Groundwater Sapping and Valley Formation on Mars

Most small valleys in the ancient highlands of Mars are probably the result of erosion by groundwater sapping. This conclusion together with the fact that liquid water is thermodynamically unstable on the surface of Mars today is frequently used to argue that the planet was warmer in the past. The strength of these arguments is tested by examining the effectiveness of the sapping mechanism under various climatic and hydrologic conditions, including aquifer permeability, aquifer temperature, sediment grain size, wind speed at the surface, latitude, and obliquity. Numerical simulations of groundwater sapping under different conditions indicate that the link between the small valleys and a warmer early martian climate is not as strong as is often assumed. If water in the upper few hundred meters of the martian regolith had been kept above the freezing point through hydrothermal circulation or conduction of geothermal heat, then the small martian valleys could have formed by groundwater sapping under climatic conditions like those that prevail today, especially if the permeability of the martian regolith is like that of terrestrial gravels. Geothermal warming to temperatures well above the freezing point would have greatly facilitated sapping. The current low-pressure atmosphere is more conducive to the sapping process (though not necessarily to subsequent stream flow) than a high-pressure atmosphere that is only slightly warmer.