GEOGRAPHIC VARIATIONS IN THE THERMAL AND DIFFUSIVE STABILITY OF GROUND ICE ON MARS

To investigate the stability of ground ice within the top several meters of the Martian regolith, time-dependent models of the thermal and diffusive behavior of the regolith have been developed. The geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation have been included in calculations of surface and subsurface temperatures between +/-60-degrees latitude. Ground ice was found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point. This generally occurs poleward of the mid-latitudes. The latitude poleward of which ground ice is stable varies by about 20-degrees to 30-degrees from one longitude to another. Geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stability. Calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between +/-60-degrees-latitude at low obliquity. Thermally driven diffusion of atmospheric water vapor within the regolith was modeled accounting for both ordinary molecular and Knudsen transport and equilibrium between ice, vapor, and adsorbed phases. Atmospheric water vapor was found to be able to supply the top few meters of the regolith with ice in regions where the annual mean surface temperature was below the atmospheric frost point. Ice was found to begin condensing in as short as 1000 Martian years. Rapid accumulation of ice in the pore space of the upper layers of the regolith acted to choke transport to lower layers and slow the diffusion process. Even so, after 10(5) Martian years, in some cases, as much as 30% to 40% of the available pore space accumulated ice. The total amount of subsurface ice ranged from a few to more than 25 g/cm2 within the Lop few meters in regions of stability. Ground ice was found to form below a depth where the annual average vapor pressure over ice was equal to the annual average atmospheric vapor pressure near the surface. Atmospheric vapor was not found to accumulate as ice below a depth where the seasonal temperature oscillations gave way to the geothermal gradient. The time scales for condensation of ground ice were found Lo be comparable to that of orbital oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.