The Effect of Mars‐Relevant Soil Analogs on the Water Uptake of Magnesium Perchlorate and Implications for the Near‐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 Mars-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 salt-mineral mixture behaved similarly, within estimated uncertainties, to pure magnesium perchlorate in all cases. Experiments were performed in both N 2 and CO 2 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 Mars-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 Mars-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).Deliquescence is the transition that occurs when a dry crystalline particle absorbs water vapor and turns into an aqueous droplet, while efflorescence is the opposite phase transition where an aqueous drop loses water PRIMM ET AL.2076 Hogancamp, J. V., et al. (2018). The effect of Mars-relevant soil analogs on the water uptake of magnesium perchlorate and implications for the near-surface of Mars.

Experimental Investigation of the Atmosphere-Regolith Water Cycle on Present-Day Mars

“…Recent research has focused on the interaction of salts in the soils with atmospheric water. There have been numerous studies that suggest that sulphates, perchlorate and chloride salts in the regolith can absorb water from the atmosphere, forming hydrates, by absorption, and then liquid brines, through deliquescence and hydration [10][11][12][13][14][15][16][17][18][19][20][21][22]. Brines are aqueous saline solutions where the presence of salts decreases the freezing temperature of the solution and its saturation water vapor pressure [23], allowing it to capture water from the atmosphere when the relative humidity (RH) is higher than the threshold, which is known as deliquescence relative humidity (DRH) [24].…”

Temperature and humidity monitoring to identify ideal periods for liquefaction on Earth and Mars – data from the High Andes

“…Un der the con di tions mea sured by the REMS in stru ment at Gale Crater on Mars, some chlo rine-con tain ing salts can hy drate and oth ers de hy drate on daily cy cles based on lab o ra tory tests (Gough et al, 2019), how ever, at mo spher i cally based hydration of salts may be very slow. Aque ous so lu tions from the in ter ac tion of var i ous per chlor ates are ex pected to emerge around the Phoenix land ing site but not prob a ble at the Cu ri os ity land ing site of Gale crater (Primm et al, 2018); how ever, del i ques cence is expected there (Rivera Valentín et al, 2018). Ana lys ing the Phoenix land ing site, RH was usu ally <5% in the day and >95% at night-time, the tran si tion be tween the two val ues be ing rapid (Zent et al, 2016).…”

Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry‐Flow Trigger Mechanism for Recurring Slope Lineae on Mars

“…Chlorine-containing salts may be enhanced at RSL locations (Ojha et al, 2015), although this has been questioned recently (Leask et al, 2018;Vincendon et al, 2019). It has been suggested that salts in the Martian soil may uptake water and then lose water during diurnal or seasonal cycles (Fischer et al, 2014;Gough et al, 2011Gough et al, , 2019Heinz et al, 2016;Nuding et al, 2014;Primm et al, 2018). This exchange of atmospheric water by salts may be due to deliquescence and efflorescence (solid-aqueous transitions) or hydration and dehydration (solid-solid transitions).…”