Cryovolcanism and the recent flow of liquid water on Mars

The surface of Mars is too cold and dry to permit stable liquid water, yet fresh, apparently water-carved gullies and seepage features have been identified in high-resolution imaging of canyon and crater walls by the Mars Global Surveyor spacecraft. Here, a model of nonequilibrium hydrological activity and liquid water cryovol-canism explains the paradoxical appearance and observed properties of these landforms: Aquifers within a porous crust are confined by growing ice and impermeable volcanic layers or basement rock. Freeze-thaw cycles driven by changes in heat flow or climate create high ambient pore pressures and lead to catastrophic expulsion of water through cracks to the surface. The gullies are thus indicators of the geologic and thermal state of the crust, not the present surface environment. Investigations of deposits formed by this cryovolcanism are a means of probing the deep subsurface of Mars for potential biological activity. (C) 2001 Academic Press.