Download Oxygen Isotope Heterogeneity in the Solar System The Molecular

Oxygen Isotope Heterogeneity
in the Solar System
The Molecular Cloud Origin Hypothesis and
its Implications for the Chemical Composition of
Meteorites and Planetary Oxygen Isotopes
Kiyoshi Kuramoto Hokkaido University
&
Hisayoshi Yurimoto Tokyo Inst. Tech.
Outline
• Introduction
– Problem of oxygen isotopic
heterogeneity in the solar
system
– Basic concepts
• Molecular cloud origin
hypothesis
– Isotope fractionation due to
photochemistry in molecular
clouds
• Gas-dust fractionation
processes
– Enrichment of H2O in the inner
solar nebula
– Interpretation of O-isotopic
heterogeneity
• Implication to chemistry of
meteorites
– Lack of simple correlation
– Evolution of nebular chemical
environment
– Significance of recycling
• Gas planets
– Predicts O-isotopic
composition as a future
diagnostic of the present
model
O-isotopic composition
• Oxygen
– Most dominant element in solid bodies in the solar
system
• Earth’s Matters
17O/16O=0.038/99.757
18O/16O=0.205/99.757
• δ notation
17,18
16
17,18
16
(
O/
O)

(
O/
O)standard
sample
17,18
3
 O

10
(‰)
17,18
16
( O/ O)standard
• Mass dependent fractionation processes
 O  1/ 2   O (for small 
17
18
17,18
O)
O Isotopic Heterogeneity
in the Solar System
Solar wind data
after Ireland et al.
This WS
• CAIs
– Ca,Al-rich refractory
inclusion
• Chondrules
– Spherical grain (mmsize)
– Main constituents of
primitive meteorites
Solar wind data
after Hashizume and Chaussidon (2005)
Nature, in press
Characteristics of O isotopic
compositions among
Earth and meteorites
• Deviated from the terrestrial composition
50  
17,18
O  10 (Typically)
• Mass independent features
• Significant deviations are observed among
CAIs (calsium-aluminum rich inclusions)
and chondrules.
– Interpreted as mixing line connecting 16O-rich
and –poor end-members
• Deviations are smaller for whole rock data.
Nuclear Processes ?
• Unlikely
– Other major elements such as Si show much
weaker isotopic heterogeneity.
– Not correlated with O isotopic composition.
• We need another explanation
Molecular Cloud Origin Hypothesis
Yurimoto & Kuramoto Science 305 (2004)
13CO/C18O
• based on the observations which reveal isotopic
fractionation of CO molecules.
• CO is the most dominant O-bearing gas species.
Typical for
low mass star
formation
Likely caused
by selective
ultra-violet dissociation
Lada et al. (1994)
Mechanism of selective photodissociation
• Predissociation by line absorption of UV
– CO+hv (913<λ<1076Å)→CO*, CO*→C+O
– Each isotopomer has own labs due to difference in
vibrational –rotational energy levels.
• Self-shielding
– C16O (major): UV at labs attenuates in the surface
zone of MC.
– C17,18O(minor): UV at labs penetrates the interior of
MC.
Selective photo-dissociation of minor isotopomers
– Causes “mass independent” fractionation
CO becomes “light” (C16O enriched, C17,18O depleted)
Oxygen Isotope Heterogeneity
in the Solar System
Diffuse cloud data
after Sheffer et al (2002)
Where heavy O goes ?
• Water ice is most likely.
– produced by reaction with H on grain surface
• Mass balance calculation
assumption: O partitioned as
CO:H2O:silicate =3:2:1 (solar)
mean δ17,18OMC = 0
CO: －60 > δ17,18OMC > －400
(↑from obs. & calc.)
H2O: ＋100 < δ17,18OMC < ＋250
Gas-dust fractionation
• Case of no fractionation
– Heterogeneity may be erased
– Bulk system should be reset to original
isotopic composition under high T conditions
where silicate reprocessing occurs
• Mechanisms of fractionation
– Enrichment of icy dust
