Chlorine on the Surface, Chlorine in the Air, What Is the New Global View of the Martian Chlorine Cycle?

Abstract Atmospheric hydrogen chloride (HCl, or hydrochloric acid) has strong links to volcanic activity on Earth. If it is present in the atmosphere of Mars, it could hint at active magmatic processes, or the outgassing from the remnants of recently dormant volcanoes. It has been sought in the Martian atmosphere using terrestrial telescopes and was a target for the ExoMars Trace Gas Orbiter (TGO) mission. Since it was found by TGO, the terrestrial telescopes have returned to their hunt, and the recent study by Faggi et al. (2025), https://doi.org/10.1029/2025je009105 presents the results of a multi‐year campaign to study the global distribution of HCl across the Martian surface. In this commentary, we will examine the importance of HCl, its context in the broader chloride cycle on Mars, how we have gotten to this point, and the implications the new study has on our understanding of its origins.