Constraints on the formation of comets from D/H ratios measured in H2O andHCN

This report is the follow-up of the paper of A. Drouart et al. (1999, Icaru s 140, 129) in which it was demonstrated that appropriate models of the sol ar nebula permit us to interpret the deuterium enrichment in water with res pect 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 similar to4 x 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 th e turbulent part of the nebula, isotopically exchanged with hydrogen, and m ixed with water vapor coming from the inner part of the nebula. Subsequentl y, 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 r esults, we discuss the story of molecules detected in comets coming from Oo rt cloud. Most molecules detected in Comet Hale-Bopp originated from ices e mbedded in the presolar cloud. Ices vaporized prior to entering into the ne bula or in the early nebula, and subsequently recondensed, except highly vo latile molecules. According to A. Kouchi et al. (1994, Astron. Astrophys. 2 90, 1009), water ice condensed in crystalline form. We discuss the possibil ity that the most volatile species were then trapped in the form of clathra te 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, As trophys, suppl. Ser. 58, 493). Comets formed in the Kuiper belt may contain amorphous water ice and have kept the isotopic signature of the presolar c loud. 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 ic es are in agreement with those measured in comets. This confirms the curren t assumption that cometesimals and planetesimals that formed the cores of U ranus and Neptune had similar compositions. (C) 2000 Academic Press.