– Enrichment of H2O vapor
Sedimentation and inward migration
of dust grains
Dust sedimentation to
nebular midplane
z-component of stellar gravity
Inward Migration
frictional loss of angular
momentum
z
High P
Low P
Gas rotation: slightly slower
than the Keplerian rotation
Dust migration in accretion disk
Inner disk: water vapor enriches
dust relative motion
Vapor Concentration
Vdust/Vgas
Dust grains migrate faster than
gas toward disk center
17,18O change along mixing line
150
17OMC
100
50
Inner disk
enriched
In H2O
H2O Ice
0
Silicate
-50
CO
-100
-100 -50
0
50
18OMC
100
150
1
10
100
1000
H2O enrichment
Yurimoto
and Kuramotofactor
(2004)
Interpretation of O-isotopic
heterogeneity
•
16O-rich
components such as CAIs
– formed before H2O-enrichment
– escape from later reprocessing in H2O-enriched
nebular gas
– End-member represents “solar O-isotopic
composition”
• Consistent with one data of solar wind implanted into lunar
metal grains (Hashizume & Chaussidon, 2005)
• Most of terrestrial & meteoritic matters
– Enriched in heavy oxygen isotopes
– reprocessed in H2O-enriched nebular gas
• isotopic exchange between metallic oxide and nebular gas
• oxidation of metals (mainly Fe) by water vapor
Relationship with
chemistry of meteorites
• Oxidation state v.s. O isotopic composition
– simple expectation
• More oxidized matter is more enriched in 17,18O.
– But such simple correlation is NOT observed.
metallic Fe is abundant
• Contradict
Oxidized meteorites
to simple
no metallic Fe
expectation
Relationship with
chemistry of meteorites (contd.)
• Other factors affecting chemical composition
– Variation of T,P, and C/O ratio
– Recycle of refractory components such as CAIs
(16O-rich) and/or SiC
induced by bipolar flow
Nebular inner edge
Shu et al. (1997)
Star
Accretion disk
• Time dependent
simulation of vapor
enrichment in the
inner nebula
• Assuming
– instantaneous decline
of accretion rate
• Vdust/Vgas=
1 for t < 0
5 for t > 0
– Half of C is
partitioned into
refractory organics
Organics vaporizes H2O vaporizes
O-isotopic composition of gas planets
• Gas planets
– O-isotopic compositions are unknown
– Enriched in heavy elements
• Water ice and silicate are the major sources
• Expected to have 17,18O-enriched composition
relative to the Sun
relative to Sun
Predicted O-isotopic composition
Uranus/Neptune
Saturn
Jupiter
Sun
“Lower limit”
Enrichment factor of heavy elements
O-isotopic composition of gas
planets (contd.)
• Regular satellites as a window for isotopic
observation
– Probably share the same isotopic composition
– Formed in circum-planetary disk expanded from gas
envelope of proto-parent planet.
– Worth to observe volcanic gas (Io) and ice (ring &
satellites)
• Would provide key constraints for the O-isotopic
evolution
– Predicted composition is actually model dependent
• Confirmation of solar wind composition is
primarily crucial
Summary
• Solar system is significantly heterogeneous in
oxygen isotopic composition.
• Such heterogeneity may be originated in parent
molecular cloud.
• Gas-dust fractionation serves heterogeneity in
oxygen isotopic and chemical compositions
within the inner nebula.
• Sun is predicted to be 16O-rich but gas planets
to be 16O-poor.
Evolution of C/O ratio in the
accreting solar nebula
• Instantaneous decline of mass accretion
– Vdsut/Vgas increases
• Enrichment of H2O and reduced C-bearing
vapors starts to evolve from each
evaporation front
• Variation of C/O ratio allows formation of
reduced and oxidized matters
• Recycling of SiC possibly occurs