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A. P. Zent (NASA Ames)
The 2007 Phoenix mission will land near 70 N latitude, and excavate the regolith to sample ground ice. Chemical analyses will search for dissolved species, chemical sediments, and organic compounds that may indicate the chemical and environmental history of the ground ice. Mars periodically experiences high obliquities, leading to warming of the polar latitudes. This raises the possibility that thin films of unfrozen water could serve as habitats for microbial communities in a manner analogous to terrestrial permafrost communities. However, Phoenix can only access the uppermost millimeters of the ground ice. We model the likely history of the ground ice that Phoenix will access. The model tracks surface energy balance, including cap latent heat. The upper boundary condition is the annual average H2O vapor density, which is a sensitive function of H2O ice albedo and conductivity, as well as the albedo, emissivity, and thermal conductivity of the seasonal CO2 cap. Relatively small variations in these properties can introduce considerable uncertainties in the actual depth of the ground ice. We bracket these uncertainties by assuming either that: a) the annual average H2O vapor density is invariant with time, or b) the annual average H2O vapor density is proportional to the column abundance determined by radiative balance. For most thermophysical properties, the model predicts that ground ice has become more shallow over the past 25 ka, corresponding with migration of perihelion from northern summer to northern winter. The ice Phoenix will access has probably been recently deposited from the atmosphere, and has not experienced heating at high obliquities. Therefore, it is unlikely to have constituted a viable habitat. Nonetheless, Phoenix may encounter materials relict of earlier periods of high obliquity when thin unfrozen films were possible.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.