Stability of Ground Ice on Mars
Howard Houben (Space Physics Research Institute)
Martian atmospheric water vapor diffuses into the regolith where a considerable water inventory can accumulate over a few decades in the uppermost 10 cm. The diffusion will of course continue to lower depths over longer timescales. If temperatures are low enough at depth, large deposits of ice can be built up in this manner. This process is of great interest in assessing the total water inventory of Mars. A number of simulations of ground ice accumulation have been conducted in the past. However, they all share the shortcoming of assuming an annually averaged atmospheric water column as the upper boundary condition on the diffusion process. Thus, these models find that ground ice is stable wherever the vapor pressure over ice at the annual average ground temperature is lower than the assumed atmospheric amount. Recent fully three-dimensional transport simulations of the Martian water cycle, which couple the atmosphere and near-surface regolith, suggest an improved approach to this problem. To correspond to the observed water cycle, the transport model requires an adsorbing regolith. The adsorption process slows the diffusion of water vapor through the soil so that the diurnal wave of moisture penetrates only to about 1 cm depth (whereas the diurnal thermal wave penetrates about 10 cm). Water in a deeper layer (to 11 cm) exchanges with the atmosphere on a much longer timescale and shows little variation over the annual cycle. It is the water amount in this layer that is best used as the upper boundary condition on diffusion to greater depths. Indeed, these water amounts are also the best measure of the annually averaged atmospheric column. (Atmospheric water vapor amounts can vary by orders of magnitude in the mid and high latitudes over the course of the seasons because of the high temperature sensitivity of the vapor pressure curve.) The vapor pressure curve over an adsorbing regolith is a much less steep function of temperature than the vapor pressure curve over ice. In practice this means that ice will not form at low temperatures until all the available adsorption sites on the soil have been filled. Comparison of this vapor pressure curve with the modeled water contents of the regolith for the current annual water cycle leads to the conclusion that ground ice is not stable at present on Mars except in the near vicinity of the permanent north polar water ice cap.