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


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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Fig. 1: Spectral fitting with ACS MIR.
Fig. 2: Solar occultation latitudes distribution over latitude and solar longitude of ACS MIR occultation using position 7 and all other ACS occultation points.
Fig. 3: CO VMR vertical profiles at different latitudes.
Fig. 4: Modelled CO distributions.
Fig. 5: Time evolution of CO VMR vertical profiles.

Data availability

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,, 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

Code availability

The GGG software suite is maintained by NASA’s Jet Propulsion Laboratory (JPL) and the California Institute of Technology. GGG is available at 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:


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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.

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Spectral fitting of ACS MIR spectra was performed by K.S.O. using the GGG software suite. Input and aid on spectral fitting was given by J.A., C.F.W., D.A.B., A.A.F. and F.M. Processing of ACS spectra was done at LATMOS by L.B. and at IKI by A.T. A.A.F. supplied ACS NIR retrievals of PT profiles and preliminary CO VMR profiles for comparison. The LMD GCM was run by F.L. with support from F.F. The ACS instrument was operated by A.T., A.P., A.V.G. and A.S. All coauthors have contributed to the preparation of the manuscript, written by K.S.O., F.L., F.M. and O.K.

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Correspondence to K. S. Olsen.

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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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Supplementary Methods, Figs. 1–7 and Table 1.

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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).

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