Following the HCl Cycle Over Three Martian Seasons in Mars Year 36

Abstract Hydrogen chloride (HCl) was recently discovered in the Mars atmosphere using the ESA's ExoMars Trace Gas Orbiter (TGO) onboard ESA's ExoMars mission. Its discovery is the first confirmation of an active presence of any chlorine‐bearing species in the modern Mars atmosphere. TGO permitted investigations of HCl altitude profiles with high precision and showed that water vapor and ice clouds play an important role in the production and temporary loss of HCl. TGO cannot always sample the Martian atmosphere near the surface, and when those measurements are possible, they are highly affected by the increase in dust opacity, nor can TGO observe at equatorial latitudes with high cadence, due to orbital constraints, so its measurements are not suitable to obtain instantaneous global coverage. In this work, we present a methodic investigation of the Martian atmosphere, in support of the ExoMars TGO mission, targeting HCl and water using iSHELL at NASA/InfraRed Telescope Facility. Our observations mapped the Martian atmosphere, exploring three seasons in Martian Year 36. We observed the beginning of an increase in the HCl abundances around L S = 249°–301°, followed by a drop in the abundances around L S = 319°. We confirmed a strong correlation between the spatial distribution of water vapor and HCl—both globally and locally—suggesting that water vapor plays an important role in the production of HCl, in agreement with previous studies. Our observations also suggest the presence of two competing processes involving the dust, one supporting HCl production and another one contributing to its destruction.