Download Allen et al. Science submission Apr2013-distrib..

Title: Cosmic pollution on Mars
Authors: Mark Allen1,2*, Inge Loes ten Kate3, George Cody4, Edwin Kite2, Karen
Willacy1, Jason Weibel5
Affiliations:
1
Science Division, Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Drive, Pasadena, CA 91109, USA.
2
Division of Geological and Planetary Sciences, California Institute of Technology, 1200
East California Boulevard, Pasadena, CA 91125, USA.
3
Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD
Utrecht, The Netherlands.
4
Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road,
NW, Washington, DC 20015, USA.
5
Chemistry Department, Shenandoah University, 1460 University Drive, Winchester, VA
22601, USA.
*Correspondence to: [email protected].
Abstract: As for Earth, Mars is regularly bombarded by particles derived from asteroids
and comets that contain organic material to varying degrees. Due to the Martian thin
atmosphere, much of this organic material reaches the Martian surface relatively
unprocessed. We have modeled the accumulation of this exogeneous organic material—
cosmic pollution in effect—on the surface and burial to depths beneath the surface, taking
into account decomposition due to ultraviolet radiation and galactic cosmic rays. The
computed abundance of exogeneous organic material is a few parts per billion to a few
parts per million by mass at the surface and for many meters beneath the surface. This
material is predominantly insoluble polymeric organics. At these abundances, the
meteoritic organics should be detectable by present and future landed experiments. As
such, this amount of cosmic pollution can confound current and future plans to detect
endogenous organic material that might be evidence for extinct or extant Martian life.
One Sentence Summary: Exogeneous organic material regularly deposited on the
Martian surface may overwhelm attempts to detect endogeneous organic material that
could be evidence for extinct or extant life.
Main Text: The Martian atmosphere is regularly bombarded by organic-containing
particles derived from asteroids and comets, as is the case for the Earth (1). Due to the
thin Martian atmosphere, many of these particles reach the surface relatively unaltered
(2). The predicted total mass of cosmic material landing on the Martian surface is
between 17 and 370 gm m-2 my-1 (2). If the particles are of asteroidal origin, this would
imply a flux of carbon to the surface between 35 and 767 mg m-2 my-1, while if of
cometary origin, between 4.2 and 92 gm m-2 my-1 is landing on the Martian surface (3).
The chemical composition of the carbon-containing phases of these particles is fairly
similar whether derived from asteroids or comets—80% insoluble polymeric organic and
20% soluble organic molecules, such as amino acids.
With the advent of modern searches for organic chemicals on the surface or in the
near subsurface including the recent landing of the NASA Curiousity mission, a joint
ESA-Roscosmos mission later in this decade, and planning for sample return, we chose to
reexamine the implications of the influx of this cosmic pollution. We computed the
expected surface abundance and abundances at depths beneath the surface of exogeneous
organic material. The abundance at the surface and below was computed as a balance
between the exogeneous flux to the surface, chemical decomposition as a function of
depth beneath the surface, and physical burial due to aeolian or sedimentation processes.
The model used for the calculation herein was derived from a coupled
atmospheric chemistry/vertical transport model used to simulate atmospheres throughout
the solar system (4). However, for the purposes of this paper, the surface is the top
boundary of the model and transport to the subsurface is what counterbalances local
chemical processes. In addition, the vertical transport is treated as advection, not
diffusion. The numerical values for the parameters in the model were derived from the
literature.
As the chemical loss rates are a function of the chemical composition of the
carbon-containing components of the exogeneous material, we treated separately the
insoluble polymeric fraction and the soluble fraction. Of the possible molecules
comprising the soluble fraction, we only could find in the literature measured or
computed decomposition rates for amino acids, so we treat the whole soluble faction as if
it were simply composed of amino acids. For the purposes of the calculation herein, we
assume that the organic fractions are isolated from and not protected by the inorganic
matrix in which the organic fractions are delivered to the surface (if otherwise, the
organic fractions will be protected, more stable, and even more abundant on the surface
or in the near subsurface than estimated below).
There are three forms of chemical degradation discussed in the literature—
oxidation processes, photolytic decomposition, and decomposition by ionizing radiation,
particularly galactic cosmic rays. While oxidative decomposition of organic molecules on
the Martian surface as been discussed since the days of the Viking mission (5), there are
no published oxidative decomposition timescales. In previous work, the consequences of
ultraviolet and cosmic ray decomposition were treated separately; in this work we
consider the consequences of these processes acting simultaneously. Ultraviolet
decomposition of amino acids has been quantified for Mars conditions and we adopt a
value for the inverse mean loss timescale of 1.7 x 10-6 s-1. The published value was
calculated assuming overhead illumination (6), so the value we have adopted has been
adjusted downward by a factor of three to be consistent with a global diurnal average. As
the insoluble fraction is not a specific molecular compound, there have been no published
measurements for ultraviolet decomposition. However, from the work of (7), we derived
an inverse mean decomposition lifetime of 1.3 x 10-10 s-1. Cosmic ray decomposition has
been studied for amino acids; we adopt of value of 1.3 x 10-16 s-1 based on the work of
(8). Again there is no work published on insoluble cosmic ray decomposition, but (9)
have shown a scaling with mass that allows a insoluble cosmic ray inverse loss timescale
to be estimated from lower mass values (10), yielding a value of 5.3 x 10-15 s-1 based on a
stoichiometry of C100H120O60N4 for the insoluble fraction (11).
