Multiphase volatilization of halogens at the soil-atmosphere interface on Mars

The Martian critical zone, especially its soil-atmosphere interface, may host an active halogen cycle affecting habitability, as inferred from halogen trends observed in situ at Gusev and Meridiani, along with oxychlorine species detected in situ. However, the sinks and sources of the Martian halogen cycle remain poorly constrained, especially for Br, including the broader geographic significance of halogen volatility in soils. Here, we statistically analyze Br, Cl, and S distribution and associated cations in the soils of Gale Crater and compare the results with soils of Gusev Crater and Meridiani Planum. We further place the overall in situ soil trends in the framework of terrestrial natural evaporative halides and Mars-analog evaporative experiments. We observe three lines of evidence that support preferential volatilization of Br versus Cl and S in Martian soils: (a) Unlike Cl and S, Br abundance decreases from subsurface to surface. (b) Br and Cl abundances correlate strongly at the relatively dust-free and freshly exposed surface of the active Bagnold dunes, but Br-Cl correlations weaken and Br concentrations decrease in less mobile surface soil. (c) Laboratory experiments show preferential Br loss over Cl and S under ultraviolet (UV) irradiation, consistent with the observed Br loss relative to Cl in Martian soil samples. Overall, these findings generally suggest that soil serves as a source for volatilized halogens. Specifically, soil-atmosphere multiphase reaction pathways constitute a crucial component missing from purely gas-phase photochemical models of the halogen cycle.