Chlorate as a Potential Oxidant on Mars: Rates and Products of Dissolved Fe(II) Oxidation

Oxychlorine species are globally widespread across the Martian surface. Despite their ubiquitous presence, the ability of oxychlorine species to serve as oxidants on Mars has largely been unexplored. While perchlorate is kinetically inert, chlorate may be a critical Fe(II) oxidant on Mars. However, the timescale over which chlorate may oxidize Fe(II) and the mineral products formed in Mars-relevant fluids are unclear. Fe(II) oxidation by chlorate was thus investigated in magnesium chloride, sulfate, and perchlorate fluids under neutral to acidic conditions for different total Fe(II) and background salt concentrations. The results show near-complete Fe(II) oxidation within approximately 2 to 4 weeks, accompanied by formation of the Fe(III) minerals goethite, lepidocrocite, akaganeite, and jarosite. The Fe(II) oxidation rate and the mineral products depend on Fe(II) concentration, the composition and concentration of the background salt, and the acidity of the solution. Calibration of an existing rate law to lower temperatures well reproduces the observed oxidation kinetics in all fluid compositions and allows prediction of the rate of Fe(II) oxidation by chlorate under diverse Mars-relevant conditions. Rate comparisons demonstrates that chlorate can oxidize Fe(II) substantially faster than O₂ and on similar or shorter timescales than ultraviolet light. Notably, chlorate causes rapid oxidation under acidic conditions, unlike other oxidants. Chlorate may thus represent an important abiotic Fe(II) oxidant on Mars. The expected coassociation of chlorate with perchlorate may allow for its percolation into the subsurface during brine migration, leading to oxidation in regions that are cutoff from ultraviolet radiation and atmospherically derived oxidants.