transcript
- Slide 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.
- Slide 2
- 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
- Slide 3
- 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
- Slide 4
- 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
- Slide 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 16 O-rich and poor end-members Deviations
are smaller for whole rock data.
- Slide 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
- Slide 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. Lada et al. (1994) Likely caused by selective
ultra-violet dissociation Typical for low mass star formation 13
CO/C 18 O
- Slide 8
- 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
- Slide 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 H 2 O vapor
- Slide 12
- 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
- Slide 13
- 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
- Slide 14
- 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
- Slide 15
- 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
- Slide 16
- 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.
- Slide 17
- Oxidized meteorites no metallic Fe metallic Fe is abundant
Contradict to simple expectation
- Slide 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 ( 16 O-rich)
and/or SiC induced by bipolar flow Nebular inner edge Shu et al.
(1997) Star Accretion disk
- Slide 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 H 2 O vaporizesOrganics vaporizes
- Slide 20
- 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
- Slide 21
- Predicted O-isotopic composition relative to Sun Jupiter Saturn
Uranus/Neptune Sun Lower limit Enrichment factor of heavy
elements
- Slide 22
- 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.)
- Slide 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 16 O-rich but gas planets to be 16
O-poor.
- Slide 24
- 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