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D.H. Grinspoon, M.A. Bullock (Southwest Research Institute)
The surface temperature of Venus is a sensitive function of the abundances of greenhouse gases and also of cloud structure. In previous work we have studied 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). These studies showed that in outgassing events 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. The largest warming typically arises several hundred million years after an 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. It seems likely that large comet impacts should also produce such a situation. The atmosphere of Venus currently contains 7 x 1018 grams of water, about as much as in a 25 km diameter comet. Comets may have been an important contributor to the current water inventory on Venus. Much of this may have been supplied by a few large comet impacts in the last several hundred million years (3). We will report on new runs of our Venus Evolutionary Climate Model which simulate the volatile input from large comet impacts and investigate the climate effects of these events. Calculation will be done with cometary delivery alone, and in conjunction with various outgassing scenarios. This allows us to examine how the vulnerability of the Venusian climate system to impact induced climate change is affected by the relative timing of large magmatic and impact events. (1) Bullock, M.A., and D.H. Grinspoon, J. Geophys. Res. 101, 7521-7529, 1996. (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.