Modelling the Variation of HCl in the Martian Atmosphere

The ExoMars Trace Gas Orbiter (TGO) has characterised trace gases in the Martian atmosphere over several Mars years, improving the accuracy of species concentration measurements and observing temporal, vertical and spatial variations. Hydrogen chloride—detected for the first time with TGO [1,2]—has been investigated recently using the mid-infrared channel on the Atmospheric Chemistry Suite (ACS MIR). HCl observations show a strong seasonal variation, with almost all of the detections occurring during the latter half of the year (solar longitudes 180-360°) in the dusty season, when water vapour is present in the Martian atmosphere and ozone concentrations are low. Chlorine-bearing species such as HCl are important to understand in Mars’ atmosphere because on Earth they are involved in numerous processes throughout the planetary system, including volcanism, and they play a key role in atmospheric chemistry, e.g., by influencing concentrations of oxidative species such as oxygen (O₂) and ozone (O₃).

Here, we use the Mars Planetary Climate Model—a 3-D global climate model that includes a photochemical network—to explore the atmospheric HCl observations. We build on existing chlorine photochemical networks [3,4] to investigate potential source and sink mechanisms, focusing in particular on heterogeneous chemistry involving ice aerosols, and exploring the possibility of its role in direct release of HCl to the atmosphere. We also explore how chlorine species are affected indirectly by changes in the abundances of oxidative species (e.g., OH and HO₂,and by extension, O and O₃),driven by heterogeneous chemistry. Understanding the role of oxidative chemistry on HCl and other trace gases is key to achieving a more complete picture of processes occurring in the present-day Mars atmosphere, as well as processes that have shaped its evolution and habitability.

[1] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [2] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [3] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [4] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059.