recognition that the
outer proto-Solar Nebula was probably a chemically
active region has motivated several detailed studies.
These began with simply applying simple interstellar gas-grain chemistry, either in a steady state for an
assumed disk structure, or a time-dependent calculation at a fixed point under nebular conditions. Because the dynamical and chemical evolution can be
decoupled in this region, there have been several
time-dependent chemical models that assume a stationary density and temperature structure31. The
chemistry is very temperature-sensitive and is greatly influenced by the dynamical model adopted. For
example, in deuterium fractionation by ionmolecule reactions it really matters whether the gas
is at 10K, 20K, or 30K.32,33
A key issue is how much these chemistries contributed to the composition of comets. A product distribution similar to that found in Fischer-TropschType (FTT) catalysis, for example high molecular
weight aliphatic hydrocarbons, could be produced in
the inner nebula39. If this is the origin of ethane in
comets then it requires outwards radial mixing in the
nebula. Alternatively, reduction of acetylene on
cold (10K) grains could form ethane; this would
require that it survive transport to the comet-forming
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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region from its place of origin (natal cloud or 100
AU in the nebula).
Objectives and Motivation:
We propose to perform detailed theoretical studies
of the molecular chemistry of proto-stellar disks.
Radial Mixing of Nebular Material
Turbulent motions, whether convective or magnetohydrodynamic in origin, lead to a situation where
there is an outward diffusion of material from the
inner nebula, and an inwards advection of material
from the outer nebula.42 This leads to the radial mixing of the products of the two chemistries described
above; the key question is how much of this mixing
actually occurred. In the proto-Solar Nebula, radial
mixing was probably most pronounced in the region
of giant planet formation. Hence, the spatial and
temporal chemical evolution of this region was crucial in determining the composition of comets (especially the inventory of organics) that bombarded
the terrestrial planets.
Calculations will be undertaken to determine the
spatial and temporal chemistry of the gas and dust
within the 5-40 AU comet-forming region of the
proto-Solar Nebula. These studies will aim to quantify how important for the organic composition of
comets was outward radial mixing of matter from
the hot inner nebula with pristine and partiallyprocessed interstellar material from the cool outer
nebula. We will compare the results with the composition of comets and with astronomical observations of proto-stellar disks.
Scientific Approach
Our goal is to develop a suite of dynamicalchemical models that will allow us to routinely calculate the radial and vertical chemical structure of
proto-stellar disks in detail. We will employ stateof-the-art chemical models. Many studies have addressed the dynamical evolution of the scenarios of
interest to us27 and mathematically extending these
models to include chemical reactions is straightforward, if somewhat involved44. However, obtaining
‘ground truths’ from the chemistry results requires a
great deal of care and experience - it is not simply a
matter of including a reaction network. One example of this concerns the treatment of grain-surface
reactions in the outer nebula. The previous approach
has been shown to be fundamentally flawed48 and a
full stochastic Monte Carlo simulation of this problem is essential.49 However, this is presently not
practical for disk chemistry due to the number of
processes involved; we describe our adopted approach below.
Theoretical models of radial mixing involve solving
the related systems of advection-diffusion equations,
usually for a stationary disk structure.42 Mixing
models of disk chemistry involve incorporating
chemical reaction networks into these models as
source and sink terms. This leads to a system of
reaction-advection-diffusion equations.43 As the
age of proto-stellar disks is longer than the diffusion
time-scale in the outer disk, non-stationary disk
models are required for a long-term treatment of the
mixing process. In the case of a turbulently viscous
disk, the disk evolution27 is also solved with the
chemical evolution.44
Amongst the attractive features of radial mixing
models is that they offer a means of transporting
crystalline silicates into the comet-forming region45.
However, nebular shocks are another possible candidate for the origin of annealed silicates at around
10 AU.46 Whichever explanation is correct, one
might expect that the organic chemistries associated
with each will be different. Also, Gail47 showed
that while mixing of oxidized C-dust can explain the
presence of some hydrocarbons in comets (CH4 and
C2H2), it can not explain the presence of others
(C2H6 and CH3OH). A major part of the theoretical
work proposed here aims to address these and other
issues through quantitative chemical modeling.
Chemical modeling of the inner nebula has a long
pedigree and for this we will employ the models of
Fegley50. These contain a comprehensive chemical
equilibrium description of the gaseous and solid
chemistry appropriate to this region, as well as gasgrain catalytic processes such as FTT. These chemical codes are the state-of-the-art. We do not anticipate they will require any substantial development
work, at least not in the early phases of the project
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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when the outer nebula chemical models will be developed.
gas phase atomic O/H ratio. Thus, we may expect
significant differences with published models even
at this stage.
Modeling the chemistry of the outer nebula has now
reached a high level of sophistication. We envisage
a methodical build-up of increasing dynamicalchemical coupling starting from the simplest, that is,
none. In developing these models, we will initially
employ a very simple chemistry of around 30 species. This is sufficient to comprehensively model
the major C-H-O species in molecular clouds and
contains all the major cometary volatiles: H2O, CO,
CO2, H2CO, CH3OH, CH4. A carefully constructed
and validated chemical network is essential for these
studies, especially since many complex reaction
pathways will have to be considered for the organic
species we are interested in (e.g., carbon chains). As
in other studies, we will use the UMIST and NIST
databases. Here again, caution must be exercised
since these are compilations and even the UMIST
one, geared for astrochemical modeling, cannot be
reliably used in an off-the-shelf’ way. We will also
take reaction data from the chemical literature
where appropriate.
Model II will be a straightforward extension to include inward radial transport by solving reactionadvection system for the chemical trace species.
For the case when matter is still being accreted on
the disk surface, the accretion shock calculations
will provide the chemical composition of this material; with no vertical transport considered, we will
simply assume that it is initially transported to the
mid-plane unaltered. Model III will include both
advection and outward radial diffusion from the
inner regions.43 Model II will be developed so that
we can implement the numerical treatment of reactive transport in a simpler case. It is an intermediate
step45 in reaching Model III that will be the basic
model we will use for implementing complex organic chemistries from the inner and outer nebular
environments. We anticipate that we can reach this
stage at the end of the first year of the project.
In Model IV we will treat the chemistry in the vertical plane above the comet-forming regions of the
disk. In contrast to previous models of this problem30,34 we will consider the convective transport of
chemical tracer in the vertical plane of the disk, at
radii of 5-40 AU, at several epochs. It is clear how
significant this process could be for the chemistry
and therefore the organic composition of the disk
mid-plane: trace chemical species can be transported to higher regions of the disk where there is more
ionizing radiation, undergo chemical reactions, and
then return towards the mid-plane, where the radiation does not normally penetrate, to re-condense as
ices. These effects have never been studied before.
The vertical structure can be obtained from hydrodynamical models of a viscous disk51 and we can
apply the same principles to calculate the chemistry
under transport. Radiation transport is important
only for the upper layers and we will adopt the same
prescriptions as in the steady models.
We divide the development of these models depending on whether the underlying dynamics is
steady or unsteady.
Stationary Disk Models
Under the assumption of a stationary disk, we will
begin by developing four models (I-IV) that will
include increasingly realistic transport processes.
These models will initially only involve the simple
30 species chemistry. First, we will take a given
physical structure and calculate the radial distribution of molecules. The aim of model I will be to
ensure that all the relevant chemistry and microphysics have been correctly incorporated (e.g., Xrays, UV flux, 3-body processes, gas-grain exchange of molecules and surface chemistry), and the
results benchmarked against similar codes31.
One important difference is that we will not follow
previous treatments of grain chemical kinetics. Instead, since surface timescales are so fast, at each
radius we will simply ‘oxidize’ or ‘reduce’ the accreted surface composition in proportion to the local
Non-Stationary Models
The stationary disk models are only valid over a
fraction of the disk evolutionary age. Nevertheless
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Theme 1: Evaluate delivery of pre-biotic organics and water to the young Earth.
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magnitude changes in the opacity43. However, as
grain coagulation is not treated even in some of the
more comprehensive dynamical simulations27, we
shall ignore it here also.
they are very useful - those published to date have at
least been able to reproduce the broad characteristics
of observed disks. Hence, we will develop a model
of the chemistry in a viscously evolving nebula
(Model V). We will couple chemical model III
with a calculation of the disk dynamical evolution.
This was done recently by Wehrstedt & Gail44 and
is the state-of-the-art in nebular chemistry modeling.
This can be done in straightforward fashion since
the same algorithm can be used to solve for the dynamics.
The numerical method we shall use to solve the associated systems of reaction-advection and reactionadvection-diffusion equations in Models I-V will be
based on the fully-implicit method of Richtmeyer &
Morton54. This allows the construction of very
powerful, stable and flexible algorithms and it found
early applications in computer simulations of turbulent transport in fusion plasmas.55,56 It is easily extended to diffusion in multi-species problems and
ideally suited to solving highly nonlinear diffusion
equations. Charnley is very experienced in solving
these problems and has recently employed this
method to model multi-species reaction-diffusion
systems in cometary nuclei.57 Simple tests on published interstellar reaction-diffusion problems yield
similar results. In fact, the finite-differencing employed by Gail and his collaborators is very similar
to this method44.
The spatial dependence of the opacity is a key input
for the dynamical modeling of these disks because it
determines the disk temperature. The opacity depends sensitively on composition of the disk material. Initially, to develop these codes, we will employ
simple power law relations.52 However, eventually
we will employ results from more sophisticated
treatments that consider the chemical composition
of the gas/dust mixture.53 This is an important issue
since, for example, by simply annealing any silicates in the mixture this alone produces order of
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2.0 From Molecular Cores to Planets: Our Interstellar Heritage
The molecular heritage of our Solar System stretches back to the natal cloud that preceded our Sun.
The complex energetic and chemical evolutionary
history of planetary bodies like Earth has by now
obscured direct vestiges of this early chemical soup
on them. As presented in Section 1, comets represent one way to access this early molecular composition in our nebula. There are two other vital
sources for information about our history: young
planetary systems that are themselves currently in
formation in the Galaxy around us1 (e.g., Figure
1.0), and other evolved planetary systems discovered over the past ten years2.
about the variety of possible outcomes from the
formation process. We can learn about the molecular complexity of material falling into our early system, about the ability of pre-biotic molecules to survive to incorporation into the first pebbles, and
about the modifications to interstellar molecules
exposed to energetic processes in the early Solar
System. By studying the spectra of planets around
other stars we can learn about the atmospheres of
non-Solar System planets and look for bio-indicator
molecules.
We propose to use a wide array of current astronomical instrumentation to study the chemical,
structural, and energetic evolution of material that is
incorporated into planetary systems. The work
funded primarily by this proposal will focus on linking astronomical observational work to the other
parts of this proposal and to the general astrobiology
community.
By studying these systems, at all stages of evolution
from collapsing pre-stellar clouds to currentlyforming stellar system to evolved planetary system
(e.g., Frontispiece, Figure ES.1, Figure 1.1), we can
learn what conditions must have been like during
the formation of our Solar System and we learn
2.1 The Beginning of our Heritage: The Initial Conditions for Planet Formation
Investigators: Geoffrey Blake, William Irvine, Michael Hollis, Lee Mundy, and Jeffrey Pedelty
There is a clear broad-brush picture of how Sun-like
stars form inside molecular clouds in our Galaxy
(Frontispiece).1-4 Pieces of these clouds become
dense enough to become unstable to gravitational
collapse. The material falls inward, ever increasing
the central density until enough mass is accumulated in the central region to form a stellar core. Continued collapse and in-fall of material in the surrounding cloud adds material to the forming star.
Due to conservation of angular momentum, more
and more of this material cannot fall directly onto
the star; rather it falls onto a rotating disk around the
star, through which material then feeds onto the star.
Over time the reservoir of surrounding material is
used-up and the star plus circumstellar disk emerges
from the natal cloud. This process takes 1-5 Ma
from initial collapse to exposed stellar system (Frontispiece).
extends from a few stellar radii out to hundreds of
AU from the star5,6. Observations also indicate that
these disks initially form as early as 104 years after
formation of the first stellar core and that the disks
persist for the first 10 to 20 Ma of the star’s lifetime7. These circumstellar disks are the birth places
of planets, comets, and all components of planetary
systems.
Understanding the molecular heritage of our Solar
System and other planetary systems is directly
linked to understanding the evolution of cloud material feeding into the growing star and the evolution
of disk material around the star8. This direct connection is amplified by the new theories of giant
planet formation and the diverse isotopic heritage of
individual grains found within some meteorites.
In particular, the discovery of many “hot Jupiter”
systems9 has driven reconsideration of the scenarios
Current observations and theory suggest that most in which giant plants form10,11. Now, it is expected
of a star’s mass is built-up from material that passes that giant planets commonly migrate during forthough the circumstellar disk. This disk typically mation; it is even speculated that the earliest giant
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Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
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frared Space Observatory’s Short Wavelength
Spectrometer (which covered 2.4-45 m) tremendously advanced our understanding of ices
in molecular clouds and star forming regions.
With this instrument, the overall composition
of icy grain mantles in the envelopes of massive young stellar objects was determined15.
Of key interest are the vibrational absorptions
due to H2O (3 and 6 m), CO (4.27 m), CO2
(4.27 and 15.2 m), CH4 (7.7 m), NH3 (~9
m), OCS (4.9 m), OCN- (4.62 m), CH3OH
(3.53 and 9.7 m), and an as yet unidentified
organic feature at 6.85 m. These molecules
are potential precursors to biogenically important species. In addition to these, PAH
emission (3.3, 7.7, 8.6, 11.2 m) and silicate
absorption (9.7 and 18 m) may also contribute to this spectral region.
planets may merge into the star, with the final planetary system just the one left standing when the disk
ran out of gaseous material; but all such scenarios
are strongly dependent on the radial and vertical
density and temperature profiles of the disk.
In meteorites, isotopic studies clearly show that
some individual grains were able to maintain their
integrity through accretion onto the disk and inclusion into small bodies. Could the same be true for
ices and embedded complex molecular species?
