The Chemical Reactivity of the Martian Soil and Implications for Future Missions

Possible interpretations of the results of the Viking Biology Experiments suggest that >1 ppm of a thermally labile oxidant, perhaps H2O2, and about 10 ppm of a thermally stable oxidant are present in the martian soil. We reexamine these results and discuss implications for future missions, the search for organics on Mars, and the possible health and engineering effects for human exploration. We conclude that further characterization of the reactivity of the martian regolith materials is warranted—although if our present understanding is correct the oxidant does not pose a hazard to humans. There are difficulties in explaining the reactivity of the Martian soil by oxidants. Most bulk phase compounds that are capable of oxidizing H2O to O2per the Gas Exchange Experiment (GEx) are thermally labile or unstable against reduction by atmospheric CO2. Models invoking trapped O2 or peroxynitrates (NOO-2) require an unlikely geologic history for the Viking Lander 2 site. Most suggested oxidants, including H2O2, are expected to decompose rapidly under martian UV. Nonetheless, we conclude that the best model for the martian soil contains oxidants produced by heterogeneous chemical reactions with a photochemically produced atmospheric oxidant. The GEx results may be due to catalytic decomposition of an unstable oxidizing material by H2O. We show that interfacial reaction sites covering <1% of the available soil surfaces could explain the Viking Biology Experiments results. Due to the low concentration of reactive agents, the fact that they are probably composed of elements that are common in the soil, and their observed instability, it is unlikely that a sample return mission would be effective in identifying them. Methods for in situ characterization of the oxidant must be developed that are consistent with the dry state of the soil. Traditional terrestrial methods of characterizing the soil Eh are unsuitable. Alternative methods must be developed; one approach under development involves exposing a wide variety of materials to the martian regolith and monitoring their reactions.