Gas-phase and heterogeneous chlorine chemistry in the contemporary martian atmosphere

Introduction: Atmospheric hydrogen chloride (HCl) was first detected in the martian atmosphere in 2021 by the ExoMars Trace Gas Orbiter (TGO) [1], and more recently also studied by ground-based telescopes [2]. Observed HCl occurs in abundances of parts per billion by volume (ppbv) and shows strong seasonal and spatial variation. The vast majority of confirmed detections have been during Mars’ dustier perihelion season and there is a notable paucity of detections during the clearer aphelion season; HCl abundances show steep increases/decreases at the corresponding equinoxes [3,4].

When confirmed to be present, observed atmospheric HCl shows a bias towards higher abundances in the southern (summer) hemisphere. Measured HCl profiles also display vertical structure across their retrieved altitudes (generally 10-50 km above the surface), including sharp drops in abundance coinciding with the presence of water ice clouds [5].

Analysis of observed profiles has indicated a weak positive correlation between HCl abundance and dust loading and a stronger positive correlation between HCl and water vapour abundance [6]; similar correlations are apparent from analysis of ground telescope measurements [2].

Consideration of the observed sharp temporal and spatial gradients in HCl, together with its apparent relationship to aerosol and vapour abundances, has led to the idea that heterogeneous chlorine chemistry might explain these strong variations. This is supported by modelling of gas-phase chlorine chemistry in a Mars global climate model (GCM), which shows that in the absence of heterogeneous reactions the HCl distribution remains more uniformly mixed in time, latitude, and height [7].

Candidate heterogeneous chlorine chemical reactions, involving dust and water ice aerosol, have recently been incorporated into both 1D models [8,9] and a GCM [10]. The resulting modelled HCl profiles and global distributions show a marked improvement relative to observations and are able to reproduce important aspects of the observed distribution including the southern hemisphere bias, enhanced abundances during the perihelion season, and strong vertical structure. However, modelling also reveals features which either conflict with observations or have not yet been observed, and the exact nature of the significant reactions involved remains an open question.

Results & Discussion: We discuss results of our recently published GCM modelling work [7,10] and the gas-phase and heterogeneous reactions involved. We also discuss results from ongoing GCM modelling to better constrain the nature of proposed heterogeneous chlorine reactions.

 

Figure 1. Comparison of atmospheric HCl profiles for TGO/ACS observations, gas-phase only model outputs, and heterogeneous model outputs. All model data is masked to best match the observation times and locations. Subplot (a) shows mean (solid lines) and population standard deviation (shaded area) for all observations/data within the perihelion seasons of MY 34–36. Subplots (b–d) show comparisons for individual observed profiles in MY 34–36, with error bars representing standard errors. Adapted from [10].

 

Our results show that gas-phase chlorine chemistry alone cannot reproduce the observed global HCl distribution in a GCM [7]. Inclusion of idealised heterogeneous chlorine chemistry improves representation of HCl abundance and is able to reproduce key features of the observed distribution, suggesting a crucial role for heterogeneous chemistry in the contemporary chlorine cycle on Mars [10]. Figure 1 displays selected model profiles (with and without the idealised heterogeneous reactions) compared to TGO observations, showing the more realistic vertical structure obtained when heterogeneous chemistry is considered.

Further investigation of more specific and realistic potential heterogeneous reactions, adapted from [8] and [9] for our GCM, indicates that some reactions are more plausible than others as significant drivers in the contemporary Mars chlorine cycle. In the wait for further laboratory and observational work to better characterise these reactions under martian conditions, we discuss which reactions are more likely to be prominent based on analysis of their structural effects.

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