What fraction of the ices and organic materials in
small bodies represent material locked-up when the
coagulating particles were only hundreds of microns
or millimeters in size?
Our group has the core expertise and on-going research programs in the areas of star- and planetformation, the evolution of planet-forming disks,
chemistry of complex molecules, and detection of
pre-biotic and bio-molecules. In collaboration with
other members of our node we can address the following areas.
Due to issues of sensitivity, the ice mantle
composition around low- and intermediatemass YSOs and cold molecular clouds are not
well studied. Ground based spectrographs
covering the 1-5.5 m region at low to moderate resolution are available at the VLT
(ISAAC), Keck (NIRSPEC), UKIRT (CGS 4),
Subaru (IRCS), and IRTF (SPEX). The 5-40
m region will be available with SIRTF. With
this combination of ground and space based
observatories, we can cover the entire region
of solid state ice features that are of astrobiological interest with enough sensitivity to investigate the chemistry in young solar analogs.
Once we know the composition of icy grain
mantles in low mass YSO’s, we can evaluate
the potential detectability of ices in the outer
regions of protoplanetary disks. Laboratory
studies are vital to interpretation of these data.
The position, width, and profile shapes of ice
features are sensitive to history of thermal and
energetic processing and mantle composition
(pure ice vs. mixtures). Eventually, the launch
of JWST and the hoped-for launch of the Astrobiology Explorer (ABE) will provide opportunities to investigate ice chemistry with unprecedented sensitivity.
• The Creation of Seed Planetesimals: When does
it occur? How big do dust grains grow in the
relatively benign environment beyond 50 AU?
Observations of other young stellar systems at centimeter, millimeter and infrared wavelengths are
supplying answers to these questions. For example,
there is now clear evidence of strong depletions of
gas-phase molecules in the dense portions of clouds
that are in the early stages of star formation12,13.
Depletions of molecules that form simple ices with
high sublimation temperatures (e.g., water, methane,
methanol) can be wide-spread throughout regions of
clouds shielded from UV light; CO2 ice, which has
a low sublimation temperature, tends to be concentrated within the darkest and densest regions14. The
combination of molecular line observations of gasphase species and infrared observations of ice absorption features tracks the build-up of ices on grain
mantles and yields insights into the history of its
creation.
Knowing the composition of interstellar ice
mantles is critical to studies of chemical evolution in protostellar envelopes. The recent In-
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In addition to icy buildup, there are a few young
systems whose dust emission properties suggest the
formation of dust grains up to hundreds of microns
in size within the in-falling material, at radii of hundreds of AU, before incorporation into a disk16. The
best-documented case is the NGC 1333 IRAS 4
system. NGC 1333 IRAS 4 is a very young forming star (likely age < 10,000 years) that is embedded
in core of in-falling material. This material extends
out to 3,000 to 5,000 AU, and its dust emission
properties are very similar to those inferred for material in typical circumstellar disks. The implication
is that significant grain growth has occurred out at
distances where gas and dust temperatures are expected to be 15 K or less. Such grains are expected
to be fluffy, icy composites. Theoretical opacity
coefficients calculated for icy coagulated grains17
have yielded promising results.
throughout the present Solar System. However, it is
difficult to document exactly how, when, and why
that buildup occurred. Observations at millimeter
and centimeter wavelengths can play a key role in
tracing the buildup from micron-sized grains to
many-centimeter sized bodies because the dust continuum emission at these wavelengths is sensitive to
the emitter’s size and because it is only at these
wavelengths that the young disks become optically
thin enough to permit measurements of the full disk.
Our key goal will be to document grain growth observationally and to attempt to document opacity
loss as the bulk of the material gets incorporated into
bodies larger than a few millimeters. There are
many past observational studies that have inferred
grain growth from the spectral index of the dust
emission at millimeter wavelengths, but such a simple conclusion is not likely to be correct in many
cases (see Beckwith et al.18 for a full discussion).
The critical point is that spatially-resolved imaging
at multiple wavelengths is needed to resolve ambiguities introduced by dust properties, source opacity, and source structure.
Understanding the growth of grains prior to incorporation in the disk, and understanding the process
of incorporation into the disk are critical to assessing
the dominance of interstellar heritage. To accomplish these goals, we propose to link on-going work
on gas and dust in forming stars with laboratory
work on ices (Section 3), and with numerical modeling of coagulation during infall (Section 1.4). We
will emphasize physical and chemical conditions of
the gas and dust properties in material at 100 to
5,000 AU. The observational work will include
dust continuum imaging of young stellar environments at wavelengths from 1.3 mm to 1.3 cm, molecular line imaging of selected gas tracers (to monitor molecular depletions and physical conditions in
the gas), and SIRTF infrared observations of dust
continuum emission and absorption features.
Mapping of young circumstellar disks with linear
resolutions as good as 30 AU is possible with current millimeter wavelength arrays such as the
BIMA array (our Team includes members from two
of the three owner institutions); linear resolutions as
good as 7 AU have been achieve with the NRAO
VLA19.
Figure 2.1 displays an image of the dust emission
from the disk around HL Tauri with 0.3” (42 AU)
resolution superimposed on an HST 1.1 micron image of the nebulosity associated with the star (the
star itself is barely visible in the HST image)20. This
emission at 1.3 millimeter wavelength, as well as
the image of emission at 2.6 mm wavelength, is
well-fitted by a simple power-law surface density
distribution. Radial dependences in the range of R0.8
to R-1.5 with outer disk radii of 80 to 140 AU fit
the current data adequately. The two wavelengths
are best fitted with a relative emissivity of 2 (1.3
mm/2.6 mm) supporting the evidence for grain
growth in this system.
• The Creation of Seed Planetesimals: The Evolution of Planetesimals and Gas in the Disk: How
does it proceed and how is it tied to the gas content of the disk? Is there a simple evolution with
time or does giant planet formation and migration wipe the disk clean periodically?
The buildup of pebbles and rocks from small dust
grains, and their subsequent accumulation into gravitationally significant bodies, must have occurred in
the early Solar disk: we see the resulting bodies
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Dr. Mundy and his collaborators are continuing
studies of circumstellar disks on the BIMA array
and starting new work at 7 mm wavelength on the
NRAO VLA. The long term goal of this observational work is to characterize the disk structure and
dust properties in a number of young disk systems
with ages from 1 to 10 Ma. These results will help
to direct and interact with modeling of the growth of
planetesimals (Section 1.2).
exploration of this chemistry will provide insights
for pre-Solar Nebular chemistry (Section 1.4), and
provides a chemical boundary condition for nebular
material (Frontispiece; Figure 1.0; Section 1). Together, these studies can establish the biological potential for seeding newly-formed planets with exogenous organics.
This interstellar chemical heritage has become increasingly important with the confirmation of icy
mantles on dust grains and the growth of grain size
by coagulation. Such large fluffy grains provide a
vehicle for delivering delicate molecules to icy
planetesimals and for incorporation of these molecules, intact, onto planets, moons and other bodies.
For terrestrial planets, the delivery of organic material into the early atmosphere via comets holds special interest (e.g. Section 1). For icy moons, Pluto,
trans-Neptunian objects, and comets, material delivered any time after surface formation might be preserved, and form the initial biochemical soup for
life.
• The Creation of complex organic molecules and
bio-molecules: What molecules can be detected
in different environments? How far can interstellar and circumstellar disk chemistry take us
on the road to life?
More than 100 organic compounds have been found
to date in interstellar clouds of gas and dust grains
throughout our Galaxy21,22. The unexpected complexity of interstellar astrochemistry, and the detection of interstellar molecules of potential biological
interest (or biomolecules), have fascinated biologists, astronomers, chemists, and physicists for several decades. It is now generally accepted that interstellar molecular clouds contain heavy, complex
molecules, including biomolecules. Observational
Our group includes some of the world’s premier
biomolecule hunters. The leaders, Drs. Hollis and
Snyder, have three decades of work under their
belts. They have used interferometric arrays to detect large molecules such as ethylene glycol23, acetone24, ethyl cyanide, vinyl cyanide, and methyl
formate in hot young molecular cores25. They used
arrays to search a dusty molecular core near the
Galactic Center for the important biomolecules acetic acid and glycine (the smallest amino acid). As a
result, interstellar acetic acid was detected for the
first time26, and the first interstellar sugar, glycolaldehyde, was also discovered by this group27. There
are also promising suggestions and continuing work
on interstellar glycine.
Figure 2.1 Image of the disk around the young star HL Tauri as
seen in the 1.3 µm dust continuum emission (contours). The
background image is the HL Tauri reflection nebulosity as seen at
1.1 microns wavelength with the HST. The dust disk is roughly
200 AU in diameter and is viewed a little inclined from edge-on.
Our broad goal will be to measure the sources and
chemical origins of biogenic compounds in the interstellar medium in order to advance our understanding of the similarities that exist in the compositions of the biogenic compounds in interstellar,
cometary, and meteoritic samples. We will achieve
this by using interferometric arrays, laboratory
measurements of molecular frequencies, and image
processing to examine large biogenic molecules in
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Laboratory measurements are particularly important
to us and to the general community. Such measurements provide fundamental spectroscopic data
such as measured and predicted centimeter- and
millimeter-wave transition frequencies, intensities,
and hyperfine structure that are essential for planning search strategies and for making sound astronomical line identifications in the complex spectra
obtained with the current generation of sensitive
telescopes and arrays. Collaborator Dr. Frank
Lovas will continuously consult the literature of laboratory spectra for data on pertinent biomolecules,
and will obtain new laboratory measurements from
NIST and other spectroscopic laboratories as needed for identifications.
Figure 2.2 Comparison of the UV flux of the Young Solar
Analog,  Ceti (750 Ma) with the Sun. Absorption crosssections for various greenhouse gases are shown in schematic
form. (The cross-sections for ammonia are truncated at 1500 Å
for lack of readily available data.) The flux of Lyman- has not
been corrected for interstellar absorption.
star-forming regions of the interstellar medium.
2.2 Developing Heritage: Photo-Processing of Molecular Precursors in Young Planetary Systems
Investigators: Sally Heapand Rob Petre
phere – the chromosphere, transition region, and
corona – which are not well understood.
The evolutionary history of the Sun is well understood from stellar evolutionary theory. At its
“birth,” the Sun was more than 10 times more luminous and substantially cooler than it is today. According to evolutionary models1, its pre-main sequence phase lasted about 50 Ma. Upon its arrival
on the zero-age main sequence (ZAMS), the Sun
was only about 70% as luminous as it is now2 and
slightly cooler (Teff ~5,550 K). Over the course of
the last 4.6 Ga, the Sun has slowly evolved to its
present state.
Observations by Einstein, Rosat, and IUE have
shown that XUV emission is characteristic of virtually all “Solar-type” stars3,4 and is thought to have a
common origin: the outer layers of the stellar atmosphere, which are heated and confined by magnetic fields generated by a rotation-powered dynamo. Furthermore, these space observations showed
that “stellar activity” declines with stellar age5, as
would be expected from magnetic breaking in the
Solar wind 6.
The radiation history of the Sun, particularly the
history of its X-ray and ultraviolet (XUV) emission,
is less well known. Most of the Solar flux is emitted
in the optical-IR and shows a fairly smooth continuum. Given the temperature and surface gravity at
some point in time from evolutionary models, stellar-atmosphere models can reproduce the optical-IR
flux distribution nearly exactly.
This age-rotation-activity paradigm has spurred astronomers to trace the history of the Solar XUV
emission by using young stars of Solar mass as
proxies of the young Sun (Young Solar Analog’s, or
YSA’s). Such an approach has its limits – we do
not know whether the Sun was originally a slow or
rapid rotator – so we cannot put strict constraints on
the XUV emission of the early Sun. Nevertheless,
within these limits, there is much that observations
of YSA’s can teach us about the radiation from the
young Sun.
At about 2000 Å, however, the spectrum changes to
a much weaker continuum with bright emission
lines superposed, making the XUV flux much
stronger than would be expected from its effective
temperature (Figure 2.2). These XUV emission
lines arise in the outer layers of the Solar atmos-
The most comprehensive study of YSA’s is the
“Sun in Time” project7. This study indicates that
4.5 Ga ago, the Sun was rotating more than 10 times
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faster than today and consequently, had up to ~1000
times stronger coronal X-ray and EUV emissions
and up to ~100 times stronger chromospheric emission in the far-UV (900-2000 Å). The decline in the
XUV fluxes with age (Figure 2.3) or rotational period can be fit with power laws of different slopes: the
more energetic the emission, the steeper the slope.
Despite this early observational work, it is only
more recently that two important inferences have
emerged: the importance of X-ray emission in shaping the environment of PMS stars, and the detection
of ubiquitous, strong X-ray emission from all classes of young stellar objects (YSOs); proto-stars as
well as PMS stars (Figure 2.4). Proto-stars have
been difficult to detect in the X-ray band because
the enshrouding cloud absorbs most of the X-rays
below ~2 keV, where the first generation of X-ray
imaging observatories were sensitive.
More recent observatories with imaging capability
up to 10 keV (ASCA, Chandra, XMM-Newton)
have shown that strong and highly variable X-ray
emission is a ubiquitous feature of proto-stars. In
the best-studied field, the Orion Nebula, Chandra
detected 70 percent of the known proto-stars (as
well as all the classical and weak line T Tauri
stars)10. As expected, these sources are more absorbed than more evolved cluster members, but they
also have higher temperatures. The duty cycle of
flaring is higher in them than in more evolved stars,
as is the ratio of flare luminosity to quiescent luminosity.
Figure 2.3 Evolution of XUV irradiance of Young Solar Analogs as obtained from observations by ASCA, Rosat, and IUE.7
The decline in energetic radiation is well correlated with stellar
age and rotation.
