Abstract EANA2024-52

The Martian surface and shallow subsurface lack stable liquid water. However, hygroscopic salts in the regolith, such as perchlorates and chlorates, can facilitate the transient formation of liquid brines. Perchlorate salts have been detected on Mars 1, and chlorate salts are also likely present, as evidenced by the detection of chlorate in the Martian meteorite EETA790012 2. Recent research suggests that Mars' hyperarid climate and the abundant presence of iron (hydro)oxide, chloride oxidation should yield significantly more chlorate than perchlorate 3, highlighting the importance of chlorate salts for the habitability of the Martian environment. Perchlorate and chlorate salts can form liquid brines through deliquescence, where the salts attract atmospheric water and dissolve in it, or through the contact of these salts with water ice. In the shallow subsurface, a thin regolith layer can prevent water ice sublimation, allowing brines to persist longer. Additionally, regolith layers can shield microbes from harmful UV radiation, making the shallow subsurface a potential habitat for microbial life on Mars.

This study investigated the combined effects of perchlorate or chlorate salts, UV radiation, water scarcity, and regolith depth on microbial survival under simulated Mars-like conditions. Mars simulation experiments were conducted in the Mars Environmental Simulation Chamber (MESCH)4. We exposed vegetative cells of Debaryomyces hansenii and Planococcus halocryophilus, and spores of Aspergillus niger, to simulated Martian conditions (-11°C, 6 mbar pressure, CO2 atmosphere, and 2 hours of daily UV radiation). Colony Forming Units (CFU) and water content were evaluated at three regolith depths (0-0.5 cm, 1-3 cm, 10-12 cm) after 3- and 7-day exposure periods. Each organism was tested under three conditions, where Mars regolith simulant was inoculated with cell suspensions of the three model organisms containing either: 1) 0.5 mol/kg NaClO3, 2) 0.5 mol/kg NaClO4, or 3) no additional salt.

The results showed a with regolith depth increasing residual water content in all three exposure experiments and under all tested conditions. Remarkably the survival rates of the organisms also increased with regolith depth in the NaClO3 and salt-free samples. However, survival rates in the NaClO4 samples were consistently lower across all depths, with the most significant difference observed at 10-12 cm, the depth with the highest residual water content. This discrepancy is likely due to the emergence of enhanced salt concentrations in the NaClO3 and NaClO4 samples resulting from the freezing of water retained in the regolith. This likely mirrors changes in brine concentrations in the Martian shallow subsurface. The higher survival rates in chlorate samples suggest that, for these organisms, perchlorate brines are more toxic than chlorate brines under the experimental conditions.

These findings, along with the potential widespread presence of chlorate salts on Mars, underscore the necessity for further research on this oxychlorine species. Environments enriched with chlorate salts should be more habitable and be considered in the search for microbial life on Mars, given that most existing research has focused on the more toxic perchlorate salts.

References: 

1.         Hecht, M. H. et al. Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site. Science 325, 64–67 (2009).

2.         Kounaves, S. P., Carrier, B. L., O’Neil, G. D., Stroble, S. T. & Claire, M. W. Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: Implications for oxidants and organics. Icarus 229, 206–213 (2014).

3.         Qu, S.-Y. et al. Preferential Formation of Chlorate over Perchlorate on Mars Controlled by Iron Mineralogy. Nat. Astron. 6, 436–441 (2022).

4.         Jensen, L. L. et al. A Facility for Long-Term Mars Simulation Experiments: The Mars Environmental Simulation Chamber (MESCH). Astrobiology 8, 537–548 (2008).