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.
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The datasets generated by the ExoMars Trace Gas Orbiter instruments, including ACS, and analysed during the current study are being made available in the ESA Planetary Science Archive (PSA) repository, https://archives.esac.esa.int/psa, following a six months prior access period, following the ESA Rules on Information, Data, and Intellectual Property. Data used herein can be found by searching for the ‘ExoMars 2016’ mission and then selecting the ACS and MIR instruments. Derived products (CO VMR vertical profiles) have been deposited in the Oxford University Research Archive at https://doi.org/10.5287/bodleian:wxxq2m6jo.
The GGG software suite is maintained by NASA’s Jet Propulsion Laboratory (JPL) and the California Institute of Technology. GGG is available at https://tccon-wiki.caltech.edu and distributed under a non-commercial software license. The LMD GCM is maintained at LMD. It can be obtained using the Subversion version control system following the instructions made available here: https://www.lmd.jussieu.fr/~lmdz/planets/mars/user_manual.pdf.
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The ExoMars mission is a joint mission of the European Space Agency (ESA) and Roscosmos. The ACS experiment is led by the Space Research Institute (IKI) in Moscow, assisted by LATMOS in France. This work was funded by Roscosmos, the National Centre for Space Studies of France (CNES), the Agence Nationale pour la Recherche (ANR)-MCUBE project, the Ministry of Science and Education of Russia, the Natural Sciences and Engineering Research Council of Canada (NSERC) (PDF – 516895 – 2018) and the UK Space Agency (ST/T002069/1, ST/R001502/1 and ST/P001572/1). Science operations are funded by Roscosmos and ESA.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Jun Cui and Daniel Viudez-Moreiras for their contribution to the peer review of this work. Primary Handling Editors: Tamara Goldin; Stefan Lachowycz.
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Olsen, K.S., Lefèvre, F., Montmessin, F. et al. The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter. Nat. Geosci. 14, 67–71 (2021). https://doi.org/10.1038/s41561-020-00678-w
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