Distribution and state of H 2O in the high-latitude shallow subsurface of mars

Abstract

A quantitative model of the state, distribution, and migration of water in the shallow Martian regolith is presented. Reported results are confined to the region of the planet greater than 40° lat. The calculations take into account (1) expected thermal variations at all depths, latitudes, and times resulting from seasonal and astronomically induced insolation variations; (2) variations in atmospheric P H 2O and P CO 2 resulting from polar insolation variations and regolith adsorptive equilibria; (3) feedback effects related to latent heat and albedo variations resulting from condensation of atmospheric constituents; (4) two possible regolith mineralogies; (5) variable total H 2O content of the regolith; (6) kinetics of H 2O transport through the Martian atmosphere and regolith; and (7) equilibrium phase partitioning of H 2O between the condensed, adsorbed, and vapor phases. Results suggest that the adsorptive capacity of the regolith is important in controlling the state and distribution of high-latitude H 2O; unweathered mafic silicates favor the development of shallow ground ice at all temperate and polar latitudes, while heavily weathered clay-like regolith materials leads to a deeper ground ice interface and far more extensive quantities of adsorbed H 2O. The capacity of the high-latitude regolith for storage of H 2O and the total mass of H 2O exchanged between the atmosphere, polar cap, and subsurface over an obliquity cycle is found to be relatively independent of mineralogy. The maximum exchanged volume is found to be 3.0 × 10 4 km 3 of ice per cycle. Implications for the history of the polar caps and the origin of the layered terrain are discussed. Results also suggest that seasonal thermal waves act to force adsorbed H 2O into the solid phase over a wide variety of latitude/obliquity conditions. Seasonal phase cycling of regolith H 2O is most common at high latitudes and obliquities. Such phase behavior is highly dependent on regolith mineralogy. In a highly weathered regolith, adsorbed H 2O is annually forced into the solid phase at all latitudes ≥40° at obliquities greater than approximately 25°. Seasonal adsorption-freezing cycles which are predicted here may produce geomorphologic signatures not unlike those produced by terrestrial freeze-thaw cycles.