Historical Habitability of Mid- and High-Latitude Martian Ground Ice

Shallowly buried ground ice is ubiquitous on Mars at high latitudes poleward of ~50° in both hemispheres. In the current climate, this ice is perennially cold, e.g., ~190 K on annual average at ~70° N. Despite low temperatures, thin "premelted" films of unfrozen water can exist in the shallow subsurface due to interfacial and Gibbs-Thomson pre-melting at soil-ice interfaces. The presence of salts can further depress the freezing point, increasing the volume of unfrozen pore water. There has been long-term interest in ground ice as a potential microbial habitat on Mars, supported in part by the occurrence of viable bacteria in liquid vein networks of Antarctic glacial ice.

In the decade since the habitability of high-latitude ground ice was first systematically investigated, perchlorate salts have been discovered at the Phoenix and Curiosity landing sites and our understanding of unfrozen water content in ice-cemented soils has improved substantially. Here, we have employed new numerical simulations of subsurface temperature, unfrozen water content, and water activity (a_H2O) in thin films to constrain habitability in shallow ice-cemented ground over the past 10 Ma. We use the climate model described by Zent (2008) to simulate the evolution of temperature and ice-table depth, z_i, at latitudes north of 55^o over the past 10 Ma. We use the thin film model described by Sizemore et al. (2015) to track temperatures and phase partitioning in a soil that is fully ice and water saturated.

Over the past 10 Ma, orbital conditions episodically allow mid-summer values of a_H2O to exceed the 0.61 and 0.75 thresholds for terrestrial metabolism. These "habitable" periods occur preferentially before 4 Ma, a timeframe in which there are major uncertainties in the atmospheric vapor density boundary condition, the orbital configuration, and the occurrence of precipitation. Episodes of habitability are also brief and rare - persisting for 100-300 sols/year during isolated ~10⁵ year windows. The long dormant periods indicated by our models present a major challenge to the repair of accumulated radiation damage for any extant microbes in the upper decimeters of the Martian subsurface.