Planetary atmosphere observation analysis

Abstract:

A major quest in Mars' exploration has been the hunt for atmospheric gases, potentially unveiling ongoing activity of geophysical or biological origin. Here, we report the first detection of a halogen gas, HCl, which could, in theory, originate from contemporary volcanic degassing or chlorine released from gas-solid reactions. Our detections made at ~3.2 to 3.8 μm with the Atmospheric Chemistry Suite and confirmed with Nadir and Occultation for Mars Discovery instruments onboard the ExoMars Trace Gas Orbiter, reveal widely distributed HCl in the 1- to 4-ppbv range, 20 times greater than previously reported upper limits. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere. Understanding the origin and variability of HCl shall constitute a major advance in our appraisal of martian geo- and photochemistry.

Abstract:

Carbon monoxide (CO) is the main product of CO2 photolysis in the Martian atmosphere. Production of CO is balanced by its loss reaction with OH, which recycles CO into CO2. CO is therefore a sensitive tracer of the OH-catalysed chemistry that contributes to the stability of CO2 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.