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The Limits of Theoretical Modeling and Geomorphic Interpretation in Assessing the Present Distribution of Subsurface H2O on Mars

Introduction: The search for water has been identified as the principal objective and common thread of Mars research [1]. Of the planet's estimated 0.5 –1 km global i nventory of H 2 O [2], ~0.000001% is found in the atmosphere and ~5-10% is thought to be stored as ice in the perennial polar caps and la yered terrain. This leaves ~90-95% of the planetary inventory of H 2 O that is unac-counted for, the vast bulk of which is believed to reside, as ground ice and groundwater, within the planet's crust. The expected distribution and state of subsurface water on Mars, as well as plausible values of the large-scale physical, thermal, and hydraulic properties of its crust, have been discussed by a number of investigators [3-8]. To a first order, subsurface conditions on Mars are expected to resemble those found in cold-climate regions on Earth, particularly the unglaciated, continuous permafrost regions of Antarctica, Siberia, and North America. This similarity is likely to extend to an equivalent level of geologic complexity and spatial variability in such characteristics as lithology, structure, stratigraphy, porosity, volatile content, permeability, and mechanical and thermal properties. Theoretical Modelling: Given the uncertainty regarding the amount and distribution of subsurface water, a variety of theoretical and geomporphic techniques have been employed to infer where it might reside-most notably with respect to the stability and distribution of ground ice. The very earliest efforts [9-11] considered the distribution of ice in equilibrium with the zonally-averaged mean annual surface temperature and water vapor content of the atmosphere. Later studies [12-14] examined the stability and evolution of ground ice under disequilibrium conditions, investigating how plausible values of regolith thermophysical and diffusive properties might influence the latitudinal stability of ground ice in an otherwise homogeneous crust. More recent investigations [15-16] have examined how local variations in the radiative and thermophysical properties of the surface could lead to significant geographic differences in ground ice stability, suggesting that – in some regions, under favorable conditions – fossil ground ice might survive at shallow depth even at the equator. These latter studies have suggested to some that the simple acquisition of high spatial resolution thermal data might allow us to predict the present distribution of ground ice with considerable precision. This belief is based on the assumption that the thermal and diffusive properties of the regolith are well understood and can be extrapolated, with confidence, to depths …