The calculation includes these processes varying with depth beneath the surface.
The mean attenuation depth for ultraviolet radiation of 0.078 cm (780 μm) was derived
from the work of (12) and is consistent with an experimental value of 200 – 500 μm (13).
The mean attenuation depth for cosmic ray penetration of 160 cm was derived from (10);
our value was scaled upwards by a factor of two to account for differences in density
(rock ~ 2 gm cm-3 versus Mars soil ~ 1 gm cm-3 (14)).
Low areas on Mars may be filled in with sand from wind erosion. Sediments have
been laid down over time. Either process can bury exogeneous organic material deposited
onto the surface if not otherwise decomposed when exposed on the surface. For the
calculation herein we have estimated a burial rate between 30 and 100 μm/yr from
various published estimates for dust deposition and sediment layer formation (15-17).
To test the model formulation, we used the values in (7) for surface accretion of
meteoritic carbon and ultraviolet decomposition. We have used a slow burial velocity
since (7) modeled some mixing in the near-surface volume. The computed surface
abundance for insoluble organics of 1.7 parts per million by mass (ppmm) is comparable
to the estimate of ~ 4 ppmm carbon load at low latitudes.
Table 1 summarizes the model parameters adopted for the model simulations
reported herein. Figure 1 shows the chemical loss mean timescales corresponding to the
photodecomposition and cosmic ray decomposition processes in Table 1. Also in Figure
1 is the timescale for burial to 1 cm depth at a burial rate of 30 μm/yr. These timescales
will be useful in understanding the model simulation results that follow. If oxidative loss
timescales are ever computed for the Martian surface, these values can be compared to
the timescales shown in Figure 1 from which the effect of oxidative loss on exogeneous
organic abundances can be estimated.
Fig. 1. Mean timescales for loss of meteoritic organic fractions deposited on the Martian
surface as a function of depth below the surface. Shown for the insoluble organic fraction
(red lines) and soluble organic fraction (blue lines) are the photodecomposition (solid
lines) and cosmic ray decomposition (dashed lines) timescales. In addition the black line
is the timescale for burial to a depth of 1 centimeter at a rate of 30 μm/year.
Table 2 shows the specific parameters used in the different model simulations.
Within the range of parameter values in Table 1, the maximum surface meteoritic organic
abundance is calculated by using the maximum surface deposition fluxes for insoluble
and soluble organics and the slowest burial rate (model simulation 1) (Figure 2). Note
that the chemical loss processes for insoluble organics are comparable to or much slower
than burial so the insoluble organic abundance is large all the way to several meter
depths. On the other hand, chemical loss for the soluble organic fraction exceeds burial
with the result that the soluble organic fraction fully decomposes within ~40 μm of the
surface. Scaling up the burial rate by a factor of ~3 (model simulation 2) leads to opposite
effects for the two different meteoritic organic fractions. For the chemically long-lived
insoluble fraction, faster burial just redistributes material from the surface reservoir to
depths below the surface and thus the reduction in the near-surface abundance. On the
other hand, the faster burial redistributes some of the short-lived soluble organic fraction
to depth below the surface where the material is more protected from ultraviolet
decomposition, thus leading to a small enhancement in abundance in the near-surface
layer. When the surface deposition fluxes are reduced to the low end of the values in
Table 1, the model simulation 3 results show the abundances being reduced accordingly.
Fig. 2. Mixing ratios for meteoritic insoluble organic fraction (red lines) and soluble
organic fraction (blue lines) as a function of depth below the surface. Model simulations
1-3 (Table 2) are indicated by solid, dashed, and dotted lines, respectively.
Figure 2 shows that meteoritic organic infall flux can result in ppbm to ppmm
mixing ratios that are well within Mars lander instrument detection capabilities. The
Viking Organic Analysis Experiment had a nominal detection limit of ppbm for simple
organic compounds (18). However, since this experiment only heated samples up to 500
°C, it was recognized at the time that complex polymeric material would not have been
detected. As the meteoritic insoluble organic fraction is the dominant exogeneous
contribution to surface materials and decomposes in laboratory experiments only above
600 °C (19), the presence of organics on the surface was undetectable. In these laboratory
experiments, thermal decomposition of the meteoritic insoluble organic phase yields a
range of lighter organic compounds. Since the Curiosity Sample Analysis at Mars
instrument suite heats samples well above 600 °C and has ppbm sensitivity (20), the
predicted meteoritic organic surface abundance should be detectable and might even be
the dominant signal.
