Nonequilibrium melting of icy soil in confined geometries on Mars

While applicable to natural phenomena such as landslides, the study reported here was motivated by concerns about radioactive power sources (RPS) that might be emplaced just below the martian surface as a result of a landing "anomaly." Mars is best described as a cold, dry desert in the sense that water (in any form) is not circulated by precipitation or other means. Thus, while the phase diagram supports liquid water in many places, over time it would all migrate to the coldest locations, primarily at the poles or just beneath the surface at high latitudes. Transient water can be formed, however, when this equilibrium is disturbed by introduction of a heat source. Since the crash of a spacecraft might deposit a halo of microbial contamination in the vicinity of an RPS, such transient sources of water could provide breeding areas that would violate planetary protection treaties. The Mars Odyssey spacecraft has identified vast stretches of high latitude terrain as zones where liquid water could easily be formed in this scenario. To address both the extent and the duration of wet soil, certain scenarios have been modeled in two dimensions using both a finite difference time-marching method and by one dimensional analytical approximation. Considered in the analysis are the diffusion of both heat and water vapor, capillary forces on liquid water, latent heat exchange, surface processes such as radiation, evaporation, and convection, and continuous equilibration between the liquid, vapor, and solid phases. Results indicate that the initial ice content of the soil, a proxy for thermal conductivity, exerts the greatest influence on the progress of the wetting and drying cycle. Ice can be melted at distances of almost a meter from a 250W power source and may, under certain circumstances, persist for months. It is not yet clear, however, whether the result suggests a planetary protection risk.