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 O₂. 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.

在火星上进行的原位漫游车调查已经在 Gale 和 Endeavor 陨石坑发现锰氧化物作为裂缝填充材料。先前的研究将这些矿物质解释为早期火星大气中氧气的指标。相比之下，我们认为锰被氧气氧化的可能性很小，因为在类似火星的条件下反应动力学极其缓慢，因此需要更多的活性氧化剂。在这里，我们进行了动力学实验，以确定氧卤素物种氯酸盐和溴酸盐对氧化火星类流体中溶解的 Mn( ii ) 的反应性。我们发现，广泛存在于火星表面的卤氧物质引起的锰氧化率明显高于 O ₂. 通过比较所有可用氧化剂（包括活性氧过氧化物和超氧化物）的潜在氧化速率，我们认为卤氧物质是火星上最可能的锰氧化剂。此外，我们的实验沉淀了锰氧化物矿物 nsutite，它在光谱上与火星上报道的暗锰积累相似。我们的研究结果提供了在火星早期预期的地球化学条件下形成锰氧化物的可行途径，并表明这些阶段可能记录了活跃的卤素循环而不是大量的大气氧化作用。