Patrick Russell



Evolution of hydraulic head of groundwater on Mars: testing the model with geologic evidence in the northern lowlands.

Clifford (1993) has proposed a model for the major transport pathways and repositories of water on Mars. Two important aspects of this model are the distribution of water in the Martian subsurface and its potential for accessing the surface, both of which may have changed over the planets [thermal] history. In this project I would like to test the specific predictions of this model concerning the thickness of the cryosphere and the presence of underlying groundwater, possibly under hydrostatic pressure, by examining geologic features on the surface. There are three specific areas on which I would eventually like to focus: the source of the outflow channels, the highland/lowland boundary scarp and the associated anomalously young, smooth terrain, and the northern lowlands, all of which may have been [differently] affected by ground and/or surface water (per Cliffords model) at different stages of Mars history.

I will first look at the northern lowlands. According to Clifford and Parker (1999), a thin cryosphere early in Martian history would have permitted communication of groundwater with the surface, resulting in the potential for flooding the northern lowlands with an ocean. Geologic evidence for a northern ocean has been provided by Parker et al. (1989), Head et al. (1999), and others. As Mars cooled, the cryosphere would have grown, effectively sealing off ground water from the surface and thus giving rise to the potential for the development of hydrostatic head beneath the cryosphere. Clifford (1993) suggests this might have lead to the outbursts of the outflow channels. Presumably, hydrostatic pressure under the northern lowlands late in Martian history would be even greater due to their lower elevation. What would have been the result of a breach in the cryosphere here, by an impact, for example? Would water have flown onto the surface (hydraulic head, permeability, re-freezing rate)? Is there geomorphologic evidence for this in any northern lowlands craters (smooth, flat crater fill, terraces, burial, erruption)? If so, what is the minimum size (depth) crater that exhibits such evidence? Examining these questions would test Cliffords hydrological model and constrain estimates of the distribution of water in the subsurface (thickness of cryosphere, depth to water table). (Long-shot implication: could an impact piercing the cryosphere have supplied enough water to fill an ocean?!)

Step 1: What do we expect from the model?: fit Cliffords schematic predictions of subsurface water distribution to actual typical MOLA topographic profiles.
Step 2: Is there geologic evidence to support this model?: look at craters in northern lowlands.