Global distribution and seasonality of martian atmospheric HCl explained
through heterogeneous chemistry
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 and rapidly decreasing afterwards faster than
projected from photolysis and gas-phase chemistry. HCl also shows
anti-correlation with atmospheric water ice. 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 (ACS). We found that the heterogeneous
scheme significantly improved the modelled 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 characterise the exact aerosol reactions
involved.