Origin and evolution of the atmosphere of Venus

Implications for the origin and evolution of the terrestrial planets are drawn from a comparison of the Venus, earth and Mars atmosphere volatile inventories. Attention is given to the possible loss of an appreciable amount of water from Venus, in light of recent evidence for a 100-fold deuterium enrichment. Ar-40 and He-4 abundances suggest that outgassing has been inefficient for much of Venus's lifetime, in keeping with evidence for a lower level of tectonic activity on Venus than on the earth. Attention is also given to Venus's CO2 geochemistry. The picture now emerging is that of a Venus that began to evolve along a path similar to that of the earth, but suffered a catastrophic, runaway greenhouse effect early in its lifetime. How early the castastrophe occurred may be suggested by the presently low inventories of radiogenic argon and helium in its atmosphere.