FIGURE 4.1 Hypothetical pole-to-pole cross section of the present day martian crust along 157° longitude, illustrating the potential relationship between surface topography, ground ice, and groundwater. The depicted thickness of frozen ground (the cryosphere) assumes the present latitudinal range of mean annual surface temperatures (~154 to 218 K), a column-average thermal conductivity of 2 W m–1 K–1, and a global mean geothermal heat flux of 30 mW m–2 (modified from Clifford and Parker, 2001).

result in a present-day groundwater system that is in effective equilibrium (i.e., characterized by a relatively flat water table)—except where it may be locally perturbed by tectonic, seismic, or thermal processes. Because of the low porosity expected at depth,6 comparatively little water is required to produce a groundwater system of substantial extent. Therefore, if a subpermafrost groundwater system is present on Mars, it may underlie much of the planet’s surface—although the extent to which it may be interconnected is unknown (Clifford, 1993; Carr, 1996).

The distribution of ground ice is expected to follow the thermal structure of the crust, whereas the distribution of groundwater, under the influence of gravity, will drain and saturate the lowermost porous regions present at depth. For this reason, the vertical distance separating these subsurface reservoirs may vary considerably, such that the intervening unsaturated zone is maximized in regions of high elevation and minimized, or absent, at lower elevations (see Figure 4.1). Within the unsaturated zone, water vapor will tend to diffuse from the higher-temperature (higher-vapor-pressure) depths to the colder (lower-vapor-pressure) region just below the base of the cryosphere. As this moisture-laden air rises and cools, some of the vapor will condense, creating a low-temperature hydrothermal convection system of rising vapor and descending liquid condensate. Such a system may have resulted in complicated variations in saturation state between the base of the cryosphere and the regional ground-


Porosity decreases exponentially with depth because the pressure of the overlying rock collapses the pores that might otherwise be present.

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