Humidity Enhancement in Dry Permafrost: The Effects of Temperature Cycles on Habitability.

Abstract

The search for life in the solar system often focuses on water and on environments where habitable conditions exist, persistently or occasionally. In this search, dry permafrost (ice-free frozen soil) has received minimal attention. It was previously proposed that within martian dry permafrost the water activity ($a_w$, an essential property for habitability) could be enhanced by diurnal thermal cycles and water desorption from soil grains, but the details remain unexplored. We examined $a_w$ in dry soil (which contained only vapor and adsorbed water) through experiments and numerical simulations and contrasted the results with a habitability threshold for terrestrial organisms ($a_w\hspace{0.17em}\hspace{0.17em}>\hspace{0.17em}\hspace{0.17em}0.6$). We found that heating cycles in a soil raised $a_w$. As water vapor desorbs from warming soil grains, it diffuses toward cooler adjacent soil, where a fraction of this incoming vapor enhances the local $a_w$. In laboratory tests with loess and clay soils, we observed $a_w$ to increase by 0.06-0.12. Extrapolating from laboratory to permafrost conditions by using numerical simulations, we found that some Antarctic soils can be boosted periodically into a habitable range. In contrast, the current martian climate is too dry or cold for this $a_w$-enhancement process to impact habitability. However, high-obliquity periods on Mars are analogous to the Antarctic case.