The impact of X-rays
The emission of X-rays by young stars is a phenomenon that has been known for nearly a quarter
century (see extensive list of references in review by
Feigelson and Montmerle8). The discovery of nearly ubiquitous X-ray emission by pre-main sequence
(PMS) stars was made using the first imaging X-ray
observatory, the Einstein Observatory. In fact, Einstein advanced the study of PMS stars by establishing the existence of a distinct evolutionary class, the
so-called weak line T Tauri stars, PMS stars whose
disk has disappeared en route to the main sequence9.
It can be presumed that planets are associated with
the disappearance of the disk in at least some of
these systems. The X-ray emission is highly variable and thermal. Correlation between X-ray luminosity and rotation rate and other indicators leave
little room for doubt that the emission arises from
magnetic processes.
Figure 2.4 Schematic of the evolution of the X-ray luminosity
of Solar-type stars (adapted from Randich11). The bars labeled
“Disk” schematically show disappearance of the disk mass with
time12. The quiet Sun is also shown.
The X-ray emission from YSOs is thermal in nature. The highly variable flux arises from magnetic
activity. The temperature of the plasma responsible
for the emission falls in the range 2-10x107 K. Quiescent luminosities fall in the range 2x1028 erg s-1 to
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40
Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
———————————————————————————————————————
5x1030 erg s-1, while flare luminosities are typically
4-20 times higher, reaching in some stars a luminosity of ~1032 erg s-1. The quiescent and flare luminosities can be compared with the Sun, whose quiescent X-ray luminosity is ~2x1025 erg s-1. The typical
Solar flare luminosity is ~1x1027 erg s-1, with the
most powerful flares reaching ~1x1028 erg s-1. The
much higher values in younger stars have been attributed to stronger magnetic fields and faster rotation rate. The flare luminosities in proto-stars are
higher than those in later evolutionary stages (T
Tauri), suggesting a higher magnetic field in protostars .
ratio of some deuterated molecules (NH2D/NH3 and
DCN/HCN).
The X-rays have a significant influence on the circumstellar and interstellar environment. They produce a variety of effects, most notably ionization
and modification of the chemistry of the gas and
dust. Because of their penetrating power, X-rays are
likely to play a central role in the heating and ionization of the disks surrounding young stars.13,14 The
ionization is important because it allows the coupling of the gas with the magnetic field. X-rays will
create an extended region of low ionization and Xray heating, which is bounded by the absorption
length of X-rays through the disk. The primary ionization mechanism is the photo-ionization of inner
shell electrons, which produces cascades of lower
energy photons (1 keV photon can produce 30 such
photons with energy 35 eV). A 1 keV photon can
penetrate to AV ~ 2.5; a 5 keV photon to AV ~100.
At the densities encountered in the disk, the recombination time is ~10 years.
More exciting is the claim that excess emission at
95 µm measured by ISO in the 50-200 µm continuum of the Solar-type protostar NGC 1333-IRAS 4
is a calcite dust feature.18 The observed intensity
indicates that calcite represents 1 percent (a substantial fraction!) of the warm (~23 K) dust. What is
particularly exciting is that calcite requires liquid
water to form. The authors suggest that the calcite
forms at the surface of the grains where water ice
layers may have locally enhanced mobility caused
by heating due to hard X-rays emitted by the central
object. They go on to speculate that under the conditions in which calcite forms, the possibility exists
that pre-biotic or biotic molecules that need aqueous
solutions to form might also form on grains due to
localized X-ray heating.19
The effects of X-ray heating and ionization must be
detected in other bands. In particular IR and millimeter observations are used to measure molecular
abundances and search for new species. Recent
observational results suggest that the results of irradiation by X-rays are observable. Weintraub et al.
detected vibrationally-excited H2 at 2.2 µm in the
disk around the X-ray bright classical T Tauri star
TW Hya16. The only viable mechanism for exciting
the H2 is X-ray ionization.17 The intensity of is
emission consistent with X-ray excitation models.
No direct connection between the X-rays produced
by young stars and the formation and destruction of
pre-biotic molecules has been established. However, the promising traces of evidence for the chemical
effects of X-ray irradiation of disks raise the possibility that the ubiquitously strong X-ray emission
in young Solar-type stars directly affects the prebiotic chemistry of circumstellar disks.
The irradiation of molecules by X-rays will produce
a complicated series of chemical reactions, which
are difficult to calculate in a fully consistent way. In
warmer regions, the X-ray heating will promote a
variety of neutral reactions. In cooler regions, Xrays stimulate molecular synthesis by ion-molecule
reactions. There are recent calculations of the overall distribution of molecular material in disks taking
into account X-ray heating, and predictions.15Chemical tracers of X-ray heating and ionization include the HCN, C2H, CN and HCO+ column densities (enhanced by X-ray heating), and the
Making a more direct connection is a challenge,
both observationally and theoretically. Observationally, this connection will come from a combination of a measurement in the IR or mm band of an
unusual abundance of a pre-biotic molecule in the
environment of a YSO that can be correlated via
models and laboratory simulation with the ionizing
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41
Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
———————————————————————————————————————
environment characterized by the X-ray observations.
Obviously observations in the X-ray band will not
reveal the abundances of pre-biotic molecules. As
X-rays are a crucial environmental influence, however, it is essential to fully characterize the X-ray
properties of young stars as they evolve to the main
sequence. A substantial amount of phenomenological work has been done in this area20 but much
more remains to be done. In particular, it is only
with the most recent, powerful X-ray observatories,
Chandra and XMM-Newton, that modest numbers
of Class 0 and Class I proto-stars have become detectable. A true understanding of the properties of
these objects requires observations of many more of
them, as well as long term monitoring of a subset.
Figure 2.5 Evolution of CIV resonance emission. The irradiance measurement of the T Tauri star is a lower limit (no correction for interstellar absorption), and the plotted age is an
overestimate (the object is a pre-main sequence star).
will be used (see Section 3.3) to assess the influence
of X-rays on pre-biotic molecules and to establish
measurements that can be made in other bands of
stellar systems that will reveal this influence.
The impact of ultraviolet emission
High-resolution spectroscopy using the grating
spectrometers on these observatories and the XRS
on the pending Astro-E2 mission can provide qualitative and quantitative insight into the emission
mechanism and ionization conditions. Observations
during flares might detect hard tails of non-thermal
radiation. This spectral component is indicative of
the presence of high-energy particles. The presence
of a significant, high-energy particle flux, thought to
be the source of some of the isotopic anomalies
found in micrometeorites, and the influence on disk
chemistry of penetrating particle flux (observable
indirectly via X-ray and radio) needs to be understood as fully as the influence of X-rays.
The Space Telescope Imaging Spectrograph (STIS)
installed on the Hubble Space Telescope has become the premier UV-optical instrument for studying the ultraviolet emission of young Solar-type
stars. As shown in Figure 2.1, Lyman- is by far
the strongest chromospheric feature, contributing
60-80% of the far-UV flux between 1200 and 1500
Å.
The high-resolution of Hubble/STIS spectra enables
us to correct for absorption by interstellar gas and
thereby, to obtain the intrinsic strength of the
Lyman- emission. They also enable us to validate
atmospheric (photosphere + chromosphere + transition region + corona) models, before extrapolating
them into the unobservable EUV region of the spectrum. Such extrapolation is necessary in order to
calculate the photolysis rates of important greenhouse gases like methane, whose absorption crosssection extends into the EUV, to about 500 Å. The
EUV emission is also needed to estimate the rate of
loss of hydrogen through the atmosphere to space,
and hence, the rate at which Earth’s surface was
oxidized. The rate of hydrogen loss is determined
by the structure and chemical composition of the
upper atmosphere, and the former is influenced by
the heating of the thermosphere by Solar EUV radiation (100-900 Å).
Finally, it might be possible to couple X-ray monitoring observations of specific stars with sufficiently
high X-ray flux with infrared and mm interferometric observations in an attempt to measure directly a
change in the chemical abundances as a consequence of strong flaring.
Independent of establishing an observable connection between X-rays and pre-biotic molecule formation, extensive and detailed X-ray observations
of YSOs have tremendous value in supplying accurate input to chemical modeling codes and to laboratory simulations. Thus far, modeling has relied on
typical X-ray fluxes, and has not incorporated the
effects of flaring. The analytic and laboratory work
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Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
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Recent STIS+FUSE studies of the far-UV emission
of PMS stars and YSA’s add new information about
the global UV photochemistry of the Earth’s early
atmosphere21. As illustration, Figure 2.5 shows the
decline in flux of the CIV 1550 resonance doublet
of Solar-type stars of different ages. From this plot,
we infer that the 1550-Å luminosity of the Sun at
the time when life started on Earth was about 7
times greater than at present.
proposal include Kenji Hamaguchi, an NAS/NRC
Resident Research Associate, whose research has
concentrated on observations of star forming regions using the Chandra X-ray Observatory; Tim
Kallman, whose XSTAR code is the premier X-ray
photoionization modeling program and who was a
pioneer in investigating the effects of X-rays on
chemistry in disks24; and Rob Petre, the head of the
X-ray group, with expertise in a broad range of Xray observational and instrumentation issues. The
kinds of studies described above, detailed spectroscopy of bright YSOs and systematic characterization of YSO properties, are already underway, and
will continue. Coordination with other team members will lead to consensus objects for multi-band
observations.
New observations with STIS are certain to add important new information about the impact of UV
radiation on the Solar disk. For example, CfA scientists are presently obtaining Hubble/STIS spectra
of LkCa 15, GM Aur, and DM Tau, three protoplanetary disk systems whose chemistry has been
studied at millimeter wavelengths. Their studies
indicate that disks have a rich molecular chemistry
that appears to be controlled by Far-UV radiation.22,23
Likewise, the GSFC UV-optical astrophysics group
has developed STIS and is using it to study circumstellar disks. Bruce Woodgate (the PI of STIS) co-I
Sally Heap, and Carol Grady are conducting an energetic program of coronagraphic imagery of dust
disks, and coronagraphic spectroscopy of the gas in
these disks. Heap is also conducting a study of the
influence of UV Solar radiation on the photochemistry of the early Earth’s atmosphere. The UVoptical group maintains an archive of all publicly
available STIS data and plans to continue its research on the structure, content, and composition of
circumstellar disks.
Goddard Institutional Support
Goddard scientists associated with this proposal
are in a unique position to connect XUV astronomy with astrobiology, as a natural outgrowth of
the close proximity of GSFC’s X-ray astrophysics, UV-Optical astrophysics, and astrochemistry groups.
The GSFC X-ray astrophysics group has expertise
in all aspects of X-ray astronomy, observational,
instrumental, and theoretical. Participants in this
2.3 Organic Signatures of Extra-Solar Planets
Investigators: Geoffrey Blake and Drake Deming
orbital periods as short as 3 days (the so-called “hot
Jupiters”). However, the Doppler surveys are now
beginning to detect gas giants in multiple-year orbits, i.e., true Jupiter analogs2.
Ground-based stellar Doppler spectroscopy has inferred the existence of ~100 planets orbiting Sunlike stars,1 and the list of extra-Solar planets is growing almost daily. Many of the initial discoveries
were gas-giant planets quite close to their star, with
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Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
Terrestrial planets like Earth have not yet been detected, but it is known that some planetary systems
(e.g. the Upsilon Andromeda System, and others3)
contain regions where the orbit of a putative terrestrial planet would be dynamically stable within the
habitable zone (where water can exist in liquid
form). There are obvious astronomical and biological reasons to directly detect and study extra-Solar
planets.
The fact that the orbit is edge-on removes the orbital
inclination ambiguity in the Doppler observations,
giving the planetary mass (0.69 ± 0.02 Jupiter masses). The relative depth of the transit light curve is
closely equal to the fractional area of the star
blocked by the planet, yielding the planetary radius
(1.42 ± 0.12 Jupiter radii). The resultant mass density (0.3 g cm-3) shows that the planet is a gas-giant,
and must be composed primarily of hydrogen.
We propose to search for pre-biotic organic signatures in the IR spectra of HD 209458b, and other
yet-to-be-discovered transiting gas-giant exoplanets. This work is an expansion of D. Deming’s current program (focused on continuum flux peaks,
and simple molecules like methane and CO).
Charbonneau et al.7 demonstrated that the eclipse
was slightly deeper (by ~ 2 parts in 10,000) in a
wavelength band centered on the strong sodium D
doublet at 589 nm wavelength. This occurs because
free sodium in the planet’s atmosphere provides an
additional absorption, making the effective blocking
area of the planet slightly greater at the sodium
wavelength. This was the first direct detection, and
crude “spectroscopy”, of an extra-Solar planet atmosphere.
One — but not the only — method to directly detect
and study an extra-Solar planet is to image it. The
principal challenge in extra-Solar planet imaging is
to eliminate or greatly reduce the light of the parent
star, which would otherwise overwhelm the planet
by factors of 104 to 1010 (depending on wavelength,
angular separation, etc.). NASA is currently evaluating a visible-light coronagraphic mission, as well
as an infrared interferometer, as complementary
space-borne approaches to extra-Solar planet imaging. The actual imaging of an extra-Solar planet
from such missions will not likely be accomplished
in this decade. However, direct detection of extraSolar planets - and spectroscopic measurement of
their composition - does not strictly require imaging.
The fact that many of the known extra-Solar planets
are massive gas-giants in orbits close to their stars
has great significance for Astrobiology, in the following sense. A massive extra-Solar “hot Jupiter”
close to its star will unavoidably have a large gravitational cross-section for accretion of extra-planetary
material, including exogenous primitive organics
from collisions with comets and other primitive
bodies. Not only does the mass of the hot Jupiter
Direct detection only requires that the photons from
the planet be distinguished in some fashion from the
stellar photons. Another way to do this is to exploit
the situation when the planetary orbit is, by chance
alignment, oriented edge-on as seen from Earth. In
this geometry, the planet passes in front of (“transits”) the star once per orbit, and disappears behind
the star once per orbit. In 2000, the planet orbiting
the star HD 209458 was found to transit4,5. The
transit was subsequently observed with very high
signal-to-noise ratio using the STIS instrument at
visible wavelengths on HST6, as shown in Figure
2.6.
