The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter

Carbon monoxide (CO) is the main product of CO 2 photolysis in the Martian atmosphere. Production of CO is balanced by its loss reaction with OH, which recycles CO into CO 2 . CO is therefore a sensitive tracer of the OH-catalysed chemistry that contributes to the stability of CO 2 in the atmosphere of Mars. To date, CO has been measured only in terms of vertically integrated column abundances, and the upper atmosphere, where CO is produced, is largely unconstrained by observations. Here we report vertical profiles of CO from 10 to 120 km, and from a broad range of latitudes, inferred from the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter. At solar longitudes 164–190°, we observe an equatorial CO mixing ratio of ~1,000 ppmv (10–80 km), increasing towards the polar regions to more than 3,000 ppmv under the influence of downward transport of CO from the upper atmosphere, providing a view of the Hadley cell circulation at Mars’s equinox. Observations also cover the 2018 global dust storm, during which we observe a prominent depletion in the CO mixing ratio up to 100 km. This is indicative of increased CO oxidation in a context of unusually large high-altitude water vapour, boosting OH abundance. The CO mixing ratio in Mars’s atmosphere increases towards the poles because of downward transport of CO from the upper atmosphere, according to an analysis of data from the ExoMars Trace Gas Orbiter.