Manganese Oxide Formation by Oxyhalogens: Faster Alternatives to Oxygen as Mn Oxidants on Mars

High concentrations of manganese were detected at two locations on Mars by in situ rover measurements and are attributed to the presence of Mn(III/IV) oxides formed by Mn(II) oxidation. Oxidized Mn has been cited as unique evidence for the presence of atmospheric O₂ on early Mars, but this argument fails to recognize that there are no plausible oxidants on Mars with redox potentials between those of Fe(II) and Mn(II). The occurrence of iron oxides already indicates that a "high-redox potential" oxidant was present. An additional challenge is that while Mn(II) oxidation by O₂ is thermodynamically feasible, the reaction is kinetically slow except under alkaline conditions. Mn oxidation on Mars may thus indicate the presence of some other strong oxidant(s) instead of O₂. Strong oxidants, such as chlorate (ClO₃^-) and bromate (BrO₃^-), are inferred to be present on Mars and previously shown to oxidize Fe(II) faster than O₂. Here we investigate the ability of BrO₃^- and ClO₃^- to oxidize Mn(II) via laboratory experiments under anoxic conditions in Mars-relevant fluids and also characterize the mineral products that result. Dissolved Mn(II) in magnesium chloride or sulfate fluids at near-neutral to acidic pH was aged with equimolar dissolved sodium chlorate and/or sodium bromate. The Mn(II) concentration was then monitored over time. All experimental solutions containing BrO₃^- displayed Mn(II) oxidation and a drop in pH within ~30 days, producing primarily nsutite (γ-MnO₂). Solids formed from chloride fluids contain only Mn(IV) while sulfate fluids produced a mixed Mn(III/IV) phase. ClO₃^- did not produce observable Mn(II) oxidation on a similar timescale. These studies show that BrO₃^- is an effective Mn(II) oxidant in Mars-relevant fluids in near-neutral to acidic solutions. The rate of Mn(II) oxidation by BrO₃^- is likely proportional to BrO₃^-, Mn(II), and H⁺ concentrations. The presence of BrO₃^- as a major Br species on Mars and its ability to oxidize Mn(II) in diverse fluids suggest that BrO₃^- may be a key oxidant responsible for the observed Mn oxides. Importantly, it is a faster oxidant than O₂, especially under acidic conditions. Mn oxides detected on Mars thus do not provide unequivocal evidence of past O₂ in the Martian atmosphere-hydrosphere as viable pathways exist using other plausible oxidants.