Temperatures in a runaway greenhouse on the evolving Venus: implications for water loss

Abstract

Recent work has established that Venus once had at least 100 times its present complement of outgassed water. An original complement of water comparable to the earth's is not inconsistent with our present knowledge (and is worthy of consideration since it involves no special assumptions concerning the differences in origin of the two planets). We use a one-dimensional calculation to investigate temperatures in the "runaway greenhouse" which may have ensued if Venus once had more than a few percent of the earth's water complement. We find that the atmosphere must exhibit an unusual structure, with condensation and presumably cloud formation taking place at high altitudes, while deep in the atmosphere the gas is strongly unsaturated with respect to water vapour. The necessity of including clouds introduces considerable uncertainty into the calculation of surface temperatures. However, for certain reasonable values of the cloud parameters, very high temperatures can be sustained, approaching the temperature of the basalt solidus. We speculate that such high temperatures may have promoted water-rock reactions, releasing free hydrogen and "burying" oxygen. A plastic or molten surface could have promoted rapid exposure of fresh rock, significantly easing the problem of disposing of the oxygen released by the dissociation of water.