Figure 2.6. The transit of the extra-Solar planet HD 209458b in
front of its star, as observed at visible wavelengths. Note that
the transit depth is ~1.7%, and the noise level in these spaceborne data is a few time 10-4, relative to the stellar signal.
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Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
itself provide a large gravitational trap, but its close
orbit places it deep in the gravitational potential well
of its parent star.
One has only to recall the hundreds of “Sungrazing” and “Sun-colliding” comets observed by
the SOHO spacecraft8, to realize that massive inner
planets will receive a high accretion flux from their
outer Solar Systems. Arguably, the atmospheric
temperatures of these hot Jupiters (heated to T ~
1000 K by irradiation from the star) will prevent
condensation. Also, these planets are believed to
have an outer radiative zone, where convection and
mixing are absent, so much of the accreted material
should remain to enrich the atmosphere. Interestingly, Atreya et al. have recently argued that the
sodium observed in the atmosphere of HD 209458b
is not primordial, but is indeed the result of accretion9. As we pointed out in Section 1.1, many simple organic compounds have strong spectral features
in the IR. If the “surface” composition of hot Jupiters can be observed using IR spectroscopy, then we
may be able to delineate the accretion history of
close-in extra-Solar planets, and by implication the
likely extraterrestrial flux onto the young Earth.
Figure 2.7. Observations of the 2-micron spectrum of the extra-Solar planet HD 209458b, obtained by Richardson, Deming
& Seager, using “occultation spectroscopy”. The ordinate is in
units of the stellar continuum; notice that an amplitude as small
as 0.1% would be off-scale on this plot.
is a Univ. Colorado graduate student working with
Deming) recently reported an upper limit for methane in HD 209458b.
More puzzling recent results (still under interpretation by Richardson, Deming & Seager) are shown
in Figure 2.7. This Figure shows the 2-micron spectrum of HD 209458b, where virtually all models for
this extra-Solar planet predict a broad flux peak due
to lowered opacity between water absorption
bands11. The observations have sufficient sensitivity to detect this flux peak. But the data firmly rule
out the presence of the peak at the modeled amplitude - it must be at least a factor of two less prominent than predicted.
Is it currently feasible to measure the infrared spectrum of an extra-Solar planet? Co-Investigator
Deming already has an active program pursuing
such measurements using ground-based IR spectrometers. The technique relies on the eclipse geometry to separate the spectra of star and planet (the
systems cannot be spatially resolved). Spectra taken
during “secondary eclipse” - when the planet is behind the star (“star only” contributing to the spectra),
are subtracted from spectra taken outside of eclipse
(“star plus planet” contributing). Subtracting the
two sets of spectra (“star plus planet” minus “star
only”) gives the planetary spectrum. Observations
of a nearby reference star are used to remove effects
due to the time-variable terrestrial atmosphere, and
suitable normalizations, calibrations, etc. are performed.
The spectrum in Figure 2.7 represents ~ 1200 individual spectra taken during two secondary eclipses;
half of the spectra were taken outside of eclipse, and
half during secondary eclipse. The difference yields
the putative planetary spectrum (plotted points).
Different symbols represent different echelle orders.
The solid line is the predicted11 spectral flux peak;
the data show that this peak does not occur at the
predicted amplitude.
These are very difficult observations, and no actual
planetary absorption spectrum has been extracted
(yet), only upper limits. However, the upper limits
are significant. Richardson et al.10 (L. J. Richardson
One interpretation is that a high-altitude haze layer
(which some investigators have already advocated)
is reducing the visibility of the IR continuum flux at
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45
Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems
this wavelength. Could such a layer be composed
of primitive organic compounds, such as occur on
Titan12? Does the intense UV flux from the star
drive an active photochemical cycle, processing
simple exogenous organics into more complex
compounds? If so, broad spectral absorptions due
to tholins, amines, aromatic and aliphatic hydrocarbons, and nitriles, may conceivably be present in the
spectrum of this extra-Solar planet.
planet HD 209458b using space-borne cryogenic
infrared spectrometers to increase sensitivity. At
least one suitable spectrometer will fly on SIRTF to be launched in April 2003, and other sensitive IR
spectrometers may fly on the proposed “Astrobiology Explorer,” and on the JWST.
We propose to search for pre-biotic organic signatures in the IR spectra of HD 209458b, and other
transiting gas-giant planets (the Kepler mission will
discover hundreds of transiting gas-giant planets13).
A new graduate student will work with D. Deming
to define the potential infrared signatures, calculate
the optimum spectral resolution, estimate the expected signal-to noise ratios, and otherwise assist in
executing this research.
Given the sensitivity already achieved in Deming’s
ground-based IR observations (see Figure 2.7), it is
quite plausible that application of these specialized
observational techniques (“transit spectroscopy” and
“occultation spectroscopy”) could detect organic
signatures, in the atmosphere of the extra-Solar
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46
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
3.0: Organic Material from Laboratory Simulations of Astrophysical Environments
Investigators: Jason Dworkin, Reggie Hudson, Marla Moore, and Joseph Nuth III
As discussed in Section 1, the evidence is strong At NASA’s GSFC, the Grain and Cosmic Ice Lathat the early Earth was seeded with pre-biotic or- boratories were designed to address both gas/grain,
ganics created in the interstellar medium, proto- and ice or ice/grain molecular syntheses in simulatSolar Nebula, and asteroidal/cometary parent bod- ed cosmic environments1-9. In particular we address
ies. It is possible that this extraterrestrial material the following questions:
represented a significant fraction of the total organic
inventory on the early Earth. Furthermore such ma- • What molecules form under what conditions?
terial may have had chemical and physical proper- • What is their likely formation pathway?
ties required to play an important role in the subsequent emergence of life. Fortunately, the elucida- • What are the yields of new molecules?
tion of the chemical characteristics of the ISM can • What are the rates of formation and destruction?
be accomplished via millimeter and IR observations
Our past experiments generated basic data for astrocoupled with sophisticated chemical modeling.
physics7,10-13, predicted the formation of new undeThe analysis of meteorites and IDPs, coupled with tected molecules 11,14,15 and processes, and stimulatobservations of modern proto-stars greatly enhances ed new and innovative work in the area of astrobiour understanding of the chemistry of the Solar ology16-18. Our proposed astrobiology research will
Nebula. We know much less about the atmospheric combine the Grain and Cosmic Ice Laboratories
composition of the ancient Earth. It is thus possible expertise with our newly formed Astrobiology Anato constrain laboratory simulations of exogenous lytical Laboratory to provide detailed information
material in the ISM and proto-stellar nebulae to on the synthesis of biogenic molecules.
make them relevant to understanding the composition of materials delivered to the early Earth. This Simulations on Grains
Section of the proposal describes a unique set of Fischer-Tropsch reactions catalyzed by metal or
well-defined laboratory simulations for the synthesis mineral surfaces have been proposed as a source of
of complex organics in grains, ices, and ice/grain endogenous organic compounds on the early Earth.
composites.
For example, it is well known that CO can be converted to hydrocarbons in the presence of hydrogen
3.1 Introduction: Laboratory Simulations
and an iron catalyst18,19. Such synthetic reactions
The formation mechanisms of the complex inter- have been proposed to occur in both the Solar
stellar molecules of interest to astrobiologists are not Nebula20 and under certain planetary conditions21.
well understood. Laboratory astrophysics continues Goddard’s Grain Laboratory simulations have
to provide both qualitative and quantitative chemical shown that heterogeneous catalytic reactions involvand physical data about processes that affect the ing gas-grain interactions form complex organics
formation and stability of molecules in space envi- that remain on grain surfaces (Figure 3.1)16. We
ronments. Using the most realistic simulations of expect that similar organics will undergo consideraprocessed cosmic grains and ices possible in the ble change within asteroidal-sized bodies, and we
laboratory, we iterate the results with modelers, ob- hope to follow the general progress of these metaservers, and those who analyze authentic samples morphic reactions in the laboratory. The results of
(meteorites, IDPs, and retrieved extraterrestrial ma- such laboratory studies will then be compared to
terial) so that they can better interpret their data, organics found in primitive and more highly metamodels, spectra, and samples. In turn, their data morphosed meteorites.
allow us to better simulate the ISM to improve the
realism and relevance of our laboratory simulations.
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47
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
a rich set of organics. The HPLC technique has not
yet been applied to most ion-irradiated ice residues,
but is included as a key feature in this proposal.
Many of the organic molecules produced by interstellar ice simulations are potentially relevant to the
origin or early evolution of life. These include simple molecules such as HCN and NH4CN10, oligomers of simple molecules28, numerous simple ketones and alcohols4,13, and more complex compounds. These include amino and hydroxy acids23,
quinones27 and other functionalized aromatics29,30,
and membrane forming amphiphiles25.
The organics synthesized during the gas-grain simulations include simple molecules such as methane,
propane, acetone and benzoic acid as well as a variety of complex polyaromatic hydrocarbons including cyano- and hydrocarbon-side groups. Even
more complex compounds seem likely if relevant
ices are allowed to interact with grains on which
they are accreted in the ISM or if these organiccoated grains interact with liquid water.3.2 Work
in Progress
Figure 3.1 Total ion current chromatogram of pyrolysate released from the surface of an Fe-silicate catalyst after exposure
to ~1 atm H2 + CO + N2 (~12:1:1) at 200-600°C.
Simulations in Ices
A mantle of ice of various compositions condenses
around interstellar grains in the dense molecular
cloud environment. These interstellar ices are exposed to ionizing radiation in the form of cosmic
rays (and from secondary radiation generated by
cosmic-ray interactions with matter), the attenuated
diffuse ISM UV field, and UV photons from stars
forming within dense clouds. A similar radiation
environment exists for ices in proto-planetary disks
with the addition of a keV X-ray flux that is thought
to be significant for Class I and II sources22. Table
3.1 summarizes predicted photon and ion fluxes in a
variety of cosmic environments.
The Grain and Cosmic Ice Laboratories at NASA’s
GSFC have a long history of experimental studies
aimed at understanding chemical processes in astrophysical environments. In recent years we have
followed the chemical evolution of silicate grain
analogs and irradiated ices, and we elucidated reactions that could have important influences on the
composition and IR spectra of similar cosmic materials. These same ice and grain analogs are prime
candidates for synthesis of the complex pre-biotic
molecules thought to be necessary for the origin of
life on Earth and possibly elsewhere.
Our laboratory experiments have shown that energetic processing of interstellar or pre-cometary ices
leads to more complex molecular species4,6,13,17,23-27.
A surprisingly large number of new compounds are
synthesized from ices that initially contain only a
few simple molecular components. Figure 3.2
shows examples where results were analyzed using
IR spectroscopy of a proton-irradiated ice (H2O +
CO). IR spectra reveal the most abundant ice products. A high precision liquid chromatography
(HPLC) analysis of a UV processed ice residue
(H2O + CH3OH + CO + NH3) (100:50:1:1) reveals
We next describe our progress to date in both the
Grain and Cosmic Ice Laboratories to synthesize
organic materials, our on-going (funded) research
projects in these areas, and our intended accomplishments as part of the NASA Astrobiology Institute.
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48
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
* *
2.0
Absorbance
* HCOO
Irradiated H2O + CO Ice
Reference
H2O + CH3OH
Absorbance at 270 nm
2.5
1.5
H2O + HCOOH
1.0
H2O + H2CO
Ice Residue (x10)
0.5
0.0
H2O + H2CO3 (from irradiated H2O + CO2)
1600
1500
1400
1300
Murchison Extract
1200
1100
0
1000
-1
5
10
15
20
25
HPLC Retention Time (min)
Wavenumber (cm )
(A) IR Spectra of Irradiated Ice
(B) HPLC Analysis of UV Photolyzed Ice
Figure 3.2. (A) Infrared (IR) spectra of proton irradiated H2O + CO (5:1) ice reveals the formation of a number of simple organic
species4. (B) High precision liquid chromatograph (HPLC) of the 1:1 CHCl3:CH3OH extract of a residue from a UV processed H2O
+ CH3OH + NH3 + CO (100:50:1:1) ice and Murchison meteorite extract (Dworkin, unpublished results). This demonstrates that
photolysis of a fairly simple ice can generate levels of apparent chemical complexity similar to those seen in a meteorite. Note that
the sigmoid shape of the magnified residue plot is due to changing solvent concentrations during the HPLC elution protocol25.
Table 3.1. Estimated Fluxes for Ice Processing Environments Compared to Laboratory Experiments
Environment
residence time
(years)
Diffuse ISM
(105 - 107)31
Dense cloud
(105 - 107)31
Proto-planetary nebula
(105 - 107)c
Oort cloud
(4.6 x 109)
Laboratory
(4.6 x 10-4)g
Flux,
1 MeV p+
(eV cm-2s-1)
1 x 107
1 x 106
1 x 106
(E)f
8 x 1016
Ion Processing
Energy ab-
Photon Processing
Energy
sorbed
(eV cm-2s-1)a
1.2 x 104
Dose
(eV molec-1)
<1 - 30
1.2 x 103
0.02 µm ice
1.2 x 103
0.02 µm ice
f
<< 1 - 3
2 x 1015
1 µm ice
<< 1 - 3
~150 (0.1 m)
~55-5 (1-5 m)
<10 (5-15 m)
10
Flux
(eV cm-2s-1)
9.6 x 108
at 10 eVb
1.4 x 104
at 10 eV
2 x 105
at 1-10 keVe
9.6 x 108
at 10 eV
absorbed
(eV cm-2s-1)
5 x 108
Dose
(eV molec-1)
104 - 106
1.7 x 103
0.02 µm ice
5x104
0.02 µm iced
9.6 x 108
0.1 µm ice
<1-4
2.2 x 1015
at 7.4 eV
2.2 x 1015
1 µm ice
2 - 240
2.7 x 108
10
a
The absorbed energy dose from 1 MeV cosmic-ray protons assumes a 300 MeV cm2 g-1 stopping power and an H2O-ice density of
1 g cm-3. Protons deposit energy in both the entrance and exit ice layer of an ice-coated grain.
b
10eV photons = 1200 Å, vacuum UV (UV-C).
c
Typical disk longevities.32
d
Absorbed energy dose from 1 keV X-rays assumes a 1 keV electron production in 1 g cm-3 H2O-ice with a 127 MeV cm2 g-1 stopping power.
e
Typical flux at 0.1 pc22; 1 keV photons = 12 Å, soft X-rays.
f
An energy dependent flux, (E), was used to calculate the resulting energy dose at different depths in a comet nucleus for an H2Oice density of 1 g cm-3. For details see reference 33 and citations therein.
g
Typical proton and UV data from our Cosmic Ice Laboratory.
