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Project Progress

Lithoautotrophic microbial ecosystems have been documented in crustal environments up to 2.8 kilometers below the surface of Earth reportedly sustained by high concentrations of dissolved sulfate and hydrogen, however the origin of these metabolic resources remains enigmatic. Radiolysis of water coupled to oxidation of sulfide minerals in uranium-bearing rocks could produce gradients of hydrogen and partially to fully oxidized sulfur species in suitable proportions for maintenance and growth of microbial organisms. A set of experiments exposing deoxygenated water and pyrite mixtures to gamma radiation were previously conducted to test this hypothesis. Molecular hydrogen was found to be the dominant gaseous species (highest yields with high water to pyrite ratio experiments), with molecular oxygen two orders of magnitude lower than hydrogen. Aqueous sulfate and elemental sulfur (S⁰) were the only sulfur bearing products recovered from these radiolysis experiments and their sulfur isotopic compositions measured. Additional quantitative understanding of reactions resulting from radiation of water with other sulfide minerals is needed to recognize and predict products of radiolysis on Earth and other rocky planets.

A new set of radiolysis experiments will soon be carried out using the mineral pyrrhotite and deoxygenated water in varying ratios. X-ray diffraction was used to characterize a pyrrhotite sample from a massive sulfide ore deposit in Labrador, Canada. The sample is predominantly composed of pyrrhotite (78%) and troilite (18%) with about 4% magnetite (Fig. 1). This pyrrhotite/troilite sample is appropriate as an analog for common sulfide minerals in basalts on Earth and andesitic basalts inferred on Mars. Quartz-glass reaction vessels have been constructed for these experiments to recover gaseous, liquid, and solid products for identification and isotopic measurement. Specifically the vessels have been designed to capture any hydrogen gas product for isotopic measurement. Additionally, determining the ratio of reduced (Fe²⁺) to oxidized (Fe³⁺) iron in solution will be critical for understanding the role of iron species as oxidants in sulfate production and reactants in back reactions that limit the formation of iron oxides. The Fe²⁺/ Fe³⁺ratio will be determined using ion chromatography and a visual wavelength detector.

Through further assessment of reactions resulting from radiolysis, isotopic fingerprints for sulfur, oxygen, and hydrogen can be identified for a fundamental geochemical process that is significantly understudied as a source of energy for life. Radiolysis of water coupled to oxidation of metallic sulfides could be a significant source of reduced and oxidized sulfur and iron species in crustal environments of rocky planetary bodies with levels of radioactivity less than half of the average value on Earth.

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