The Effect of Marsâ Relevant Soil Analogs on the Water Uptake of Magnesium Perchlorate and Implications for the Nearâ Surface of Mars

Abstract

The water uptake and release by perchlorate salts have been well studied since the first in situ identification of such salts in the Martian soil by the Phoenix mission in 2008. However, there have been few studies on the effect of the insoluble regolith minerals on the interaction of perchlorate with water vapor. In this work, we investigate the impact of a Marsâ relevant mineral, montmorillonite, and a Mars soil analog, Mojave Mars Simulant (MMS), on the deliquescence (transition from dry crystalline to aqueous via water vapor absorption), ice formation, and efflorescence (transition from aqueous to dry crystalline via loss of water) of pure magnesium perchlorate. We studied mixtures of magnesium perchlorate hexahydrate with either montmorillonite or MMS. Although montmorillonite and MMS are materials that may serve as nuclei for either ice nucleation or salt efflorescence, we find that these soil analogs did not affect the phase transitions of magnesium perchlorate. The saltâ mineral mixture behaved similarly, within estimated uncertainties, to pure magnesium perchlorate in all cases. Experiments were performed in both N2 and CO2 atmospheres, with no detectable difference. We use data from the Mars Science Laboratory Rover Environmental Monitoring Station instrument and the Phoenix Thermal and Electrical Conductivity Probe, as well as modeling of the shallow subsurface, to determine the likelihood of these perchlorate phase transitions occurring at Gale Crater and the northern arctic plains (Vastitas Borealis). We find that aqueous solutions are predicted in the shallow subsurface of the Phoenix landing site, but not predicted at Gale Crater.Plain Language SummaryMost previous studies on Marsâ relevant salts have looked at the water uptake and release of the pure salts, but few have looked at the effect that insoluble minerals might have on the water uptake and release. This is an important potential effect because the surface of Mars is mainly composed of (~99%) mineral dust and we might not be accurately predicting if liquid solutions are possible on Mars today. However, this study shows that a Marsâ relevant mineral (montmorillonite) and a Mars surface analog (Mojave Mars Simulant) did not have a significant effect on the water uptake of magnesium perchlorate. In addition, the Phoenix landing site is more favorable to support liquid solutions of magnesium perchlorate, rather than Gale Crater (Curiosity’s current site).Key PointsThis paper discusses the water uptake and release of Martian salts, mixed with regolith analogsThe DRH, ERH, and ice RH of magnesium perchlorate were not affected by Marsâ relevant regolith analogsBrines are predicted in the subsurface at PHX site, but not at Gale Crater