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

To investigate the stability of ground ice within the top several mete rs of the Martian regolith, time-dependent models of the thermal and d iffusive 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 t emperatures were below the atmospheric frost point. This generally occ urs poleward of the mid-latitudes. The latitude poleward of which grou nd ice is stable varies by about 20-degrees to 30-degrees from one lon gitude to another. Geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stabil ity. Calculations of temperatures at high and low obliquity suggest th at 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 mo deled accounting for both ordinary molecular and Knudsen transport and equilibrium between ice, vapor, and adsorbed phases. Atmospheric wate r vapor was found to be able to supply the top few meters of the regol ith with ice in regions where the annual mean surface temperature was below the atmospheric frost point. Ice was found to begin condensing i n as short as 1000 Martian years. Rapid accumulation of ice in the por e space of the upper layers of the regolith acted to choke transport t o lower layers and slow the diffusion process. Even so, after 10(5) Ma rtian years, in some cases, as much as 30% to 40% of the available por e space accumulated ice. The total amount of subsurface ice ranged fro m 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 atmos pheric vapor pressure near the surface. Atmospheric vapor was not foun d to accumulate as ice below a depth where the seasonal temperature os cillations gave way to the geothermal gradient. The time scales for co ndensation of ground ice were found Lo be comparable to that of orbita l oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.