Preferential Formation and Efficient Preservation of Perchlorate With Iron Salts on the Martian Surface

Abstract

Chlorine-bearing salts mixed with other minerals exposed to ultraviolet light participate in chlorine redox cycles on the Martian surface. Previous studies have shown that Fe^III sulfates can exclusively produce perchlorate by chloride photooxidation, but the mechanisms and effective scopes remain unclear. In this study, we investigated this perspective by conducting two main photochemical experiments using ultraviolet light 254 nm. Chloride oxidation experiments examined the effects of different Fe minerals (i.e., Fe^II sulfates, Fe^III sulfates, Fe^III chlorides, Fe^III nitrates, pyrrhotite, siderite and nontronite) and acidified non-Fe sulfates (Ca-, Mg-, Na-, and K- sulfates). Photocatalytic conversion experiments assessed the conversion products of perchlorate and chlorate in the presence of different sulfates (Fe^III, Ca, Mg, Na, and K). Our results showed that the ClO₃⁻/ClO₄⁻ molar ratios <<1 reported for Fe^III sulfates did not occur in any non-Fe sulfates, even after acidification by concentrated H₂SO₄. Other Fe salts, such as Fe^II sulfates, Fe^III nitrates, and Fe^III chlorides, also showed preferential ClO₄⁻ production, whereas pyrrhotite, siderite and nontronite produced more ClO₃⁻ than ClO₄⁻. Photocatalytic conversion experiments starting with NaClO₃ and NaClO₄ demonstrated that Fe^III can facilitate the direct NaClO₃-to-NaClO₄ conversion without producing Cl⁻ and inhibit the photolysis of NaClO₄. Our study highlights the unique role of hygroscopic Fe salts (both Fe^II and Fe^III) in the production and preservation of perchlorate. Mineral surfaces and water vapor may play essential roles in the chlorine redox cycle. The likely coexistence of perchlorate and Fe^III salts has important implications for liquid water on the present cold and arid Mars.