The high level of cosmic pollution possibly on the surface of Mars and to large
depths below the surface can likely interfere with attempts to detect endogenous organic
compounds that could serve as evidence for extinct or extant life. It may not be possible
to drill to depths that are not dominated by the exogeneous organic material. Even in
samples brought back to Earth, the organic component likely may be primarily
exogeneous and overwhelm any remnant from a endogeneous source. Finally, the
bombardment of Mars by organic-containing cosmic material extends back to the origin
of Mars with the consequence that the organic content in rocks and sediments, and
Martian meteoritic material found on the Earth surface (21), may be dominated by cosmic
pollution.
References and Notes:
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G. D. Cody et al., Establishing a molecular relationship between chondritic and cometary organic
solids. Proceedings of the National Academy of Sciences of the United States of America 108,
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M. Allen, Y. L. Yung, J. W. Waters, Vertical Transport and Photochemistry in the Terrestrial
Mesosphere and Lower Thermosphere (50-120 Km). Journal of Geophysical Research-Space
Physics 86, 3617 (1981).
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H. P. Klein, Viking Mission and the Search for Life on Mars. Reviews of Geophysics 17, 1655
(1979).
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I. L. ten Kate, J. R. C. Garry, Z. Peeters, B. Foing, P. Ehrenfreund, The effects of Martian near
surface conditions on the photochemistry of amino acids. Planetary and Space Science 54, 296
(2006).
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J. E. Moores, A. C. Schuerger, UV degradation of accreted organics on Mars: IDP longevity,
surface reservoir of organics, and relevance to the detection of methane in the atmosphere. Journal
of Geophysical Research-Planets 117, (2012).
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stability under gamma radiation and preservation of the enantiomeric excess. Monthly Notices of
the Royal Astronomical Society 410, 1447 (2011).
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G. Kminek, J. L. Bada, The effect of ionizing radiation on the preservation of amino acids on
Mars. Earth and Planetary Science Letters 245, 1 (2006).
10.
A. A. Pavlov, G. Vasilyev, V. M. Ostryakov, A. K. Pavlov, P. Mahaffy, Degradation of the
organic molecules in the shallow subsurface of Mars due to irradiation by cosmic rays.
Geophysical Research Letters 39, (2012).
11.
J. Kissel, F. R. Krueger, The Organic-Component in Dust from Comet Halley as Measured by the
Puma Mass-Spectrometer on board Vega-1. Nature 326, 755 (1987).
12.
C. Sagan, J. B. Pollack, Differential Transmission of Sunlight on Mars - Biological Implications.
Icarus 21, 490 (1974).
13.
A. C. Schuerger, R. L. Mancinelli, R. G. Kern, L. J. Rothschild, C. P. McKay, Survival of
endospores of Bacillus subtilis on spacecraft surfaces under simulated martian environments:
implications for the forward contamination of Mars. Icarus 165, 253 (2003).
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K. E. Herkenhoff et al., in The Martian Surface: Composition, Mineralogy, and Physical
Properties, J. F. Bell, Ed. (Cambridge University Press, New York, 2008), pp. 451-467.
15.
R. Arvidson, E. Guinness, S. Lee, Differential Aeolian Redistribution Rates on Mars. Nature 278,
533 (1979).
16.
J. R. Johnson, W. M. Grundy, M. T. Lemmon, Dust deposition at the Mars Pathfinder landing site:
observations and modeling of visible/near-infrared spectra. Icarus 163, 330 (2003).
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K. W. Lewis et al., Quasi-Periodic Bedding in the Sedimentary Rock Record of Mars. Science
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Experiment. Journal of Molecular Evolution 14, 65 (1979).
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Acknowledgments: We thank Y. L. Yung and R.-L. Shia for their assistance with the computer model
used in this work. This research was carried out at the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with the National Aeronautics and Space Administration.
Table 1. Model parameters.
Insoluble organic
fraction
Soluble organic fraction
8.9x 10-20 - 2.4 x 10-16
2.2 x 10-20 – 5.9 x 10-17
Ultraviolet
decomposition inverse
mean timescale (s-1)
1.3 x 10-10 e(z/0.078)
1.7 x 10-6 e(z/0.078)
Cosmic ray
decomposition inverse
mean timescale (s-1)
5.3 x 10-15 e(z/160)
1.3 x 10-16 e(z/160)
Burial velocity (cm s-1)
9.5 x 10-11 – 3.2 x 10-10
Surface deposition flux
(gm cm-2 s-1)
See text for references. z is distance below surface in cm.
Table 2. Model simulation parameters.
Model simulation 1
Model simulation 2
Model simulation 3
Surface deposition flux (gm cm-2 s-1)
Insoluble organics
Soluble organics
2.4 x 10-16
5.9 x 10-17
2.4 x 10-16
5.9 x 10-17
8.9x 10-20
2.2 x 10-20
Burial velocity
(cm s-1)
9.5 x 10-11
3.2 x 10-10
3.2 x 10-10