Salt Migration in Mars-like Permafrost Soil.

Soluble salts and duricrust (weakly-cemented soils) are observed at every landing site on Mars. Indeed, global remote sensing indicates that salts and duricrust are globally widespread. However, the origin and hydrological history of duricrust is presently not understood. Modern Mars is cold and dry, yet salts can help stabilize liquid water. Below freezing, thin films of liquid-like water can persist, aided by the addition of salts. These thin films can move throughout the soil column, transporting salts with them. Salts can also diffuse through mobile layers driven by concentration gradients and thermodynamic forces, where precipitation may cement loose soil grains into duricrust. Our research investigates the possible mechanism of duricrust formation via migration of thin liquid films of water and associated salts. Likewise, we examine the effect of salts on water mobility and ice formation. In our study, we focus on chloride and sulfate salts, such as those found at the Viking and Phoenix sites. MgCl₂, MgSO₄, and CaSO₄ were individually added to Mars simulant soil (Birch Hill loess from Alaska, a silt-dominated soil with minimal organic and native salt contents) at concentrations consistent with those measured on Mars. The salt-doped soils were subjected to a temperature gradient: 0 °C at the surface extending to -5 or -10 °C at the base, at 2 cm depth. A continuous flow of humidified nitrogen (frost point -1°C) was delivered to the headspace, a supply of water to condense and migrate within the soil column. After 2-4 weeks, we processed the soil samples by sectioning, observing textural changes and gravitimetric ice content. Salt concentrations were determined from measured electrical conductivity of sectioned samples in solution. Comparison of salt concentration across the column shows that MgCl₂ appeared to undergo the strongest degree of migration, in a direction toward the top of the soil column. The sulfate salts do not seem to migrate in this way. Chloride salts have higher solubility and lower eutectic compared to sulfate salts, which may explain this increased migration rate. The motion of chloride salt toward the surface is repeatable and not presently understood. Therefore, chloride salts are expected to be stronger drivers of thin film migration and duricrust formation in Mars regolith.