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

Abstract

The water content of the upper layers of the surface of Mars is not yet quantified. Laboratory simulations are the only feasible way to investigate this in a controlled way on Earth, and then compare it with remote and in situ observations of spacecrafts on Mars. Describing the processes that may induce changes in the water content of the surface is critical to determine the present-day habitability of the Martian surface, to understand the atmospheric water cycle, and to estimate the efficiency of future water extraction procedures from the regolith for In Situ Resource Utilization (ISRU). This paper illustrates the application of the SpaceQ facility to simulate the near-surface water cycle under Martian conditions. Rover Environmental Monitoring Station (REMS) observations at Gale crater show a non-equilibrium situation in the atmospheric H2O volume mixing ratio (VMR) at night-time, and there is a decrease in the atmospheric water content by up to 15 g/m2 within a few hours. This reduction suggests that the ground may act at night as a cold sink scavenging atmospheric water. Here, we use an experimental approach to investigate the thermodynamic and kinetics of water exchange between the atmosphere, a non-porous surface (LN2-chilled metal), various salts, Martian regolith simulant, and mixtures of salts and simulant within an environment which is close to saturation. We have conducted three experiments: the stability of pure liquid water around the vicinity of the triple point is studied in experiment 1, as well as observing the interchange of water between the atmosphere and the salts when the surface is saturated; in experiment 2, the salts were mixed with Mojave Martian Simulant (MMS) to observe changes in the texture of the regolith caused by the interaction with hydrates and liquid brines, and to quantify the potential of the Martian regolith to absorb and retain water; and experiment 3 investigates the evaporation of pure liquid water away from the triple point temperature when both the air and ground are at the same temperature and the relative humidity is near saturation. We show experimentally that frost can form spontaneously on a surface when saturation is reached and that, when the temperature is above 273.15 K (0 °C), this frost can transform into liquid water, which can persist for up to 3.5 to 4.5 h at Martian surface conditions. For comparison, we study the behavior of certain deliquescent salts that exist on the Martian surface, which can increase their mass between 32% and 85% by absorption of atmospheric water within a few hours. A mixture of these salts in a 10% concentration with simulant produces an aggregated granular structure with a water gain of approximately 18- to 50-wt%. Up to 53% of the atmospheric water was captured by the simulated ground, as pure liquid water, hydrate, or brine.

Present-day thermal and water activity environment of the Mars Sample Return collection

“…Previous analysis of the vertical profile at arctic Martian regions suggests that during spring and summer, a large percentage of the water column (> 25% and up to nearly 100%) was confined below ~ 2.5 km 47 . These results are comparable to those provided by the REMS instrument package on the Curiosity rover at Gale crater 24 and are consistent with previous research based on orbital and in-situ observations and modelling 44 , 48 – 54 . We conclude that similarly to what happens on other sites on Mars 55 , there is a strong rock and regolith-atmosphere exchange mechanism on Mars 56 , likely owing to the combination of adsorption–desorption of water on the regolith grain surfaces and to hydration-dehydration of salts.…”

“…The plausible existence of brines or salt hydrates on the surface or subsurface has several implications for Mars's past and current habitability. Experiments in simulation chambers have shown that for certain temperature and a w conditions, Mg, Ca, and Na perchlorates and sulfates can hydrate or deliquesce, forming stable liquid brines under present-day Martian conditions [22][23][24] . The Planetary Instrument for X-Ray Lithochemistry (PIXL) and the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instruments have investigated the abrasion patches and found hygroscopic and deliquescent salts such as Mg, Fe (hydrated) and Ca sulfates (anhydrite mostly), chlorides and perchlorates (Initial Reports-PDS; [25][26][27][28] ).…”

An Examination of Soil Crusts on the Floor of Jezero Crater, Mars

“…Water may also be absorbed to grain boundaries. Recent experiments with regolith simulant have demonstrated that a mixture containing 10 wt% ferric sulfate can absorb as much as 18% of its weight in water, and a mixture containing 10 wt% calcium chloride can absorb as much as 50% of its weight in water (Ramachandran et al., 2021). In these experiments, this water absorption changes the cohesiveness of fine regolith particles, forming granular structures.…”

Active Ground Patterns Near Mars' Equator in the Glen Torridon Region of Gale Crater

“…We suggest that the active ground patterns have broad implications for diverse geomorphic processes on Mars. Our work adds to a set of studies of a present‐day active atmosphere‐regolith water cycle (e.g., Bishop et al., 2021; Chevrier & Rivera‐Valentin, 2014; Martín‐Torres et al., 2015; Schmidt et al., 2017; Vakkada Ramachandran et al., 2021; Wang et al., 2019). Collectively, this research suggests that the surface of Mars is currently being shaped by environmentally driven volumetric changes of the near‐surface bedrock, as well as eolian, impact, insolation, and gravitational processes.…”