Constraints on the Formation of Comets from D/H Ratios Measured in H2O and HCN

This report is the follow-up of the paper of A. Drouart et al. (1999, Icarus140, 129) in which it was demonstrated that appropriate models of the solar nebula permit us to interpret the deuterium enrichment in water with respect to the protosolar D/H ratio measured in LL3 meteorites and comets. In the present report, we show that the models selected by Drouart et al. are also able to explain D/H in HCN measured in Comet C/1995 O1 (Hale–Bopp). We find that the D/H ratio in HCN entering the nebula is ∼4×10−3, which is significantly less than values measured in cold dark clouds, but consistent with values found in hot molecular cores. Both H2O and HCN ices infalling from the presolar cloud onto the nebula discoid evaporated in the turbulent part of the nebula, isotopically exchanged with hydrogen, and mixed with water vapor coming from the inner part of the nebula. Subsequently, H2O and HCN ices with D/H ratios measured in Comet Hale–Bopp condensed, agglomerated and were incorporated in cometesimals. In the light of these results, we discuss the story of molecules detected in comets coming from Oort cloud. Most molecules detected in Comet Hale–Bopp originated from ices embedded in the presolar cloud. Ices vaporized prior to entering into the nebula or in the early nebula, and subsequently recondensed, except highly volatile molecules. According to A. Kouchi et al. (1994, Astron. Astrophys.290, 1009), water ice condensed in crystalline form. We discuss the possibility that the most volatile species were then trapped in the form of clathrate hydrates. The oversolar C/N ratio and the strong depletion of Ne/O with respect to the solar abundance observed in comets are in agreement with the theory of clathrate hydrates of J. I. Lunine and D. J. Stevenson (1985, Astrophys. suppl. Ser.58, 493). Comets formed in the Kuiper belt may contain amorphous water ice and have kept the isotopic signature of the presolar cloud. New published models of interiors of Uranus and Neptune permit us to calculate that the D/H ratios in proto-uranian and proto-neptunian water ices are in agreement with those measured in comets. This confirms the current assumption that cometesimals and planetesimals that formed the cores of Uranus and Neptune had similar compositions.

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