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Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
Grain Laboratory: Condensed-Phase Organic Synthesis via Catalytic Reactions on Grains
We are exploring the potential of synthetic silicate
grain analogs as catalysts that convert CO, N2, and
H2 into complex organics. Our initial experiments
were carried out at temperatures ranging from ~500
K to ~1000 K and demonstrated that both amorphous iron silicate grains and amorphous magnesium silicate grains are efficient catalysts. Amorphous
iron silicate grains (often called smokes because of
their sub-micron size) are produced in a hydrogen
flow from a mixture of iron carbonyl and silane that
reacts with oxygen in a furnace held at about 775
K34. The reactants pass through a hydrogen flame
front (T >1300 K) in the furnace then are rapidly
quenched as they pass into the collection region
maintained at T < 275 K. Smokes collected from
this system (Figure 3.3) are then transferred to the
catalyst system (Figure 3.4) where a gas mixture (75
torr N2, 75 torr CO, and 550 torr H2) can be passed
through the dust at a controlled temperature2.
Figure 3.4 Catalyst System
form of amines and amides such as methylamine
(H2N-CH3) and N-methyl methyleneimine (H3C–
N=CH2), as well as cyano side chains on polycyclic
aromatic hydrocarbons (PAHs). This seems to imply breaking of N2 bonds by the organic coating that
is gradually deposited onto the grains as the reaction
proceeds.
Preliminary analysis of the organics deposited on
similar catalysts during 20 consecutive FTT experiments carried out at different temperatures ranging
from 475 K to 875 K were reported at the last
LPSC16. In general, the carbon in the grain coatings
comprised roughly 10% of the total grain mass
while total nitrogen was about 0.2% of the total
mass. The organics were a mixture of both shortand long-chain aliphatic compounds as well as a
variety of simple and multi-ringed aromatic hydrocarbons. Functional groups observed included nitriles, ketones, aldehydes, and acids. Overall, the
distribution of aromatic nitriles was very similar to
that observed in the Tagish Lake meteorite16.
Figure 3.3 Dust (Smoke) Generator
We find that iron silicate grains readily catalyze Haber-Bosch-type (HBT) reactions that produce NH3
from N2 and H2, as well as Fischer-Tropsch-type
(FTT) reactions that convert CO and H2 to CH4 plus
more complex organics. Amorphous magnesium
silicate grains do not catalyze HBT reactions but do
catalyze FTT reactions. We ran experiments that
used a mixture of CO, N2, and H2 as our starting gas
and were surprised to discover that both amorphous
iron silicate grains as well as amorphous magnesium silicate grains produced volatile organic molecules. These included nitrogen compounds in the
We believe that the organic coating deposited onto
the grains during FTT-HBT reactions is a good analog for the organic materials initially incorporated
into accreting planetesimals and not simply a “poison” that inhibits the reactivity of the catalyst. To
test this hypothesis, we have initiated experiments to
compare meteoritic carbon to the organic content of
the initial organic grain coating, of grain coatings
that have been thermally annealed, and of grain
coatings that have undergone various degrees of
hydrous processing. We are very encouraged by the
results of our first experiments, which demonstrated
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Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
the similarity between the organic material in our
initial grain coating and the organics found in the
Tagish Lake meteorite.
products formed in a variety of both H2O- and N2rich icy mixtures, determine the radiation or photolysis stability of molecules, examine acid-base reactions during warming, map out reaction paths for
the major products, measure intrinsic band
strengths, and calculate product yields.
Follow-on experiments to determine the evolution
of this coating with the progress of natural metamorphic events required to make an asteroid from a
freshly accreted pile of Solar System dust are in
progress. Additional experiments to determine the
efficiency of both iron and magnesium silicate grain
analogs, used as catalysts for the production of volatile organics, are also underway. These experiments
are complicated by the need to determine both the
rate per unit surface area and the temperature dependence of the reactions, as well as the product
distribution as a function of both temperature and
reaction time. Note that we assume that the extent
of the organic surface coating might easily effect the
product distribution, given our observation that the
coating can act to break the N2 molecular bond.
More complex still are the potential dependencies of
the rates and product distributions on the exact
composition of the grain catalysts (e.g. MgSiO –
FeSiO). We are just beginning to explore a set of
process variables (e.g. composition, pressure, flow
rate, temperature, time) that might eventually lead to
an understanding of the genesis of the noninterstellar portion of organic species in meteorites
and in IDPs.
We have a foundation of successes in understanding
radiation- and photo-chemical pathways. For example we have extensive experience with H2O-rich
ices and small molecules. The value of this work is
shown by our connections with recent planetary
detections of H2O235 and interstellar, ethylene glycol
(the simplest polyol (a molecule containing two or
more alcohols)) 36, vinyl alcohol37, formate ion38,
and cyanate ion39 that match what our experiments
suggest6,8,10,13,40. Recent results on N2-rich ice containing CH4 (ices relevant to outer Solar System
bodies such as Pluto and Triton) show the formation
of both HCN and HNC in nearly equal abundances
at low temperatures, and the reaction of these acids
to form NH4+ and CN- ions as the ice is warmed.10
For many irradiated or photolyzed ice samples, a
few percent of the total mass can be converted into a
room-temperature residue of low volatility that contains the most complex products. In one case, the
chemical analysis of a residue from ion-irradiated
H2O + CH3OH + NH3 + CO showed the formation
of hexamethylenetetramine, HMT17. HMT releases
NH3 and H2CO when hydrolyzed and then can
form amino acids in solution. However, we have
not had the facilities or personnel to support a continuous program to chemically analyze similar residue materials in the Cosmic Ice Laboratory.
Cosmic Ice Laboratory: Organic Synthesis in Energetically Processed Ices
Several NASA programs support research at Goddard’s Cosmic Ice Laboratory on the ion irradiation
synthesis of organics in ices. Our laboratory facility
has been described in a variety of publications6,8. In
brief, the vacuum and low-temperature environment
of space are simulated using a high vacuum chamber and a cryostat. Ice samples condensed on a
cooled mirror inside the cryostat are irradiated with
1 MeV protons to simulate cosmic-ray bombardment or photolyzed to simulate vacuum-UV exposure (see Table 3.1). The resulting ice chemistry is
followed by IR spectroscopy and products are identified. Ice spectra are recorded during warming to
examine changes from thermal processing. Using a
careful choice of conditions, we can identify major
Recently, Goddard has made a long-term commitment to our laboratory program by hiring
Dr. Jason Dworkin. He is now a permanent
member of our laboratory, and is the lead scientist
for the chemical analysis of complex materials with
access to both GC/MS and HPLC instruments in his
new Astrobiology Analytical Laboratory. Dr.
Dworkin performed much of the synthesis and nearly all the analysis of the UV processed ices that led
to the detection of picomolar quantities of biologically-relevant compounds, such as quinones27, amino acids23, and vesicle-forming compounds25. As a
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51
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
result, techniques used to analyze residues from UV
photolyzed ices will now, for the first time, routinely
be applied to ion-bombarded ices.
Grain catalyzed organic synthesis
Organics deposited on amorphous iron and magnesium silicate smokes produce more complex materials when exposed to liquid water and/or heat.
These same factors are important in converting nebular dust into the matrix materials observed in meteorites. Analysis of our smokes shows that they consist of mixtures of serpentine and smectite dehydroxalates (Mg-silicates) or greenalite and saponite
dehydroxalates (Fe-silicates) together with a variety
of metal oxides41. These inorganic constituents
should readily react in liquid water, and are known
to evolve at high temperatures to produce more ordered anhydrous silicates42.
Not only are ion-irradiated ices more representative
of the processing in dense molecular clouds, but
MeV ions process about 20 times more ice than 10
eV photons, resulting in a larger amount of residue
per experiment than in any other laboratory. This
will allow for the analysis of far more ices, under
more conditions, and in greater detail than has been
possible in the past. The combination of detailed
chemical analysis of a large number of simulations
of a wide variety of realistic ices is unique to this
laboratory. For example, the five week long syntheses of vesicle forming residues by UV photolysis25
could be accomplished in two days by proton irradiation.
We intend to follow the evolution of the organics
and the original grain analogs used as catalysts in
parallel as a function of hydration and thermal metamorphism under conditions similar to those that
might have produced a variety of meteorite types.
We will then compare the abundance of organics
and composition of our grain analogs to appropriate
meteorite types to constrain the evolutionary conditions on the parent bodies–provided that we can find
conditions that produce a match between both the
organic and grain content of the natural and analog
samples. We expect that the Solar Nebula contained many highly primitive meteorite parent bodies that had not yet had sufficient time to metamorphose into even carbonaceous chondrites–the most
primitive extant meteorite. These experiments may
provide some insight into the organics that could
have been delivered by such bodies to the ancient
Earth.
3.3 Research Plan
We propose a three-pronged approach to the study
of processes that affect complex, biomolecules in
cosmic environments.
We will study:
• Detailed chemical analysis of complex organics
formed by grain-catalyzed reactions of relevant
gas mixtures.
• Condensed-phase synthesis pathways to biologically-important organics in processed ices and
ice residues.
• Complex combinations of experimental conditions where ices and grains interact with and
without irradiation and produce residues that are
further energetically processed.
Organic Synthesis in Energetically Processed Ices
Our initial focus will be a search for the simplest
sugars (2-3 carbon aldoses and ketoses). Glycolaldehyde has been detected in the ISM and a condensed-phase route for its formation has been proposed by Hollis43. Detection of other sugars is a
Goal of Section 2.1. We will apply our knowledge
of expected reactions (Sections 1 and 2) to predict
likely pathways for the formation of glycolaldehyde
and other simple related molecules, a few of which
are shown in Table 3.2.
This will be the first systematic study of the synthesis and survival of organics from the ISM through
the formation of planetesimals in the proto-stellar
nebula. The work will be guided by observations
and modeling of such regions, as discussed in Sections 1 and 2 of this proposal.
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Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
Table 3.2: Some C-2 and C-3 Polyols, Sugars, and Sugar Acids
OH OH
H2 C
CH 2
ethylene glycol
OH O
H2 C
C H
glycolaldehyde
C
OH
glycolic acid
OH O OH
H2 C C CH2
glycerol
dihydroxyacetone
OH OH O
OH O
H2 C C C H
H
H3 C C C OH
H
glyceraldehyde
lactic acid
OH OH O
OH O
H2 C
OH OH OH
H2 C C CH2
H
OH O O
H2 C C C OH
H
H2 C C C OH
glyceric acid
pyruvic acid
Table 3.2. A small sample of interesting C-2 and C-3 poly-alcohols, sugars, and sugar acids. We have already determined the formation of ethylene glycol by proton irradiation13, glycerol, and glyceric acid by UV photolysis23. We have a tentative detection of
glycolaldehyde formed by irradiation.
The goal is to establish if these relatively complex
and reactive organics can be plausibly synthesized
under realistic interstellar conditions. Additional
experiments will examine reaction pathways for the
synthesis and destruction of amino acids and the
effect of energetic processing on racemization
(equilibrating the D and L chiral forms (enantiomers). All experiments will be performed systematically on binary and ternary ices with IR spectra of
condensed-phase and room temperature residues
augmenting the chromatographic analysis of the
volatile and/or non- volatile organics.
This list of compound classes is not selected at random. Each class is significant in contemporary biochemistry because it is likely to have been important
during the early evolution, if not the origin, of life.
In addition, each has also been either detected or
strongly suspected to be present in carbonaceous
chondrites. It would be useful to document their
formation or distinct absence from our ice and
ice/grain simulations to compare with meteoritic
data. This serves to help understand possible feed
stocks into meteoritic parent bodies, and serves as a
check of the plausibility of our simulations.
We know (from the absence of IR signatures and
preliminary analytical experiments) that complex
residual organic products, when present, are in very
low concentrations. With our in-house capability to
perform detailed chemical analyses of residues, we
will search for specific compounds of special biological or meteoritic interest. This targeted list of
compounds includes: amino acids, hydroxy acids,
C-4, C-5, and C-6 simple sugars (aldoses and ketoses), sugar acids, polyols, different biological and
alternative nucleobases, poly- and organophosphates, and membrane forming amphiphilic compounds. Upon a successful identification of robust
reactions, the formation and destruction can be analyzed more carefully and systematically, as discussed earlier in this Section.
Two classes of compounds on our “targeted list” are
distinctly absent in meteorites: aldoses and polyand organo-phosphates. It is likely that the instability of aldoses44 is to blame for their absence in carbonaceous chondrites45. However, if detected in
our simulations, it is possible that various aldoses
could serve as an indication of the level of energetic
or aqueous processing experienced by meteorites.
In the case of meteoritic phosphorus, only organophosphites have been reported46 and there are no
satisfactory pre-biotic syntheses47 of this vital48
class of compounds. It is possible that aqueous alteration of the parent body49 hydrolyzes the phosphate ester, or that extraterrestrial chemistry can be
added to the long list of pre-biotic organo- and polyphosphate syntheses that do not work47. We can use
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53
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
phosphine to place P directly into either the ices or
into the grains for this class of experiments.
ergetic processing, or do they convert into an
amorphous carbon of less biological importance?
