Preferential Formation of Chlorate over Perchlorate on Mars Controlled by Iron Mineralogy

Perchlorate (ClO₄⁻) and possibly chlorate (ClO₃⁻) are considered to be ubiquitous on Mars^1–5, and the ClO₃⁻/ClO₄⁻ abundance ratio has critical implications for the redox conditions^6,7, aqueous environments^8,9 and habitability on Mars¹⁰. However, factors that control the ClO₃⁻/ClO₄⁻ generation ratios are not well established. Here we expose mixtures of halite salt (NaCl) with Fe sulfates, Fe (hydr)oxides and Fe³⁺ montmorillonite to ultraviolet radiation or ozone in an Earth or CO₂ atmosphere and show that Fe secondary mineralogy is the dominant factor controlling the ClO₃⁻/ClO₄⁻ generation ratio: the sulfates and montmorillonite mixtures produce much higher yields of ClO₄⁻ than of ClO₃⁻, whereas the opposite is true for the (hydr)oxide mixtures. Consistent with previous studies^11–18, our results indicate that the physical state of chloride (Cl⁻) (that is, solid, liquid or gas) and the characteristics of the co-occurring minerals (for example, semiconductivity, surface area, acidity) have the greatest influence, whereas oxidation sources (ultraviolet radiation or ozone) and atmospheric composition induce only secondary effects. We conclude that, under the hyperarid climate and widespread Fe (hydr)oxide abundances prevailing on Mars since the Amazonian period¹⁹, Cl⁻ oxidation should produce yields of ClO₃⁻ that are orders of magnitude higher than those of ClO₄⁻, highlighting the importance of ClO₃⁻ in the surficial environments and habitability of modern Mars compared with ClO₄⁻.