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. Plain Language Summary Soil records the complexity of processes at the Martian surface and is a key component of the critical zone that sustains interactions between the atmosphere and crust of Mars. Previous in situ explorations of the Mars landing site at Gusev Crater and Meridiani Planum showed that Br demonstrates a preferential loss into the atmosphere among volatile elements Br, Cl, and S in the topsoil samples. Here, we examine and demonstrate that Br volatilization observed at Gusev and Meridiani is also present in Gale Crater soils, indicating ubiquitous volatilization via soil-atmosphere processes on Mars. We further place the in situ soil trends of Martian Br and Cl in the framework of natural evaporative halides on Earth and Mars-analog evaporative experiments. The framework we construct shows that the Br and Cl variations in Mars soils are consistent with halide salts mixed with halogen -low or -free components (e.g., basaltic phases) and subsequent volatilization of Br over Cl. Our findings indicate a chemically active top layer on the Martian surface, challenging the preservation of any biological matter even in deeper soil horizons.