Complex Simulations
• Do different molecules or vesicular structures
The third prong of our approach is a targeted search
for the same types of compounds listed above, but
under more complex simulations. We will examine
residues from processed ice layers that include
grains. Since laboratory residues are analogs of
sample-return-material, residue stability as a function of e.g., temperature, and humidity will be monitored. Although, investigations of residue changes
at room temperature are in their infancy50, a recent
report documents chemical changes in ammoniumcontaining residues in an uncontrolled room temperature environment51. These changes relate to the
fragility of sample-return-materials and to conditions for future archival sites of returned-material.
In addition we will analyze “reacted” ice residues
i.e. (1) residues placed in aqueous environments of
varying pH and temperature, (2) residues mixed
with grains, and (3) residues with additional ion or
photon processing.
form when residues are placed in different
aqueous environments with varying pH?
• Do the compounds that remain on grains from
gas-grain reactions interact with residues from
ice experiments?
X-Ray Processing
An extension of our processing capability will include a simulation of the Solar Nebula X-ray environment, following purchase of a low energy X-ray
tube. Although relevant X-ray doses will be modeled in this proposal (see Section 2.2), X-ray energies in the 1-10 keV range are observed for many
proto-planetary nebulae (Table 3.1). X-rays in this
energy range interacting with ice are thought to predominantly generate photoelectrons with the same
energy. Therefore we will also irradiate with 10
keV electrons which may be equivalent to 10 keV
X-rays. With electron, proton, X-ray, and UV processing, we can also compare the effects of different
sources on low-temperature ices and residues.
This more ambitious route will address questions of
ice-grain interactions:
• Is amorphous grainy material involved in the ob-
Sample Sharing
served chemistry, or is a grain just an inert substrate?
These experiments will allow us to look at syntheses
of new pre-biotic molecules and the stability of previously synthesized species under conditions reminiscent of Solar Nebula processing and under aqueous conditions possible on planetary or parent body
surfaces or interiors. This allows us to address the
questions:
In order to better understand the behavior of organic
compounds in primitive materials analyzed by
spacecraft (see Section 4), we will synthesize selected organic-rich, chemically characterized residues.
This will be an independent sample containing the
most abundantly occurring “natural” compounds
and allow for the development of remote analytical
instruments to meet the challenge of detecting the
same types of complex mixtures on extraterrestrial
icy targets. Selected residues will also be examined
by co-I Dr. Will Brinckerhoff using laser desorption
mass spectrometry (LDMS) at Applied Physics Laboratory. This technique provides ultra-sensitive
detection of specific classes of trace organics released from grains as a complimentary tool for
characterizing the residues.
• Do the pre-biotic compounds found in ice resi-
Comparison with authentic samples
• Do molecules formed in ice/grain composites differ from molecules formed in ices condensed on
inert substrates?
• Do the compounds that remain on the grains from
gas-grain reactions interact with the formation
of compounds in the ice?
dues increase in complexity with additional en-
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54
Theme 3: Conduct laboratory simulations of natal interstellar cloud cores and proto-planetary disk chemistry
To better evaluate the realism of our laboratory simulations we will compare our results not only with
compounds detected in the ISM but also to those
seen in carbonaceous chondrites and IDPs. Over
the course of the project, we plan to upgrade our
analytical capabilities. The acquisition of an nanospray LC/MS-Trap system will allow us to detect
organic compounds at greater sensitivity than had
been possible in previous meteoritic studies52. This
will require us to re-examine exogenous samples
using the same high sensitivity techniques we will
use in our laboratory simulations.
capabilities (LC-MS), via collaboration with Dr.
Antonio Manino (GSFC/Code 971) who will facilitate chemical analysis of residues when complimentary techniques are necessary (Epi-fluorescence Microscopy, GC with microwave extraction).
We also have the opportunity, with our collaborator
Dr. David Deamer of UCSC, to search for the production of microscopic vesicles in aqueous solutions
of residues from our ion-irradiated ices and ice/grain
experiments. Results will be compared with the
previous detection of vesicle formation from UVphotolyzed ice by Dworkin et al.25 Since the UV
production of material is very slow (5
weeks/experiment) compared to proton production
(~1 day/experiment), sufficient material may be
formed to allow the determination of the chemical
nature of the amphiphilic material which eluded
Dworkin et al.25. Also, it would be interesting to
test the organic material from the grains, ice/grain
mixtures, and processed residues for amphiphilic
behavior.
Collaborations
Our long-standing collaboration with Dr Raj Khanna at UMD will continue. He has extensive experience in the area of condensed-phase chemistry, matrix-isolation spectroscopy, and reaction chemistry
to form unstable species relevant to interstellar environments. He will assist in interpreting IR ice spectra.
We have close ties to the NASA Ames Astrochemistry Lab (Drs. Louis Allamadola, Scott Sandford,
Doug Hudgins, and Max Bernstein). We will share
data and exchange residues and analyses with both
the ice and PAH research efforts of that laboratory.
In addition we will continue our collaboration with
Drs. George Cody, Conel Alexander, and Larry Nittler of the Carnegie Institution of Washington. We
will have access to their analytical facilities and extensive knowledge of meteorite intractable organics,
mineralogy, and isotope geochemistry.
We will augment our current capabilities for analysis (UV-Vis, IR, GC-MS, HPLC), and expected
———————————————————————————————————————
55
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
4.0 Advanced Analysis of Primitive Materials
Investigators: William Brinckerhoff and Paul Mahaffy
The direct experimental characterization of the full
breadth of organic compounds on comets and asteroids represents a prime objective and a formidable
challenge for astrobiology. Such data would prove
invaluable to our understanding of the extent of
prebiotic synthesis in interstellar, protosolar, and
parent body environments. Ultimately, a comprehensive chemical inventory may allow us to determine the validity of models in which small body
impacts on the early Earth deliver the key compounds required for life.
suite of techniques and procedures to extract the
most information from a sample.
In the case of comets, the inherent limitations in the
remote sensing of comae strongly argue for in situ
measurements of the nucleus, and eventually analyses of returned samples. While Earth-based remote
sensing and initial flyby missions have revealed the
presence of important simple molecules in several
comets, such assays are likely strongly fractionated
relative to the nucleus. In situ and sample return
analyses can detect the entire nuclear complement
of “native” organics including those that cannot survive outgassing. Such direct measurements can also
reveal the elemental composition, the oxidation
state, and the 13C/12C and D/H isotope ratios in organics.
4.1.1 Cometary Molecules and Polymers
In this “APT” Theme we will study a set of
measurement scenarios, each comprising sample
types, analytes, and measurement techniques,
that span the most relevant astrobiology science
objectives for small bodies. The results will broadly affect future instrument and mission development.
4.1 Analytical Background
In situ experiments carried out at Comet Halley in
1986 were suggestive of the complexity of its organics, and probably those of other Oort cloud comets. Although it is very difficult to make detailed
chemical measurements in a fast comet flyby experiment, the Particulate Impact Analyzer (PIA) on
Giotto, and the Dust Impact Mass Analyzer
(PUMA) on Vega allowed inferences1,2 to be made
regarding the nature of the complex organic component in this comet.
In the PUMA experiment, the positively charged
ions released upon impact of a dust particle on a
metal target were separated according to their massto-charge ratios (m/z) in a time-of-flight mass spectrometer. PUMA allowed a determination of the
elemental composition of the dust, but also detected
complex organic species in a subset of spectra. The
PUMA spectrum shows a signal at nearly every unit
m/z value above 24 amu3. A detailed analysis of
individual dust impacts allowed PUMA investigators to generate a tentative list of candidate species.
The remote characterization of organics that may be
present in carbonaceous asteroids is even more challenging, limited by their low albedos and the likely
trace mixing ratios of organics in impact-depleted
regoliths. In situ and sample return measurements
are required to relate the extensive organics seen in
C-chondrites to asteroidal parent body processes.
Samples found in situ will be highly complex and
possibly quite fragile; their characterization with
limited resources and/or limited sample volumes is
by no means a straightforward application of current
methods. Analytical protocols and techniques
(APT) must be further developed to reduce ambiguities that will arise applying results to astrobiological
models. Separate from the miniaturization and development of experimental flight hardware itself, a
line of research is needed to determine the optimal
Other organic material detected in the coma of
Comet Halley may be polymeric4. The Positive Ion
Cluster Composition Analyzer (PICCA) heavy ion
experiment on Giotto showed repeated losses of 14
amu and 16 amu mass units. This pattern indicates
repeated losses of CH2 and O or NH2 and suggested
the presence of poly-aminocyanomethylene or polyoxymethylene (O-CH2)n chains These inferences
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56
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
were supported by laboratory studies on the thermal
decomposition of polyoxymethylene5.
niques for a determining the chirality of amino acids
and other chiral species in situ.
4.1.2 Organics in Meteorites
4.2 Work Plan
Data from the NEAR mission at asteroid 433 Eros
demonstrated that even this S-type asteroid is undifferentiated and indistinguishable from a chondritic
meteorite with regard to major element abundance6.
The evidently strong limitations on the remote determination of bulk chemistry induced by space
weathering make detailed in situ exploration of asteroids highly desirable.
The scope of this Theme covers the development of
methods for organic and isotopic analysis on both in
situ missions and returned samples. This effort will
not replace, but rather complement ongoing and
future technology and instrument development support provided by NASA and other agencies. This
approach will ensure that the expertise acquired in
ongoing instrument and technology development
programs is focused on the analytical objectives of
Astrobiology for primitive bodies.
Some meteorite types such as CI1 and CM2 are rich
in organics. For example, the Murchison meteorite
contains a wealth of complex organic compounds7
dispersed in a matrix of macromolecular organic
material and clays indicative of aqueous processing.
Understanding the sources and synthetic routes of
these organics is a vital component of astrobiology.
Many of the subunits of polymers including extended proteins, lipids, and nucleic acids have been detected in carbonaceous chondrites such as Murchison. Synthetic models for these units generally assume a processing phase in an aqueous parent body
environment. This assumption is well-motivated by
the extent of aqueous alteration found in CI1 and
CM2 chondrites. However, the absence of reaction
intermediates in the case of the Streckercyanohydrin synthesis, and the deuterium enrichment differences between amino acids and carboxylic acids8, suggest that other types of processing
may also have been important9,10. A significant
component of complex organics in meteorite parent
bodies may have been incorporated intact from ISM
and protosolar synthetic environments.
4.2.1 APT Theme Tasks
In this theme we will address three critical challenges of APT through focused tasks:
1. What are the optimal approaches and configurations for measuring both the history and the chemical state of organics in complex in situ samples?
We will approach this broad challenge by selecting
a set of specific, yet generally applicable, techniques
and applying them to a wide range of pertinent
measurement scenarios. We will study the complementarity of focused methods, such as chemical
derivatization (labeling) and chiral analysis, to general approaches, such as gas chromatography (GC)
and mass spectrometry (MS). We will also carefully
consider the relative advantages of bulk (pyrolysis)
and spatially-localized (laser or ion beam) volatilization methods for analyses of samples with the above
techniques. Measurement scenarios will encompass
a suite of sample types (synthetic analogs, ices,
mineral standards, and meteorites) paired with analytical challenges, such as detecting trace species or
understanding the spatial association of an organic
with mineral grains in a complex sample. We also
expect to take advantage of emerging protocols and
technologies as they are developed in efforts outside
the scope of this program. This task is detailed in
Section 4.3.
Early measurements of the chirality in the Murchison meteorite, showing balanced, or racemic, mixtures of D and L enantiomers were taken as evidence of their extraterrestrial origin. Although determination of the chirality of the Murchison amino
acids is complicated by possible terrestrial contamination, small non-biological enantiomeric excesses
have been reported11 hinting at a more complicated
chemical heritage than had been previously assumed9,,12,13. Thus, it is important to refine tech-
2. What are the optimal approaches to evaluating,
minimizing, and managing thermal perturbations to
Earth-return samples?
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Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
In this task we will examine sample transport methods that best maintain the structure and isotopic
composition of the most astrobiologically-pertinent
C, H, O, N, S, and P containing compounds. We
will also develop processing protocols that optimize
our ability to extract the original composition of
compounds that may have undergone various degrees of thermal processing in transport. This latter
sub-task is of critical importance to comet sample
return missions, where samples may be stable only
at very low temperatures. This task is detailed in
Section 4.4.
experience in both development of in situ experiments and the associated laboratory studies will
provide a good basis of experience for carrying out
the tasks described.
4.3. Task 1: APT for Complex Organics
4.3.1 Overview
Volatiles analysis can examine either stable organic
molecules or the pyrolysis products of more complex organic species released thermally, for example, from a collected sample of an asteroid or comet.
These GCMS studies will be extended, in collaboration with team members at the University of Paris
and Laboratoire Interuniversitaire des Systèmes
Atmosphériques (LISA), to include advanced separation and analysis methods such as chemical derivatization techniques that could be adapted for use
in a space flight environment.
3. How can we best utilize available cometary analogs obtained in the laboratory to develop and ultimately calibrate instrumentation that will analyze
real, complex cometary samples?
The success of in situ and sample return missions
will depend upon the thoroughness of our preparation with analog samples. Analogs may serve as
secondary standards, permitting us to understand
analyses of unknown samples in the context of a set
of materials with characterized chemical composition and structure. We will collaborate with investigators across all themes of this program to define,
develop, and analyze relevant analog samples. The
results will advise the direction of APT work as well
as that of mission development teams across the
scientific community. This task is detailed in Section 4.5.
These chemical transformation techniques will allow us to access an extended range of organic species. We will also examine ways to adapt chemical
transformation methods for use on spacecraft instruments, e.g., as the oxidation of organic molecules to CO2 to determine precisely the isotopic
composition of these molecules as they elute from a
GC column.
