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.