The Potential for Melting of Subsurface Ice at the Phoenix Lander Latitudes in Mars' Recent Past

The Phoenix Lander will set down on the surface of Mars in the northern plains between 65N and 75N. Odyssey's Gamma Ray Spectrometer and Neutron Spectrometer have detected large hydrogen abundances in this latitude band that are most likely due to the presence of ice within 50 cm of the surface. Here we report the results from the Ames general circulation model (GCM) on the ability of solar heating and subsurface conduction to melt these deposits. If melting did occur at some time in the past, it would make these latitudes particularly interesting for biological investigations. Melting is not possible in the present epoch because temperatures never get high enough even though surface pressures in this latitude band do exceed the triple point of water. However, about 25,000 years ago the longitude of perihelion was shifted 180 degrees such that perihelion occurred near northern summer solstice rather than northern winter solstice. Simulations of that epoch show broad regions where surface temperatures exceed 273 K enhancing the potential for melting. Indeed, surface temperatures and pressures exceed the triple point for water for as much as 90 sols of total time at some locations. A key question, of course, is whether the ice seen by Odyssey was actually present at those times and if so, at what depth below the surface. This will depend on the ability of the ice to diffuse through the soil and the water vapor content of the atmosphere. However, the short precessional time scale may limit the amount of water that can diffuse out of the soil. And the atmospheric water content will be higher with perihelion at northern summer solstice. We plan to explore these possibilities with the Ames GCM.