Mass spectrometry studies will be extended to include not only standard gas phase ionization techniques such as electron impact ionization but also
direct ionization from solid phase materials using
laser desorption and secondary ion mass spectrometry (SIMS) to analyze the more refractory organic
component. The latter two techniques lend themselves to a time-of-flight (TOF) MS system, and this
technology will be further developed in collaboration with team members at JHU/APL. Detection of
very high molecular weight species is possible with
these techniques. Focused laser and SIMS probes
also enable localized analyses of individual grains.
Such local probes may be used to study the finescale heterogeneity of cometary and asteroid samples, and are thus highly complementary to bulk
GCMS analysis.
4.2.2 APT Team Experience
Members of our team have participated in many
recent investigations and development activities for
missions such as Rosetta, Galileo, Cassini, CRAF,
and CONTOUR. The major effort of preparing the
Cassini Huygens probe for the study of organics at
Titan enabled members of our team to develop novel tools.
We presently are studying methods of exploring the
prebiotic chemical record that may be preserved in
the subsurface of Mars. The APT theme will similarly enable a future mission to a comet or asteroid
to focus on measurements that are important for
both the NASA Astrobiology Program and the exobiology research carried out internationally. This
4.3.2 Extraction, Derivatization, & Separation
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58
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
Figure 4.1 A GC spectrum obtained by LISA collaborators. This study evaluated columns most suitable for use in a comet lander
organics experiment. The mass spectrum that would result from a mass scan of one of these GC peaks is illustrated in the inset.
For analysis of solid phase materials the stable molecular pyrolysis products are analyzed.
• Use multiple GC columns to obtain the most
GCMS has long been recognized as one of the most
powerful tools for analysis of multiple molecular
species in a complex mixture. It has a history of
space use from the Viking lander search for trace
organics in Martian soil14 to the Cassini Huygens
Probe15 due to enter Titan's atmosphere in 2005.
The Huygens GCMS (developed in house at
GSFC) was designed to study the complex organic
makeup of Titan’s atmosphere detected remotely by
the Voyager IRIS experiment in 1980.
comprehensive analysis of potential organic
molecules.
• Apply derivitization techniques to stabilize molecules that would otherwise be destroyed in GC
columns.
• Develop columns capable of chiral determination.
Multiple column GCMS/pyrolysis will improve the
GC and GCMS techniques for analysis of organics
in comets and asteroids by optimizing columns and
pyrolysis conditions.
A simplified derivative of the Huygens GCMS was
originally selected for the Rosetta Champollion
Lander, and later for the Champollion ST4 New
Millenium Mission16. Although ST4 was later cancelled, the development activity led to a mature design for the use of GCMS at a cometary body. This
experience and recent technological advances now
give us the opportunity to extend GCMS-based
techniques further. The following activities will be
pursued to broaden the range of chemical, structural,
and isotopic analyses of comets and asteroids with
in situ GCMS:
Extra-National Collaborator F. Raulin, P. Coll and
associates from the LISA of Université de Paris participated in development of advanced GC techniques. Some of this work has been applied to the
Rosetta lander COSAC experiment, to definition of
the Champollion GCMS experiment, and to ongoing definition of a Mars GCMS organic analysis
experiment. In addition to material described in
Section 3 of this proposal, collaborator H. Cottin
———————————————————————————————————————
59
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
face sample18. Our team has considered sylilation or
alkylation reactions, wherein polar alcohols, carboxylic acids, amines, thiols, and amino acids are
converted in simple reactors to species that are more
easily analyzed in a GCMS.
will generate a range of cometary analogues include
polymeric material that is believed to lead to extended coma sources17.
We will continue laboratory studies to characterize
GC capillary columns and provide an optimal set
for each of these investigations. Several columns
operating sequentially or in parallel will be necessary to achieve a comprehensive analysis for a range
of heavy and light polar and non-polar molecules.
An example of a GC profile from one of these studies is given in Figure 4.1.
An example of one such reaction is shown in Equation 1, where a carboxylic acid with functional
group R (an aliphatic group of variable complexity)
reacts with the dimethylformamide-dimethylacetate
derivitizing agent to form an ester as well as reaction
biproducts.
Extensive pyrolysis and GC research and development capabilities exist in both the GSFC and LISA
laboratories. This work will leverage an ongoing
collaboration between our groups that uses these
capabilities for development of a Mars lander
GCMS/pyrolysis experiment.
O
(CH 3 O)2 CH N(CH 3 )2
2 R C OH
O
O
2 R C OCH 3 + (CH 3 )2 N CH + H2 O
(1)
The product ester will be sufficiently volatile to be
analyzed in the GC. These studies will be focused
under this program to the chemical types predicted
for comets and asteroids. A range of derivatization
agents will be investigated and tested on radiation
generated cometary analogues. This work will be
carried out by our LISA and U. Paris team members
in the first 2 years of our program.
During the first year of our study, pyrolysis studies
will be carried out on a range of polymeric materials
such as polyoxymethylene to characterize fragmentation products.In the second year of the program,
room temperature analogues such as residues from
radiation processing of ice mixtures will be obtained
from the radiation laboratory and investigated thoroughly to characterize the effects of various components of the pyrolysis and GC steps on sensitivity
and detection limits. In the third year low temperature ices and their radiation products will be produced in the cryogenic vacuum systems of our laboratory to minimize perturbations from sample
transport. Throughout these studies a variety of GC
columns will be evaluated.
Chiral-Specific GC Columns will extend methods
for determining chirality.
Numerous amino acids are present in meteorites11
that were likely produced by abiotic sources. With
only rare exceptions, chiral amino acids used by
biology are exclusively the L enantiomer. Once
complex organic molecules such as amino or carboxylic acids are found in comets or asteroid samples it is of interest to determine if racemic mixtures
are present or if exogenous samples delivered to
Earth were biased toward L. Methods of determining chirality have been studied by our LISA/U. Paris team members. An example of this separation is
illustrated in Figure 4.2. Chiral specific columns
will be flown on the Rosetta mission. The research
into methods to distinguish enantiomeric excess
with chiral GC in cases where there is a significant
complexity of (racemic) background and measurement environment will continue in the first year of
support provided from this proposal
Identification of additional molecular species will
be realized by chemical derivatization to a more
stable or more volatile molecular configuration followed by pyrolysis
Derivatization complements pyrolysis in that a
transformation may be necessary to identify a complex molecule whose pyrolysis products are too
simple for unambiguous parent identification. Development of this technique for space flight application has already started at the University of Paris for
exobiology investigations of a deep Martian subsur-
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60
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
1.-amino-isobutyric acide 2.alanine 3.glycine 4.  -alanine
5.D-valine 6.L-valine 7.norvaline 8.D-isoleucine
9.L-isoleucine+leucine 10.4-aminobutyric acid
11.norleucine 12.D-aspartic acid 13.L-aspartic acid
14.glutamic acid 15.D-phenylalanine 16.L-phenylalanine
Abundance
9400
Secondary Ion Mass Spectrometry will provide a
method for detecting fragile organic molecules that
could extensively fragment in a conventional electron ionization.
8900
15 16
8
8400
56
9
7900
7400
2
1
6900
7
4
12 13
3
6400
To detect high molecular weight organics, a “static”
ionization mode is employed where the incident
excitation ion beam current is maintained sufficiently low as not to significantly change the nature of
the surface species over the course of a mass scan.
Figure 4.3 shows a static SIMS spectrum obtained
in our laboratory of an alkyl aromatic compound.
The sample, in this case, was substantially less than
one monomolecular layer of xylene on a nickel substrate.
14
11
10
5900
5400
10
12
14
16
18
20
22
24
26
28
30
Time (min)
Figure 4.2. Chiral separation on the GC column of several
derivatized amino acids . This work was carried out by the
LISA/U. Paris members of our team.
4.3.3 Soft Ionization for Organic Analysis
This is a complementary approach to processing the
gases thermally released from the solid matrix of
dust, ice, or rock. These methods can sample yet
another range of molecular and atomic species.
Ions can be formed using a direct pulsed laser desorption or alternately by SIMS. Both these techniques can provide information on delicate organic
species as well as the refractory component of a
sample. These methods can also probe a small spot
several microns diameter, providing information on
individual grains and sample heterogeneity.
Figure 4.3. Static SIMS of xylene vacuum deposited on a
nickel substrate. The incident ion beam was Xe in this case,
with less than 1 KeV excitation energy. As is characteristic of
this analysis, metal adducts to the surface adsorbed species provide diagnostic molecular peaks.
The mass analyzer of choice for both techniques is a
TOF-MS, since the pulsed ionization provides a
sharp “start” pulse. A focus of the collaboration between researchers at GSFC and JHU/APL will be to
explore further the use of these soft ionization techniques for in situ measurements of both organics
and their chemical matrix in comets and asteroids.
These techniques are widely used in laboratory
analysis of meteoritic and other extraterrestrial samples, but inherent complexities in instrumentation
have so far precluded their use in space . However,
recent advances in low power, radiation hardened,
high speed electronics, and miniaturization of
pulsed lasers and TOF-MS ion reflector stages, have
enabled significant advancements in feasibility.
This method is suitable for refractory organics with
too low a vapor pressure to provide sufficient gas
density in an ion a source for gas phase electron ionization sampling. In addition, it is evident from the
example of
Figure 4.4 that significantly higher intensity of an
unfragmented “molecular” peak (MH+ with
M=serine) can be obtained by SIMS. The focus of
study in this area starting in the 4th year of our program will be on extending the SIMS sensitivity and
mass range for in situ mass spectrometry. Toward
this end, SIMS studies previously implemented in
the GSFC laboratories using quadrupole mass spectrometers will be carried out on the TOF-MS sys-
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61
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
tems developed for laser desorption time-of-flight
mass spectrometry (LD-TOF-MS).
resolving power to resolve isotopes of peptides with
a molecular weight >1000 amu.
Through careful attention to the design of the ion
source and reflectron components of the instrument,
it has been possible to directly examine the refractory organic composition of carbonaceous meteorites,
both as provided and distributed within an ice matrix, and obtain basic elemental assays with a common instrument configuration. This is illustrated in
Figure 4.5. Participation in the NAI will allow us to
focus on the specific analytical challenges of sampling the dust and ice of asteroids and comets (and
laboratory analogs), where even basic material
properties such as roughness and absorptivity may
not be known a priori.
Detailed attention to the various efficiencies and
fractionations in laser desorption and ionization of
primitive body analogs, organics residues, and related analytical standards will be required to address
this challenge. In one subtask, we will examine the
range of refractory compounds that are able to survive both desorption and ionization from a single
laser pulse, as a function of sample properties.
Figure 4.4. The SIMS spectrum (top frame) of the amino acid
serine obtained in the GSFC laboratory is compared with the
corresponding gas phase electron impact (EI) spectrum (bottom
frame) taken from the NIST library. Although there are some
common fragments at 70 and 60 amu, in the EI case, there is
very little signal intensity at the molecular peak (105 amu).
In another subtask, we will examine the same samples with separate laser desorption of neutrals, followed by either laser post-ionization (LPI)25,26 or
electron post-ionization (EPI)27. The LPI experiments will utilize a tunable-wavelength optical parametric oscillator (OPO) available at JHU/APL that
is able to access the prime electronic transitions of
refractory organics in the near UV range (235-390
nm)28. The EPI mode, using instrumentation already developed by GSFC and JHU/APL, will enable the study of certain less refractory compounds
desorbed by the laser that will be highly complementary to the pyrolysis-based techniques.
Laser Desorption Time-of-Flight Mass Spectrometry combines the highly-sensitive, local analysis capability of a focused laser with the high mass range
and throughput of the TOF method.
Recent advances in sampling processes and in instrument design have brought forth an enormous
recent interest in LD-TOF-MS for applications in
microbiology19, proteomics20, and exobiology21, 22.
An ongoing effort led by JHU/APL seeks to miniaturize this technique for application to planetary exploration23. GSFC and JHU/APL are presently collaborating to study the possibility of using a common TOF-MS for use in both EI gas phase analysis
and solids analysis using laser desorption24. Prototypes have been demonstrated to have sufficient
In all analyses, a very important issue that will
command much of our attention is molecular fragmentation. The fragmentation pattern observed in a
given laser mass spectrum is dependent on composition, molecular structure, matrix structure, laser
properties (energy, wavelength, pulse duration), and
analyzer details. A large number of peaks of organic
———————————————————————————————————————
62
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
compounds may also contain isobaric interferences
from fragments. Thus, it can be a challenging to
unravel complex spectra into an unambiguous
chemical assay. In biochemical applications, this
has partly motivated the development of matrixassisted laser techniques. Here analytes are mixed
with an excess of an organic acid that limits fragmentation and permits very high mass parent molecules to be ionized intact.
40
24
16
28
Mg
40
Si
Ca
O
26
LA-TOF-MS
Elemental/Isotopic
Mg
30
25
Mg
27
Al
20
(Mg2+)
10
12
29
Si
30
C
23
Si
Na
32
13
0
To avoid the potential technical complexities,
chemical reactions, reduced utility below several
hundered amu, and the loss of spatial information
associated with sample preparation steps, we intend
to concentrate on alternative protocols. The use of
post-ionization provides one such route, through
control of laser wavelength and delay. We have
also found that precise control of the laser irradiance, along with microscopic examination of the
uniformity of the analysis surface, permits an irradiance ramp to be applied.
40
C
10
S
39
44
K
42
Ca2+
20
30
Ca
Ca
40
14
LD-TOF-MS
Molecular/Organic
12
10
m = 14 amu (CH2)
8
100
150
200
250
300
350
400
450
500
550
600
m/z (amu/e)
Figure 4.5. Laser TOF-MS data from analysis of the Allende
meteorite (NMNH 3529, courtesy T. McCoy). High-power
mode (laser ablation) shows elemental assay from a laser spot
confined to a refractory inclusion. Lower-power modes (laser
desorption) in matrix regions demonstrate survival of prompt ions
of refractory organic component.
