Formation of manganese oxides on early Mars due to active halogen cycling

• Published in: Nature geoscience 2023-02, Vol.16 (2), p.133-139
• Main Authors: Mitra, Kaushik, Moreland, Eleanor L., Ledingham, Greg J., Catalano, Jeffrey G.
• Format: Article
• Language: English
• Subjects: 140/146; 704/445/209; 704/445/330; 704/445/845; Atmospheric oxygen; Chlorate; Earth and Environmental Science; Earth Sciences; Earth System Sciences; Experiments; Fluids; Geochemistry; Geology; Geophysics/Geodesy; Halogens; Kinetics; Manganese; Manganese oxides; Mars; Mars surface; Oxidants; Oxidation; Oxide minerals; Oxides; Oxidizing agents; Oxygen; Oxygenation; Reaction kinetics; Reactive oxygen species
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/s41561-022-01094-y

In situ rover investigations on Mars have discovered manganese oxides as fracture-filling materials at Gale and Endeavour craters. Previous studies interpreted these minerals as indicators of atmospheric oxygen on early Mars. By contrast, we propose that the oxidation of manganese by oxygen is highly unlikely because of exceedingly slow reaction kinetics under Mars-like conditions and therefore requires more reactive oxidants. Here we conduct kinetic experiments to determine the reactivity of the oxyhalogen species chlorate and bromate for oxidizing dissolved Mn( ii ) in Mars-like fluids. We find that oxyhalogen species, which are widespread on the surface of Mars, induce substantially greater manganese oxidation rates than O 2 . From comparisons of the potential oxidation rates of all available oxidants (including reactive oxygen species peroxide and superoxide), we suggest that the oxyhalogen species are the most plausible manganese oxidants on Mars. In addition, our experiments precipitated the manganese oxide mineral nsutite, which is spectrally similar to the dark manganese accumulations reported on Mars. Our results provide a feasible pathway to form manganese oxides under expected geochemical conditions on early Mars and suggest that these phases may record an active halogen cycle rather than substantial atmospheric oxygenation. Manganese oxidation experiments in Mars-like fluids suggest that chlorate and bromate may have been more effective oxidants of manganese on early Mars than atmospheric oxygen and explain observed manganese oxide deposits.