Experimental constraints on the formation of oxychlorine species by UV irradiation and mechanical pulverization on the lunar surface

Perchlorate and chlorate are present in various extraterrestrial celestial bodies throughout the solar system, such as Mars, the moon, and asteroids. To date, the origin mechanisms of perchlorate and chlorate on the Martian surface have been well-established; however, relatively little attention has been cast to airless bodies. Here, we experimentally investigated the potential oxidation mechanisms of chloride to chlorate and perchlorate, such as ultraviolet irradiation under H 2 O- and O 2 -free conditions and mechanical pulverization processes. Individual minerals, olivine, pyroxene, ilmenite, magnetite, TiO 2 and anhydrous ferric sulfate, and lunar regolith simulants (low Ti, CLRS-1; high-Ti, CLRS-2) and their metallic iron (Fe 0 ) bearing counterparts were examined. We found that pulverization of dry matrix material-halite mixtures, even in the presence of O 2 , does not necessarily lead to perchlorate and chlorate formation without involving water. Under photocatalytic and H 2 O- and O 2 -free conditions, olivine and pyroxene can produce oxychlorine (ClO x − ) species, although the yields were orders of magnitude lower than those under Martian-relevant conditions. Nanophase-Fe 0 particles in the lunar regolith and the common photocatalyst TiO 2 can facilitate the ClO x − formation, but their yields were lower than those with olivine. The oxides ilmenite and magnetite did not efficiently contribute to ClO x − production. Our results highlight the critical role of H 2 O in the oxidation chloride to chlorate and perchlorate, and provide essential insights into the environmental influence on the formation of oxychlorine species on different celestial bodies.