THE RECENT EVOLUTION OF CLIMATE ON VENUS: THE MOST EARTHLIKE PLANET

The Magellan radar mapping mission in the 1990s gave us a picture of the unusual resurfacing history of Earth's sister planet. This allows us to infer the outgassing history and reconstruct a preliminary climate history over the last billion years. We find that, for various reasons, climate on Venus is extremely sensitive to geophysical cycles and, conversely, surface volcanism and tectonism may be strongly affected by climate cycles. The surface temperature of Venus is a sensitive function of the abundances of greenhouse gases and also of cloud structure. I will discuss the climate impact of past and continued outgassing of greenhouse and cloud-forming gases (1) and tectonic signatures that may have resulted from volcanically induced climate change (2). In epochs of enhanced outgassing, where large amounts of both H2O and SO2 are released, the increased albedo that arises from thickening of the clouds can, to some extent, ameliorate the greenhouse warming expected from increased abundances of these IR absorbing gases. Models suggest that the largest warming typically arises several hundred million years after a large scale outgassing event, when most of the excess SO2 has been removed by reaction with surface minerals, but much of the atmospheric H2O remains (because it is removed by exospheric escape on longer time scales). This combination - enhanced H2O abundance with SO2 returned to "normal" - leads to maximum warming because the cloud thickness, and thus the albedo, is limited by the availability of SO2, whereas IR absorption in CO2 windows by enhanced H2O can cause warming on the order of 100 K. I will also discuss the climate effects of large comet impacts, which will cause temporary spikes in the greenhouse gas inventory (3). (1) Bullock, M.A. and D.H. Grinspoon, Icarus 150, 19-37, 2001. (2) Solomon, S.C., M. A. Bullock, and D. H. Grinspoon, Science, 286: 87-90, 1999. (3) Grinspoon, D.H. and J.S. Lewis, Icarus, 74, 21-35, 1988.