Cometary deuterium

Deuterium fractionations in cometary ices provide important clues to the or igin and evolution of comets, Mass spectrometers aboard spaceprobe Giotto r evealed the first accurate D/H ratios in the water of Comet 1P/Halley. Grou nd-based observations of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits for several other D-bearing molecules complement our limited sample of D/H meas urements. On the basis of this data set all Oort cloud comets seem to exhib it a similar (D/H)(H2O) ratio in H2O, enriched by about a factor of two rel ative to terrestrial water and approximately one order of magnitude relativ e to the protosolar value, Oort cloud comets, and by inference also classic al short-period comets derived from the Kuiper Belt cannot be the only sour ce for the Earth's oceans. The cometary O/C ratio and dynamical reasons mak e it difficult to defend an early influx of icy planetesimals from the Jupi ter zone to the early Earth, D/H measurements of OH groups in phyllosilicat e rich meteorites suggest a mixture of cometary water and water adsorbed fr om the nebula by the rocky grains that formed the bulk of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 tim es higher than the value in cometary H2O. Species-dependent D-fractionation s occur at low temperatures and low gas densities via ion-molecule or grain -surface reactions and cannot be explained by a pure solar nebula chemistry . It is plausible that cometary volatiles preserved the interstellar D frac tionation. The observed D abundances set a lower limit to the formation tem perature of (30 +/- 10) K, Similar numbers can br derived from the ortho-to -para ratio in cometary water, from the absence of neon in cometary ices an d the presence of St. Noble gases on Earth and Mars, and the relative abund ance of cometary hydrocarbons place the comet formation temperature near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemis try can be excluded for molecules H2O and HCN. A comparison of the cometary (D/H)(H2O) ratio with values found in the atmospheres of the outer planets is consistent with the lone-held idea that the gas planets formed around i cy cores with a high cometary D/H ratio and subsequently accumulated signif icant amounts of H-2 from the solar nebula with a low protosolar D/H.