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 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

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

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

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.

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) 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)

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)

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 Gas rotation: slightly slower than the Keplerian rotation High P Low P z

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

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 1 10 100 1000 H2O enrichment factor Yurimoto and Kuramoto (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.

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

Predicted O-isotopic composition Uranus/Neptune relative to Sun Saturn Jupiter “Lower limit” Sun 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