Experimental Constraints on Near-Surface Water Vapour Exchange on Mars

The Martian water cycle, particularly the exchange of water vapour between the atmosphere and shallow regolith, remains poorly constrained despite its importance for present-day climate, habitability, and future human exploration. On every sol, a Martian day, the near-surface regolith can act as a temporary sink and source of atmospheric water. Water vapour is adsorbed into the regoltih as temperatures drop after sunset, and released again as the surface warms after sunrise. This process has long been recognized as a key part of the near-surface water cycle, from early adsorption studies to later analyses using lander data, single-column models, and global climate models (e.g., Fanale and Cannon, 1971, 1974; Zent et al., 1993; Jakosky et al., 1997; Böttger et al., 2004; Savijärvi et al., 2016, 2019, 2021, 2024a,b; Steele et al., 2017; Rivera-Valentín et al., 2020). Laboratory studies have also measured water adsorption and desorption in Mars-relevant materials, including palagonite, basaltic analogues, clays, sulfates, and other mineral phases (Zent et al. 1993,1994, Pommerol et al. 2009, Beck et al. 2010, and Ramachandran et al. 2021).

This exchange also has astrobiological relevance. At Jezero crater, present-day environmental conditions may allow nighttime surface water activity to exceed 0.5 when temperatures fall below 190 K, while daytime conditions above 245 K correspond to water activity values below 0.02 (Zorzano et al., 2024). Despite decades of studies there are still gaps in laboratory measurements specifically under coupled Martian pressure and diurnal temperature cycles.

In this study, we present laboratory experiments designed to measure atmosphere–regolith water vapour exchange under simulated Martian near-surface conditions. This work builds on our previous single-column modelling study (Patel et al., 2025), which showed that regolith porosity can have a significant influence on the amount of water exchanged between the atmosphere and surface at Jezero Crater. We test this model prediction directly in the laboratory by exposing regolith simulants with differing porosity to Mars-like diurnal temperature cycles and pressures. The experiments are being carried out using the Dirty Under-vacuum Simulation Testbed for Icy Environments, DUSTIE, at JPL. These measurements will help test model predictions of diurnal exchange, support interpretation of rover humidity measurements, and identify which regolith properties are most important for controlling the water exchange on Mars.

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