Xerophiles on Mars: Possible Evolutionary Strategies Using Hydrogen Peroxide and Perchlorates

Introduction: The Martian surface environment has been characterized as dry, cold and harsh with respect to various forms of radiation. Also, the thin Martian atmosphere does only contain trace amounts of oxygen. If organisms are present, these could be characterized as xerophiles, psychrophiles and anaerobes. We previously suggested the presence of microbial organisms on Mars that use a mixture of hydrogen peroxide and water as an intracellular solvent as an adaptation to the harsh Martian surface environment [1]. These putative organisms would provide a consistent explanation for the observations by the Viking Biology Experiments. In contrast, it has often been concluded from the results of the Viking Biology Experiments that the Martian surface regolith is in a highly oxidized state, based on the evolution of molecular oxygen upon moisturizing soil samples in the Gas Exchange (GEx) experiment [2, 3]. Perchlorate: The finding of surprisingly large amounts (0.4-0.6 wt%) of perchlorate salts in the Martian arctic soil by the Phoenix lander [4] sheds a new light on these interpretations. First, although perchlorates are known as powerful oxidants, there is no perchlorate known to release oxygen upon moisturizing at the Viking experimental temperature of about 10 deg. C. H2O2-H2O hypothesis: The H2O2-H2O hypothesis implies that the addition of water vapor at a relatively high temperature could only be withstood by the organisms for a short time, as they perished due to hyperhydration [1]. Thus, the evolution of oxygen in the GEx experiment is explained by the high oxidative content of the organisms. The lack of detected organics by the Viking GCMS could be the result of autooxidation of the organisms as these were gradually heated. Especially heating together with perchlorate salts would result in combustion to CO2 and H2O of any organics present. Antifreeze: An interesting property perchlorate salts share with hydrogen peroxide is their effectiveness as anti-freeze. Whereas the eutectic for H2O-H2O2 freezes at -56 deg C, the water-magnesium perchlorate eutectic is as low as -70 deg C. Therefore, as Mg and Na are prevalent cations in the Martian soil, their perchlorate salts might well be employed as an antifreeze by hypothetical Martian microbes. The reason we still think that H2O2 is a more likely choice is the rapid evolution of O2 in the GEx experiment, at variance with the stability of perchlorate under its experimental conditions, and also because of the fact that H2O2 can be produced from only atmospheric constituents.