Photochemistry of Fe (II) and carbonate-bearing waters and the influence on Greenhouse Gas production in Early Mars

Aqueous iron UV photooxidation, a proposed pathway for early Martian iron deposits produced in surface environments, produces hydrogen gas, a potentially important greenhouse gas for early Mars. This experimental study focused on iron photooxidation in the presence of dissolved inorganic carbon (DIC) at a range of concentrations that represent partial pressures of CO₂ (0.1–1 bar), consistent with estimated atmospheric pressures and compositions on early Mars. We report in-situ sampling of gas headspace to understand the extent to which DIC impacts the production of hydrogen gas that may be produced from aqueous iron photooxidation. The experiments demonstrate that ferrous iron photooxidation does not occur at any of the tested DIC concentrations (3.4–35 mM). Ferrous iron carbonate minerals were precipitated at pH conditions above 6, while maintaining relatively high dissolved iron concentrations at siderite saturation. We observed hydrogen and methane gas production at varying initial conditions, but no clear relationships to either iron loss to oxidation and/or precipitation or DIC concentrations could be inferred. Instead, we suggest that hydrogen is a byproduct of methane photolysis. The methane itself was sourced from an unknown background organic carbon compound from residual organic impurities in the deionized water source utilized in the experiments and is unrelated to iron photochemistry. These observations indicate that iron photooxidation is an unlikely mechanism to have produced hydrogen and precipitated iron oxide minerals at near-neutral pH in the presence of 0.1–1 bar CO₂ on early Mars.