on Constraints on the Formation of Co Measured in H2O a

This report is the follow-up of the paper of A. Drouart et al. (1999, Icarus 140, 129) in which it was demonstrated that appro-priate models of the solar nebula permit us to interpret the deu-terium enrichment in water with respect to the protosolar D=H ratio measured in LL3 meteorites and comets. In the present re-port, 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 neb-ula, 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 de-tected in comets coming from Oort cloud. Most molecules detected in Comet Hale–Bopp originated from ices embedded in the preso-lar cloud. Ices vaporized prior to entering into the nebula or in the early nebula, and subsequently recondensed, except highly volatile molecule 290, 1009 possibilit form of c depletion comets