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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.

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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 H 2 O 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

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O-isotopic composition Oxygen Most dominant element in solid bodies in the solar system Earths Matters 17 O/ 16 O=0.038/99.757 18 O/ 16 O=0.205/99.757 notation Mass dependent fractionation processes

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O Isotopic Heterogeneity in the Solar System Solar wind data after Hashizume and Chaussidon (2005) Nature, in press 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

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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 16 O-rich and poor end-members Deviations are smaller for whole rock data.

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

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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. Lada et al. (1994) Likely caused by selective ultra-violet dissociation Typical for low mass star formation 13 CO/C 18 O

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Mechanism of selective photo- dissociation Predissociation by line absorption of UV CO+hv (913

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:H 2 O:silicate =3:2:1 (solar) mean 17,18 O MC = 0 CO: 60 > 17,18 O MC > 400 (from obs. & calc.) H 2 O: 100 < 17,18 O MC < 250

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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 H 2 O vapor

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

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

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17,18 O change along mixing line Yurimoto and Kuramoto (2004). -100 -50 0 50 100 150 -100-50050100150 18 O MC 17 O MC H 2 O Ice CO Silicate Inner disk enriched In H 2 O H 2 O enrichment factor 1 10 100 1000

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Interpretation of O-isotopic heterogeneity 16 O-rich components such as CAIs formed before H 2 O-enrichment escape from later reprocessing in H 2 O-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 H 2 O-enriched nebular gas isotopic exchange between metallic oxide and nebular gas oxidation of metals (mainly Fe) by water vapor

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Relationship with chemistry of meteorites Oxidation state v.s. O isotopic composition simple expectation More oxidized matter is more enriched in 17,18 O. But such simple correlation is NOT observed.

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

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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 ( 16 O-rich) and/or SiC induced by bipolar flow Nebular inner edge Shu et al. (1997) Star Accretion disk

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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 H 2 O vaporizesOrganics vaporizes

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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 17,18 O-enriched composition relative to the Sun

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

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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 O-isotopic composition of gas planets (contd.)

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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 16 O-rich but gas planets to be 16 O-poor.

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Evolution of C/O ratio in the accreting solar nebula Instantaneous decline of mass accretion V dsut /V gas increases Enrichment of H 2 O 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