Rain infiltration and crust formation in the extreme arid zone of the Atacama Desert, Chile

A key question in understanding life on Mars under dry(ing) conditions is how arid soils respond to small levels of liquid water. We have conducted a series of simulated rain experiments in the hyperarid core region of the Atacama Desert. Rain amounts from 0.24 to 3.55 mm were applied in the early evening to the soil. We conclude that rain events of less than 1 mm do not saturate the surface, and the soil humidity at the surface remains below 100%. Rain events of 2 mm or more generate free water in the pore space of the soil surface, which may be necessary to support biological activity in the soil. The crust on the surface of the soil is a strong barrier to the diffusion of subsurface moisture and subsequent evaporation. Our results show that once the relative humidity in hyperarid soils begins to fall below 100% the rate of decrease is quite rapid. Thus, the precise value assumed for the limits of life or water activity, do not appreciably change the time of water availability resulting from small desert rains. The Atacama Desert results may be applied to models of (H₂O) wetting in the upper soils of Mars due to light rains, melting snow and heavy precipitating fog.