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Streeter, Paul M.; Rajendran, Kylash; Lewis, Stephen R.; Olsen, Kevin S.; Trokhimovskiy, Alexander; Korablev, Oleg and Patel, Manish R.
(2025).
DOI: https://doi.org/10.1029/2024gl111059
Abstract
Recent observations from the ExoMars Trace Gas Orbiter (TGO) have revealed the presence of hydrogen chloride (HCl) in the martian atmosphere. HCl shows strong seasonality, primarily appearing during Mars' perihelion period before decreasing faster than projected from photolysis and gas‐phase chemistry. HCl profiles also display local anti‐correlation with water ice aerosol. One candidate explanation is heterogeneous chemistry. We present the first results from a heterogeneous chlorine chemistry scheme incorporated into a Mars global climate model (GCM), with atmospheric dust/water ice parameterized as an HCl source/sink respectively. Results were compared against a Mars GCM with gas‐phase only chlorine chemistry and observations from TGO's Atmospheric Chemistry Suite. We found that the heterogeneous scheme significantly improved the modeled HCl seasonal, latitudinal, and vertical distribution, supporting a crucial role for heterogeneous chemistry in Mars' chlorine cycle. Remaining discrepancies show that further work is needed to characterize the exact aerosol reactions involved.
Key Points
- Latitudinal and seasonal behavior of observed Mars atmospheric hydrogen chloride (HCl) can be explained by heterogeneous chlorine chemistry
- Observed seasonal, hemispheric, and vertical HCl variation is qualitatively captured by heterogeneous dust emission and ice absorption
- We also predict enhanced north polar HCl during the aphelion season, not currently seen in Trace Gas Orbiter observations
Plain Language Summary
The ExoMars Trace Gas Orbiter (TGO) recently detected hydrogen chloride (HCl) for the first time in the martian atmosphere. Observations show that HCl appears around southern summer, when Mars is closest to the Sun, and abruptly decreases at the end of this season. This variation occurs on timescales far shorter than the expected photochemical lifespan of HCl, implying other processes are responsible. One possible explanation is heterogeneous chemistry: reactions between gaseous species and atmospheric dust and/or water ice. To investigate we implemented such a scheme into a Mars global climate model, whereby atmospheric dust and water ice aerosol act (respectively) as a source and sink for HCl. Comparing to TGO observations, we found that including heterogeneous chemistry significantly improved HCl representation. We were able to reproduce key observed features such as the HCl seasonal cycle, the greater HCl presence in the southern hemisphere, and to an extent the complex vertical structure. This strongly suggests that heterogeneous chemistry is a crucial component of the martian chlorine cycle. However, some questions remain, such as whether there is a causal relationship between dust storms and HCl presence. Further work is therefore needed to understand the relationships between aerosols and chlorine species.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)