Here, a sequence of desorption analyses at increasing laser energy (fixed spot diameter) leads to fairly
reproducible changes in the fragmentation pattern
that correlate with the loss of signal from parent
compounds. The information on the presence of a
particular organic molecule can often then be extracted from difference spectra over the course of
many shots. This method is of interest to eventual
use in situ or on sample return where one would like
to avoid sample contact in certain cases.
tained with considerable loss in measurement sensitivity.
GSFC and APL members of this team are presently
developing under Astrobiology Science and Technology Instrument Development (ASTID), an orthogonal extraction ion source to enable highresolution time-of-flight studies using electron impact ionization to be initiated. Under support provided by this proposal we will evaluate the relative
merits of very high-resolution mass spectrometry
and. the GC/derivatization separation techniques
described previously for the analysis of complex
organic molecules.
Combined with the tailored “few-component” analog materials that would become available through
this NAI program, such laser analyses would become powerful tools.
High resolution MS by reconfiguring TOF-MS
A compact device utilizing two reflecting mirrors
can enable multiple reflections of ions before their
release to a detector. This greatly increased path
length can enable very high spectral resolution (i.e.
m/m >20,000) to be achieved. The precise measurement of m/z can enable compound identification
in many cases without the need for GC separation
since isobaric interferences are not present. However, very high spectral resolution is generally ob-
It is likely that in some space applications such as
comet rendezvous missions, a hybrid approach using both separation techniques may be optimal to
realize both high sensitivity and mass resolution.
The instrument development aspects of this task are
ongoing and the analysis relevant to the goals of our
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63
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
this proposal will be completed in the third year of
this work.
A sample return mission from a comet was among
the top scientific priorities of the recent Decadal
Study Commissions recommendations for near term
mission. Since these cold samples obtained from
below the surface of the nucleus of a comet could
contain an ancient record of early Solar System
chemical processes, a significant objective will be to
minimize chemical reactions during delivery to
Earth. While desirable, maintaining samples at extreme cryogenic temperatures may be too costly to
allow such a mission.
Chemical transformation for isotopic determination
The determination of precise isotope ratios are particularly diagnostic for understanding the extent of
nebular processing of ISM material. As discussed in
Section 1, high D/H ratios are indicative of an interstellar chemical heritage. Also, the decreasing 13C
enrichment with increasing carbon number in the
Murchison meteorite suggests that the synthesis of
the heavier hydrocarbons was via a mechanism of
sequential carbon chain build-up. However, the
comparison of the carbon enrichment with that of
deuterium in this meteorite suggests that the refractory, kerogen-like material may originate from an
entirely different reservoir.29 Thus, precise isotopic
determination for light elements provides a method
of understanding the history of formation of organic
materials in samples of comets and meteorites
Therefore, a companion study to the development
of analytical protocols will be an assessment on
conditions for sample return and preservation after
return to Earth that will maintain a substantial portion of the chemical record from these samples.
Various studies have been initiated to address the
question of the extent of physical and chemical alteration due to thermal perturbations after sample
collection.
Techniques for precisely determining the isotopic
abundances of the separate organic components
with small amounts of sample have advanced considerably in recent years30 and we propose to study
these both for application to returned samples and in
situ experiments.
For example, between -130 °C and -100 °C the vapor pressure of H2O from the icy component that
may have been sampled in such a mission is below
1x10-8 bar. Work on analog cometary ice/dust mixtures (i.e. water ice, CO2 ice, and dunite) shows
minimal physical transformation only over periods
of thousands of hours31.
For example, one approach is to direct the effluent
from a GC column into a chemical reactor that converts the hydrocarbons to CO2 before analysis by
the MS. Similarly a separate reactor could convert
the hydrogen in these same organics into H2, HD,
and D2 for precise isotopic MS analysis. The facilities to generate such an experiment are available in
our laboratory – the research associates supported
by our participation in NAI would contribute to the
laboratory experiments and their analysis. We will
initiate these studies during the first year of our program.
There are however, predicted exothermic reactions
in ice such as aminolysis and hydrolysis to form
nitriles and polymers from simple starting materials32. Examples are given in equations 2 and 3 that
occur at very low temperatures
H2CO + NH3
(H2CO)2 + NH3
NH2CH2OH
NH2CH2OCH2OH
(2)
(3)
and reactions such shown equation 4 occur at very
low temperatures if exothermic NH3 reactions provide the energy33.
4.4 Sample transport and processing protocols
HCN + H2CO
We will use the analytical techniques described and
the analog materials produced by the methods described above to better evaluate the effects of thermal perturbations on the chemical state of returned
samples.
HOCH2CN
(4)
Much of this type of chemistry may already have
occurred in the comet, but some could happen with
the increased temperature after collection.
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64
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
At higher temperatures in the range -100 °C to -50
°C significant physical transformation of cometary
analog ices may occur in hours or even minutes at
the high end of this temperature scale. The vapor
pressure of water is 1x10-9 to 1x10-5 bar in this range
and especially at the higher end considerable
transport of materials through sublimation of ices is
possible – probably with associated chemical transformation in some cases.
terials to be examined by the variety of techniques
and protocols described.
Our usual approach for evaluating the potential of a
technique or protocol will be to first apply the method to a well known sample such as a standard traceable to an accepted reference. For example, the isotopic state of carbon in solid phase material can be
referred to standards traceable to NIST. Standard
mixtures containing trace organic constituents will
be prepared. These tests will allow the dynamic
range, precision, accuracy, and linearity of our isotope MS to be assessed. Analysis will then focus on
samples either prepared in the laboratory facilities
used by the Theme 3 members or prepared in the
laboratory containing a desired instrument using the
same methods as these facilities. The separate characterization of these samples by a range of laboratory techniques by Theme 2 participants will enable a
better evaluation of the capability of our techniques
and protocols
At even higher temperatures (-50 to +75 °C) even
more substantial changes are expected. Thermal
degradation of predicted cometary polymers such as
polyoxymethylene will be significant as shown by
the measurements of Collaborator Cottin5. Diffusion
barriers in ice may be overcome to allow formation
of salts such as the NH4OCN from the corresponding ions34. Additional very complex chemical reactions may result from the heating as discussed in
Section 3.
Also, if a cometary sample contains a significant
fraction of water ice that is heated on entry to melt,
then the remaining solid phases will mix with this
fluid and be well shaken by the entry mechanical
processing. Inert mineral and molecular species
would survive, but on average, the chemical composition of the returned sample could be well removed from its state in the comet.
We have recently acquired a large cryogenic vacuum chamber that will allow temperature and pressure conditions at the nucleus of a comet to be simulated. Use of this chamber for sample preparation
using methods developed by participants in the
Theme 3 research will allow APT activities to employ an increasing range of cometary analogues.
While this task will help set the requirements for an
appropriate level of sample preservation in future
sample return missions, this work will also be relevant for understanding the chemical changes that
might occur after delivery of these materials to the
early Earth. Major milestones in the work described
will be reached in year 3 and 4 of our program.
4.6 Relationship of the ATP theme to other elements of this node of the Astrobiology Institute
Elements of this Section will benefit substantially
from the parallel activities of the other components
of this proposal. Analog materials produced by radiation processing described in Section 3 will provide
a range of samples for analysis by the in situ instrumentation under development. The remote sensing
observations will likewise provide an additional list
of candidate organic materials and a deeper understanding of the chemical environment that could
produce these observable species. Figure 4.6 schematically illustrates elements of this theme and how
they connect to other elements of the program.
4.5 Radiation processed materials as analogues
for the evaluation of analytical protocols
The methods of generating a range of cometary analog materials by radiation processing have been described in Section 3. Our research will utilize these
methods to allow an increasing range of analog ma-
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65
Theme 4: Develop analytic methods for complex organics in small bodies in the Solar System
Figure 4.6. The relationship of the research into advanced techniques and protocols, the analytes, and the samples and environments,
with links to other program elements.
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66
5.0 Management Plan
The Principal Investigator bears primary responsibility for the success of this Node of the NASA
Astrobiology Institute. He has ultimate authority in
the allocation of resources to individual projects carried out under the Node’s purview. He will be assisted by a Deputy Principal Investigator, who acts
for the Principal Investigator in his absence. Overall
Node performance will be assessed annually by a
Board of Visitors whose chair is chosen by the P. I.
The Chair then recruits three other scientists (with
the concurrence of the P. I.). The Board of Visitors
(none of whom are members of this Node), independently conducts an annual review of progress
and reports its findings and recommendations to the
P. I.
Nebular Modeling, Laboratory Chemistry, Analytical Protocols and Techniques and E/PO. Each
theme will meet monthly during the week preceding
the meeting of the Executive Committee to examine
progress in their ongoing projects. The Executive
Committee member from that Theme will chair the
meeting and will present a synopsis to the Executive
Committee at its meeting the following week. As
we expect to sponsor several cross-theme research
efforts, each such effort will be assigned a primary
theme and a secondary theme as appropriate.
Tracking of the progress of such cross-disciplinary
projects will be done under the primary theme.
The Node will convene an Annual Workshop at
which results will be presented and discussed by the
participants. All Investigators, post-doctoral associates, students, and collaborators will be strongly encouraged to attend. The Workshop will be held one
month in advance of the NAI General Meeting, in
order to enhance preparation of Node results for
presentation at that meeting. The Board of Visitors
will be expected to attend this Workshop, and will
be encouraged to conduct its Annual Review immediately thereafter.
The Principle Investigator is assisted and advised by
an Executive Committee consisting of representatives of each major theme within the node, as well
as representatives from major institutional partners.
The Executive Committee will be chaired by the
Principal Investigator or in his absence by the Deputy Principal Investigator. The Executive Committee
will meet at least once per month to review the status of all ongoing projects. The committee will recommend funding priorities and allocations to the
Principal Investigator throughout the life of the
Node.
The daily operations of the Node will be supervised
by an Executive Scientist (Dr. Monika Kress). She
will facilitate the group’s efforts to reach their overall research objectives and to interface with NASA
and the NAI. She will play a key role in formulating and developing the structure of the node’s scientific program. She will assist in implementing research priorities, drafting progress reports, and monitoring the development, progress and operation of
research investigations undertaken by the Node coInvestigators. She will maintain an ongoing record
of the research accomplishments and activities of
the Node co-Investigators, to assist the P. I. in identifying, allocating, and developing resources for scientific research. She will be responsible for assembling and editing the Annual Report to the NAI
from this Node, based on submissions from the individual co-Investigators.
The Executive Committee will be comprised of the
following permanent members: Dr. Michael
Mumma (P. I., chair), Dr. Joseph Nuth (Deputy P.
I.), and Dr. Lee Mundy (representing the Univ. of
Maryland). Individual members representing the
various themes will serve rotating terms. The initial
rotating members will be Dr. Marla Moore (Lab
Chemistry, three years), Dr. Paul Mahaffy (Analytical Protocols, three years), Dr. Michael DiSanti
(Observations, two years), Dr. Steven Charnley
(nebular modeling, two years). The Executive Scientist and the Education and Public Outreach Lead
are Ex-officio members of the Executive Committee.
The activities of the Node will be organized around
five broad Themes: Astronomical Observations,
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67
ensure that the Node’s E/PO plan is successfully
implemented.
The Executive Scientist will answer inquiries from
the general public and will handle all routine communications from the Astrobiology Institute’s Management such as submission of periodic progress
reports, notification of Institute Seminars, management of linkages to other Nodes, etc. She will be
the primary point of contact between the Node and
administrative units of the Goddard Space Flight
Center. For example, she interacts with the Assistant Lab Chief to ensure the timely availability of
office space for visitors, new students and post-docs,
communicates research results to the GSFC Public
Affairs Office, and ensures the availability of needed supplies. The Executive Scientist will be in daily
contact with the PI as well as with other members of
the Node and will keep abreast of progress in the
various projects sponsored by the Node. This individual will route inquiries from the press or from
other Node members that concern specific research
objectives to the appropriate team member. She
also liases with the E/PO officer, as needed.
This Astrobiology Node will not be “business as
usual” with only a few extra internal meetings added to otherwise normal procedures. The daily operations of researchers in this node will differ from
before, for example internal collaboration will be
enhanced by the sharing of post-docs and students
across research sub-projects, but within a Theme.
Typically, a post-doc or student is assigned to an
individual, however we plan to have that person
assigned to an idea or process. S/he would move
between several co-I’s as required for the task. For
example, Student A could explore Theme 1 with a
search for the presence of glyceraldehyde on different classes of astronomical objects and in different
spectral ranges (vibrational and rotational) as appropriate; Post-doc B could work on Theme 3 to synthesize a grain under one co-I, deposit and process
an ice with another co-I, then analyze the products
with a third co-I. This would, naturally, harmonize
the research strategies of individuals in the node
and provide a cross-discipline experience which will
strengthen the next generation of Astrobiologists.
The Education and Public Outreach Lead
(Stephanie Stockman) will oversee and be responsible for coordinating and implementing all aspects of
the E/PO program. This individual will represent
the Node in regular video and teleconferences with
other E/PO Leads, at the NAI General Meeting, and
at other educational and scientific meetings, as necessary. The E/PO Lead is responsible for entering
information on our E/PO activities into the NAI
Central database for inclusion in the NAI Annual
Report, and also provides information for the OSS
Annual E/PO Report. The Lead liases and interacts
with counterparts at other performing institutions to
In addition, “Expeditions” will be mounted in which
all students and post-docs would be expected to participate. Each Expedition would be mounted by a
given Theme (for example visiting a telescope and
analyzing the data), and two such Expeditions
would be held each year. Thus, in a two-year interval, all students and post-docs would be exposed to
cross-discipline research, and they would be exposed to each Theme on a two-year rotating basis.
———————————————————————————————————————
68
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Advertised sensitivities are in the high attomolar range. Even if realistic usage is orders-of-magnitude less sensitive this will still be the most sensitive chromatographic tool ever brought to bear.
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