Photochemistry on Mars: preparing for data from the ExoMars Trace Gas Orbiter

Abstract

Our understanding of methane on Mars is insufficient to explain the spatial and temporal variability it has been reported to experience, and the 2021 detections of atmospheric hydrogen chloride (HCl) do not, as of yet, have an explanation. A comprehensive photochemistry scheme is required to study the processes operating within Mars’ atmosphere, and to aid in reconciling modelling results with observations. To address these gaps in scientific understanding, I have developed a 1-D atmospheric photochemistry model to describe methane (CH4), ethane (C2H6), their resulting oxidation products, and chlorine-containing trace gases in the Martian atmosphere. CH4 and C2H6 are often emitted together in terrestrial surface releases. I report the first detailed study of the evolution of C2H6 in Martian conditions, and investigate the corresponding photochemical footprints of CH4 and C2H6. Using my model I have identified that formaldehyde and formic acid are common oxidation products of CH4 and C2H6, while acetaldehyde (CH3CHO) and acetic acid (CH3COOH) are unique oxidation products of C2H6. I also find that CH3CHO photolysis can lead to CH4 production at the surface. Drawing on concepts pioneered in the Earth sciences, I developed the first Tangent Linear and Adjoint model for Martian atmospheric photochemistry, which enables the study of the sensitivity of atmospheric chemistry measurements to changes in precursor gases. I use this to assess the sensitivity of O3 and H2O2 model columns to the models initial abundances of inorganic trace gas species, finding strong sensitivities of H2O2 to CO and H2O vapour abundances. O3 demonstrates strong and seasonally variable negative sensitivities to H2O vapour and O2, and strong positive sensitivities to H2O ice in the polar winters. Using aerosol and H2O profiles from the ExoMars Trace Gas Orbiter, I built a simple heterogeneous chemistry scheme for chlorine chemistry to explain the HCl profiles measured in early 2021. Using uptake coefficients used for Earth stratosphere models, the routine can produce HCl at the required abundances and shape, but a more rigorous scheme will be required for future work.