The Role of Halogens During Fluid and Magmatic Processes on Mars

Abstract

The geochemistry of halogens on Mars gives insight into the composition of the martian mantle, igneous evolution of the martian crust, aqueous processes on the martian surface, and the overall habitability of the planet. Halogen abundances have been measured from martian meteorites, in situ by landers and rovers, and from orbital missions around Mars. The bulk rock abundances of halogens have been determined for many martian meteorite samples including all petrological five types (nakhlites, chassignites, shergottites, orthopyroxenites, and regolith breccias). Measurements of basaltic martian meteorites (i.e., shergottites and regolith breccias) provide important insights into halogen abundances in mantle and crustal reservoirs. Fluorine, Cl, Br, and I have been detected in silicate, phosphate, sulfate, oxide, and halide group minerals in martian meteorites. These halogen-bearing minerals are found in melt inclusions, as secondary hydrothermal or aqueous alteration products, or in the interstices between cumulus igneous silicates. Measurements from meteorites and from martian missions indicate that Cl is the most abundant halogen on and in Mars. Measurements from landed missions suggest that Cl is commonly present in oxychlorine compounds (e.g., perchlorate and chlorate salts), whereas measurements from orbit have identified both oxychlorine minerals and halite. Halite is constrained to local depressions in the ancient southern highlands, suggesting precipitation from the evaporation of water in closed basins at ~3.5–4 Ga. The presence of oxychlorine minerals on the martian surface has important implications for the habitability of present day Mars because oxychlorine minerals may deliquesce to create seasonal deposits of liquid water. Furthermore, oxychlorine compounds are considered both a resource and potential hazard to the eventual human exploration of Mars.