Formation and Composition of Planetesimals |
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Authors: | Daniel Gautier Franck Hersant |
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Institution: | (1) Laboratoire d’Etudes et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, F-92195 Meudon Cedex, France;(2) Institut für Theoretische Astrophysik, Tiergartenstraβe 15, D-69121 Heidelberg, Germany |
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Abstract: | The composition of planetesimals depends upon the epoch and the location of their formation in the solar nebula. Meteorites
produced in the hot inner nebula contain refractory compounds. Volatiles were present in icy planetesimals and cometesimals
produced in the cold outer nebula. However, the mechanism responsible for their trapping is still controversial. We argue
for a general scenario valid in all regions of the turbulent nebula where water condensed as a crystalline ice (Hersant et al., 2004). Volatiles were trapped in the form of clathrate hydrates in the continuously cooling nebula. The epoch of clathration
of a given species depends upon the temperature and the pressure required for the stability of the clathrate hydrate. The
efficiency of the mechanism depends upon the local amount of ice available. This scenario is the only one so far which proposes
a quantitative interpretation of the non detection of N2 in several comets of the Oort cloud (Iro et al., 2003). It may explain the large variation of the CO abundance observed in comets and predicts an Ar/O ratio much less than
the upper limit of 0.1 times the solar ratio estimated on C/2001 A2 (Weaver et al., 2002). Under the assumption that the amount of water ice present at 5 AU was higher than the value corresponding to the
solar O/H ratio by a factor 2.2 at least, the clathration scenario reproduces the quasi uniform enrichment with respect to
solar of the Ar, Kr, Xe, C, N and S elements measured in Jupiter by the Galileo probe. The interpretation of the non-uniform
enrichment in C, N and S in Saturn requires that ice was less abundant at 10 AU than at 5 AU so that CO and N2 were not clathrated in the feeding zone of the planet while CH4, NH3 and H2S were. As a result, the 14N/15N ratio in Saturn should be intermediate between that in Jupiter and the terrestrial ratio.
Ar and Kr should be solar while Xe should be enriched by a factor 17. The enrichments in C, N and S in Uranus and Neptune
suggest that available ice was able to form clathrates of CH4, CO and the NH3 hydrate, but not the clathrate of N2. The enrichment of oxygen by a factor 440 in Neptune inferred by Lodders and Fegley (1994) from the detection of CO in the
troposphere of the planet is higher by at least a factor 2.5 than the lower limit of O/H required for the clathration of CO
and CH4 and for the hydration of NH3. If CO detected by Encrenaz et al. (2004) in Uranus originates from the interior of the planet, the O/H ratio in the envelope must be around of order of 260
times the solar ratio, then also consistent with the trapping of detected volatiles by clathration. It is predicted that Ar
and Kr are solar in the two planets while Xe would be enriched by a factor 30 to 70. Observational tests of the validity of
the clathration scenario are proposed. |
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Keywords: | planet formation composition of planetesimals clathrates |
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