Global distribution and migration of subsurface ice on mars

Abstract

A thermal/diffusive model of H 2O kinetics and equilibrium was developed to investigate the long-term evolution and depth distribution of subsurface ice on Mars. The model quantitatively takes into account (1) obliquity variations; (2) eccentricity variations; (3) long-term changes in the solar luminosity; (4) variations in the argument of subsolar meridian (in planetocentric equatorial coordinates); (5) albedo changes at higher latitudes due to seasonal phase changes of CO 2 and the varying extent of CO 2 ice cover; (6) planetary internal heat flow; (7) temperature variations in the regolith as a function of depth, time, and latitude due to the above factors; (8) atmospheric pressure variations over a 10 4-year time scale; (9) the effects of factors (1) through (5) on seasonal polar cap temperatures; and (10) Knudsen and molecular diffusion of H 2O through the regolith. The migration of H 2O into or out of the regolith is determined by two boundary conditions, the H 2O vapor pressure at the subsurface ice boundary and the annual average H 2O concentration at the base of the atmosphere. These are controlled respectively by the annual average regolith temperature at the given depth and seasonal temperatures at the polar cap. Starting from an arbitrary initial uniform depth distribution of subsurface ice, H 2O fluxes into or out of the regolith are calculated for 100 selected obliquity cycles, each representing a different epoch in Mars' history. The H 2O fluxes are translated into ice thicknesses and extrapolated over time to give the subsurface ice depth as a function of latitude and time. The results show that obliquity variations influence annual average regolith temperatures in varying degrees, depending on latitude, with the greatest effect at the poles and almost no effect at 40° lat. Insolation changes at the pole, due to obliquity, argument of subsolar meridian, and eccentricity variations can produce enormous atmospheric H 2O concentration variations of ≈6 orders of magnitude over an obliquity cycle. Superimposed on these cyclic variations is a slow, monotonic change due to the increasing solar luminosity. Albedo changes at the polar cap due to seasonal phase changes of CO 2 and the varying thickness of the CO 2 ice cover are critically important in determining annual average atmospheric H 2O concentrations. Despite the strongly oscillating character of the boundary conditions, only small amounts of H 2O are exchanged between the regolith and the atmosphere per obliquity cycle (<10 g/cm 2). The net result of H 2O migration is that the regolith below 30–40° lat is depleted of subsurface ice, while the regolith above 30–40° lat contains permanent ice due to the depth of penetration of the annual thermal wave. This result is supported by recent morphological studies. The rate of migration of H 2O is strongly dependent on average pore/capillary radius for which we have assumed values of 1 and 10 μm. We estimate that the H 2O ice removed from the regolith would produce a permanent ice cap with a volume between 2 × 10 6 and 6 × 10 6 km 3. This generally agrees with estimates deduced from deflationary features at lower latitudes, depositional features at higher latitudes, and the mass of the polar caps.