The solar system D/H ratio: Observations and theories

The measured D/H ratios in interstellar environments and in the solar syste m are reviewed. The two extreme D/H ratios in solar system water - (720 +/- 120) x 10(-6) in clay minerals and (88 +/- 11) x 10(-6) in chondrules, bot h from LL3 chondritic meteorites - are interpreted as the result of a progr essive isotopic exchange in the solar nebula between deuterium-rich interst ellar water and protosolar H-2. According to a turbulent model describing t he evolution of the nebula (Drouart et al., 1999), water in the solar syste m cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720 x 10(-6) as a face value for the isotopic composition o f the interstellar water that predates the formation of the solar nebula, n umerical simulations show that the water D/H ratio decreases via an isotopi c exchange with H-2. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the iso topic homogenization of the solar nebula was completed in 10(6) years, reac hing a D/H ratio of 88 x 10(-6). In this model, cometary water should have also suffered a partial isotopic re-equilibration with H-2. The isotopic he terogeneity observed in chondrites result from the turbulent mixing of grai ns, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in ag reement with these chondritic data and their interpretation (Texeira et al. , 1999).