Hydrogen Generation From Ferrous Saponite in Reaction With H₂S‐Containing Fluid: Relevance to Early Martian Habitability

Abstract Molecular hydrogen is an important gas species for understanding the early Martian climate and redox chemistry. Through ancient aqueous alterations of crustal rocks, ferrous (Fe(II)) saponite formed abundantly on Mars. Subsequent intrusions of hydrothermal fluids may have resulted in a chemical reaction between the dissolved volatiles and the nearby rocks. Here we propose a new H 2 generating reaction between ferrous saponite and H 2 S‐containing fluids, which is possible on early Mars. A series of hydrothermal experiments at a relatively low temperature of 90°C were performed under anoxic conditions using synthesized ferrous saponite to compare the resulting H 2 concentration among various gas and fluid compositions. Based on the relationship with the existence of H 2 S, reaction time, fluid pH, dissolved iron concentration, and amount of minerals, we found that high levels of H 2 (∼0.1 mmol/g ferrous saponite) were generated in the presence of H 2 S most rapidly in moderate pH conditions. Our microscopic chemical analysis of mineral phases suggested that ferrous saponite served as both the iron source of pyrite precipitation and the electron source to form H 2 . Our results suggest that intrusions of H 2 S‐containing fluids into the saponite‐containing crust of Mars would generate H 2 , which could potentially provide locally concentrated chemical energy for chemoautotrophic life.