Global distribution and migration of subsurface ice on mars

Abstract

A thermal/diffusive model of H2O 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 CO2 and the varying extent of CO2 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 104-year time scale; (9) the effects of factors (1) through (5) on seasonal polar cap temperatures; and (10) Knudsen and molecular diffusion of H2O through the regolith. The migration of H2O into or out of the regolith is determined by two boundary conditions, the H2O vapor pressure at the subsurface ice boundary and the annual average H2O 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, H2O fluxes into or out of the regolith are calculated for 100 selected obliquity cycles, each representing a different epoch in Mars' history. The H2O 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 H2O 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 CO2 and the varying thickness of the CO2 ice cover are critically important in determining annual average atmospheric H2O concentrations. Despite the strongly oscillating character of the boundary conditions, only small amounts of H2O are exchanged between the regolith and the atmosphere per obliquity cycle (<10 g/cm2). The net result of H2O 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 H2O is strongly dependent on average pore/capillary radius for which we have assumed values of 1 and 10 μm. We estimate that the H2O ice removed from the regolith would produce a permanent ice cap with a volume between 2 × 106 and 6 × 106 km3. This generally agrees with estimates deduced from deflationary features at lower latitudes, depositional features at higher latitudes, and the mass of the polar caps. © 1986.