1. 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.

2. Outline Implication to chemistry of meteorites 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

3. 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
Mass dependent fractionation processes

4. 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 (mm-size)
Main constituents of primitive meteorites
Terrestrial fractionation line
Solar wind data
after Hashizume and Chaussidon (2005)
Nature, in press

5. Characteristics of O isotopic compositions among Earth and meteorites
Deviated from the terrestrial composition
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.

6. Nuclear Processes ? Unlikely
Other major elements such as Si show much weaker isotopic heterogeneity.
Not correlated with O isotopic composition.
We need another explanation

7. Molecular Cloud Origin Hypothesis Yurimoto & Kuramoto Science 305 (2004)
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
13CO/C18O
Lada et al. (1994)

8. Mechanism of selective photo-dissociation
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)

9. Oxygen Isotope Heterogeneity in the Solar System
Diffuse cloud data
after Sheffer et al (2002)

10. 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

11. 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

12. Sedimentation and inward migration of dust grains
Dust sedimentation to
nebular midplane
z-component of stellar gravity
Inward Migration
frictional loss of angular momentum
Gas rotation: slightly slower than the Keplerian rotation
High P
Low P z

13. Dust migration in accretion disk
Dust grains migrate faster than gas toward disk center
Inner disk: water vapor enriches dust relative motion
Vapor Concentration
Vdust/Vgas

14. d17,18O change along mixing line. .
d17,18O change along mixing line
-100
-50 50
100
150
Inner disk
enriched
In H2O
H2O Ice MC O 17 d
Silicate CO
-100
-50 50
100
150 d 18 O MC
H2O enrichment factor
Yurimoto and Kuramoto (2004)

15. 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

16. 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.

17. Contradict to simple expectation
Oxidized meteorites
no metallic Fe
metallic Fe is abundant
Contradict to simple expectation

18. 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

19. 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

20. O-isotopic composition of 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

21. Predicted O-isotopic composition
Uranus/Neptune
relative to Sun
Saturn
Jupiter
“Lower limit”
Sun
Enrichment factor of heavy elements

22. 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

23. 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.

24. 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