PAST AND PRESENT WATER-BUDGET OF VENUS

A detailed analysis of Pioneer Venus large-probe neutral mass spectrometer (LNMS) data confirms an earlier report that the abundance of Venus deuterium relative to hydrogen is 2 orders of magnitude larger than that of terrestrial deuterium. These results have recently received confirmation from ground-based spectroscopic observations. The LNMS data can be analyzed to obtain height profiles for hydrogen compounds because of the very different isotopic signatures of Venus hydrogen and terrestrial hydrogen. The mixing ratio for water increases by a factor of 4 between 0 and 10 km to a value of 2.2 times the Ar-36 mixing ratio (67 ppm) and drops to 10 ppm above 50 km. Preliminary analysis has not revealed the presence of any hydrogen constituent that can compensate for the decrease in water vapor mixing ratio below 10 km. No combination of gas phase and surface chemistry has been found to account for the strange behavior, and vertical transport alone is not feasible. These results are inexplicable unless they can be interpreted in terms of large-scale temporal and spatial variations (for which other evidence also exists). It is argued that present-day deuterium and hydrogen cannot be in a steady state with stochastic cometary inputs but that these results call for an early Venus endowed with at least 2 orders of magnitude more water above the surface than is presently there. From the isotope ratio measurement alone, however, how the water got there or how much was originally present cannot be discerned.