Stability of ice on Mars and the water vapor diurnal cycle: Experimental study of the sublimation of ice through a fine-grained basaltic regolith

In order to advance our understanding of the long-term stability of subsurface ice, the diurnal martian water cycle, and implications for liquid water, we determined diffusion coefficients and adsorption kinetics for the water vapor produced by the sublimation of ice buried beneath various layers of fine-grained (<63, 63–125, and 125–250 μm) basaltic powder under simulated martian conditions. Sublimation rates at shallower depths, <10 mm, were determined to be affected by mass transfer through the atmosphere in addition to the basalt layer. For greater depths, the measured diffusion coefficients for water vapor moving through basalt grains were 1.56±0.53×10−41.56±0.53×10−4, 2.05±0.82×10−42.05±0.82×10−4, and 3.42±1.36×10−4 m2s−1 for the <63, 63–125, and 125–250 μm basaltic layers, respectively. Through the Brunauer, Emmett and Teller (BET) isotherm, which assumes multiple molecular layers of adsorbed water, we determined the adsorption constants of 52.6±8.352.6±8.3 at 270 K for <63 μm, 39.0±6.439.0±6.4 at 267 K for 63–125 μm, and 54.3±9.354.3±9.3 at 266 K for 125–250 μm, resulting in surface areas of 2.6±0.1×1042.6±0.1×104, 1.7±0.3×1041.7±0.3×104, 1.5±0.3×104 m2kg−1, respectively. These results suggest that while diffusion is too rapid to explain the purported diurnal cycle in water content of the atmosphere, adsorption is efficient and rapid, and does provide an effective mechanism to explain such a cycle. The present diffusion data suggest that very thin, <50 pr μm, shallow, 10 mm, ice deposits would last for >10 h at ∼224 K, just above the freezing point of saturated CaCl2. Temperatures can remain above ∼224 K over most of the planet, which means that water, even as saturated brine, will sublimate before the freezing point is reached and liquid could be formed. On the other hand, 1 m ice layers below 1 m of fine-grained basaltic regolith at 235 K and 10 Pa of atmospheric water could last 600 to 1300 years. At deeper depths and lower temperatures, ice could last since the last major obliquity change 400,000 years ago.