DEUTERIUM IN THE SOLAR-SYSTEM

A survey of the abundances of deuterium in planetary atmospheres and small bodies has been carried out. The observed pattern of D/H ratios in the solar system may be interpreted in terms of a few simple concepts: origin, fractionation, and dilution. There appear to be two distinct reservoirs of hydrogen in the solar nebula: the bulk of hydrogen as H2, and a smaller amount in ices and organics. The latter reservoir is characterized by a higher D/H ratio than the former, and may be the principal source of hydrogen to the terrestrial planets and small bodies. The evolution of planetary atmospheres over the age of the solar system has resulted in substantial changes in the D/H ratio in the atmospheres of the terrestrial planets. In the giant planets the abundance of D is dominated by the primordial HD, and there has been negligible chemical evolution since formation. Quantitative modeling of the D/H ratio in the solar system remains hampered by the lack of appropriate chemical kinetics data.