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D.H. Grinspoon, M.A. Bullock (Southwest Research Institute)
Venus is commonly thought to have experienced a transition, early in its history, from a wet, more Earth-like past to its current highly desiccated state (Kasting, 1988). A more recent global transition is revealed by the sparse, randomly distributed and relatively pristine crater population, which indicates a rapid decrease in resurfacing rate between 300 and 1000 Myr ago (Schaber et al, 1992; Bullock et al, 1993; McKinnon et al, 1997). The accompanying precipitous decline in outgassing rate would have caused large climate changes (Bullock and Grinspoon, 2001) and globally synchronous plains deformation (Solomon et al, 1999). We are exploring the possibility that these two transitions may be part of a single planetary transformation. The loss of atmospheric water through evaporation, photodissociation and H escape would have eventually led to a transition from plate tectonics to single plate behavior, as the shut-off of subducting hydrated sediments led to the desiccation of the mantle and consequent loss of an asthenosphere. Current estimates of the timescale for water loss are highly unconstrained, with error estimates larger than the age of the planet. Currently, we are modeling clouds in wet, hot atmospheres in an effort to better constrain the albedo, energy balance and timescale for water loss. If clouds stabilized the moist greenhouse and Venus’ oceans persisted for several billion years, rather than the canonical (but unconstrained) hundreds of millions, then the loss of water could have initiated changes in global convective style which led directly to the currently observed surface features. This might mean that Venus was a (conventionally defined) habitable planet for most of its history. This work has been supported by NASA’s Planetary Geology and Geophysics and Exobiology programs.
Bullock and Grinspoon (2001) Icarus 150, 19-37; Bullock, M.A., D.H. Grinspoon and J.W. Head (1993) GRL 20, 2147-2150; Kasting (1988) Icarus 74, 472-494; McKinnon, W.B., K.J. Zahnle, B.A. Ivanov and H.J. Melosh (1997) in Venus II; Schaber, G.G et al. (1992) JGR 97, 13,257-13,302; Solomon, S.C., M.A. Bullock and D.H. Grinspoon (1999)Science 286, 87-89.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.