IMPLICATIONS OF THE HIGH D/H RATIO FOR THE SOURCES OF WATER IN VENUS ATMOSPHERE

THE high abundance ratio of deuterium to hydrogen in the atmosphere of Venus (120 times that on Earth) can be interpreted either as the signature of a lost primordial ocean1, or of a steady state in which water is continuously supplied to the surface of Venus by comets or volcanic outgassing, balancing loss through hydrogen escape2,3. New observations4-6 of a water concentration of only 30 parts per million in Venus' atmosphere imply that the residence time of water in the atmosphere, before it escapes to space, is short compared with the age of the Solar System, casting doubt on the primordial ocean hypothesis. But a recent theoretical reanalysis of collisional ejection7 has increased estimates of the deuterium escape efficiency by a factor of 10: this means that if the venusian water budget is in steady state, the D/H ratio of the source water must be 10-15 times higher than that on Earth, ruling out cometary water, whose D/H ratio is thought to be lower than this8. Here I suggest that these observations can be understood either as the result of continuous outgassing from a highly fractionated mantle source (such as might result from severe dessication of the mantle, or massive hydrogen escape early in the planet's history) or Rayleigh fractionation after massive outgassing from catastrophic resurfacing of the planet in the past 0.5-1 Gyr.