Mobilization of H2O by humidity-dependent solid-state mineral transformations under Mars-like conditions

It is anticipated that some of the hydrogen detected at near-equatorial latitudes on Mars is bound as H2O or OH within the crystal structures of hydrated minerals (e.g., hydrated sulfate minerals and smectite clay minerals). Under water-limited conditions, these materials represent a potential source of bioavailable water and nutrients for microbial life. On Earth, saline lakes, evaporite deposits, and smectite-rich soils are inhabited by a wide variety of microorganisms. Analogous deposits on Mars may represent viable habitats for possible (extant or extinct) microbial life. However, H2O, metals, and organic molecules trapped within the crystal structure of a mineral would not necessarily be accessible to microorganisms near the martian surface. Consequently, much astrobiological research has discounted the possibility of near surface microbial communities on Mars, focusing instead on potential sources of water for biota in the deeper subsurface of the planet. Using experiments that employ X-ray powder diffraction (XRD) and scanning electron microscopy (SEM), we have determined that solid-state reactions can occur within consortia of hydrous minerals under conditions of varying relative humidity similar to those that operate at or just beneath the martian surface. These reactions mobilize structural H2O and cations. At high relative humidities (>80%), gypsum [CaSO4.2H2O] formed from dry mixtures of Ca-montmorillonite (Clay Minerals Society Source Clay SAz-1) and epsomite [MgSO4.7H2O]. At 90% relative humidity, ~10% of the epsomite in a 50:50 mixture of SAz-1 and epsomite reacted to produce gypsum within hours. Under the same conditions of relative humidity, dry mixtures of Na-montmorillonite (CMS Source Clay SWy-1) and epsomite produced lesser amounts of gypsum from the smaller amount of interlayer Ca in the Na-montmorillonite. No Na or K-sulfate minerals were produced. By adding a known amount of a well-ordered standard material with known unit-cell parameters, the volume increase that accompanies these reactions can be quantified using XRD data. Our data suggest that the behavior and composition of admixtures of smectites and hydrated sulfate minerals change in a detectable way and react in the dry state on the timescale of the martian day. Smectites commonly host organic molecules within their interlayers. Thus, organic molecules may also be released during solid-state reactions between smectites and hydrated sulfate minerals. Furthermore, short and long-term preservation of terrestrial microorganisms within evaporite minerals is well documented. Therefore, the martian water cycle could potentially have sustained microbial communities through the exchange of nutrients and water between hydrous minerals near the planet’s surface. Consequently, regolith containing both smectites and hydrated sulfate minerals could represent a potential habitat for near-surface microbial life on Mars.