Rates and Products of Iron Oxidation by Chlorate at Low Temperatures (0 to 25 °C) and Implications for Mars Geochemistry

The mean annual surface temperatures on present and early Mars likely remain well below 0 °C with episodic warming to temperatures as high as 20 °C. Temperature exerts a strong control on geochemical processes but fundamental phenomena like iron oxidation and the formation of iron oxides have not been widely studied below 20 to 25 °C. Earlier studies have demonstrated the effectiveness of chlorate in oxidizing dissolved Fe(II) to form Fe(III)-bearing minerals commonly found on Mars. Here, we determined the rate of oxidation of dissolved ferrous iron by chlorate and the resulting formation of mineral precipitates as a function of temperature in Mars-relevant fluids. The results demonstrate that chlorate oxidizes dissolved Fe(II) at temperatures as low as 0 °C on the time scale of weeks to months. Mixtures of different Fe(III) phases, including goethite, lepidocrocite, schwertmannite, and akaganeite, were produced and the minerals formed were found to be primarily dependent on the fluid type, pH, concentration, and temperature. Chloride-rich solutions favored the formation of akaganeite at 4 °C while 24 °C solutions favored lepidocrocite. In both chloride- and sulfate-rich solutions, lepidocrocite formation was favored at 4 °C whereas 24 °C fluids preferably produced goethite. Schwertmannite formed in sulfate-rich solutions that started at low pH. An established kinetic rate law model parametrized at higher temperatures was found to be accurate in determining the rate of Fe(II) oxidation at temperatures as low as 0 °C. Rate comparisons of Fe(II) oxidation showed that in an ∼1 mmol L^-1 chlorate solution at 0 °C the rate is about three times faster than under 1 bar oxygen at 25 °C, demonstrating the effectiveness of chlorate as an important Fe(II) oxidant on Mars. The impact of temperature in determining the Fe(III) oxidation products suggests that iron mineralogy in cryogenic systems on Mars may be distinct from the phases forming in more clement terrestrial analog environments.