Active Bromine Cycling Enables Bromate as an Oxidant of Mn(II) on the Low-Temperature Martian Surface

Oxidation processes in cold and freezing environments remain poorly constrained, despite their importance for redox evolution in both terrestrial cryospheric systems and planetary surfaces. Mars provides a natural laboratory: sedimentary Mn oxides indicate sustained surface oxidation, although early Mars likely remained frozen, limiting the effectiveness of atmospheric O₂ oxidation. Here we demonstrate that bromate (BrO₃^-) rapidly oxidizes Mn(II) in acidic brines under freezing conditions. In laboratory simulations from -80 to 25 °C, Mn(II) oxidation proceeds readily despite ice formation. Bromate is not quantitatively reduced to Br^-; instead, a substantial fraction is inferred to be converted to reactive and volatile bromine intermediates (e.g., BrO, BrO^-, Br₂), based on the observed bromine mass balance and comparison with previously reported bromate-involved redox systems, enabling potential atmospheric release and redistribution. These species can be efficiently reoxidized to BrO₃^- via photochemical and atmospheric processes, suggesting the operation of an active bromine redox cycle. Our results identify bromate-driven oxidation as an efficient, oxygen-independent redox pathway operating under freezing conditions, capable of maintaining long-term oxidizing environments. This mechanism provides a plausible explanation for sustained oxidation on Mars throughout its climatic evolution and highlights the broader relevance of halogen-mediated redox cycling in frozen aquatic systems. These findings advance our understanding of low-temperature environmental oxidation processes and their implications for planetary surface chemistry and potential habitability.