Lucio Baggio, Jean-Loup Bertaux, Frank Daerden, Abdanour Irbah, Denis Belyaev, Anna Fedorova, Forget Francois, Andrey Patrakeev, Jorge Pla-Garcia, Ashwin Braude, Juan Alday, S. C. R. Rafkin, Oleg Korablev, Franck Lefèvre, Colin Wilson, A. Yu. Trokhimovskiy, Gaetan Lacombe, Franck Montmessin, Kevin Olsen, Alexey Shakun, Montmessin, F. [0000-0002-4187-1457], Korablev, O. I. [0000-0003-1115-0656], Trokhimovskiy, A. [0000-0003-4041-4972], Lefèvre, F. [0000-0001-5294-5426], Fedorova, A. A. [0000-0002-4176-2955], Baggio, L. [0000-0002-9263-4937], Irbah, A. [0000-0003-3265-3148], Olsen, K. S. [0000-0002-2173-9889], Braude, A. M. [0000-0003-2437-2151], Belyaev, D. A. [0000-0003-1123-5983], Alday, J. [0000-0003-1459-3444], Forget, F. [0000-0002-3262-4366], Pla García, J. [0000-0002-8047-3937], Rafkin, S. [0000-0001-7464-1319], Agence Nationale de la Recherche (ANR), Natural Sciences and Engineering Research Council of Canada (NSERC), UK Space Agency, Science and Technology Facilities Council (STFC), PLANETO - LATMOS, Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS), Sorbonne Université (SU)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS), Space Research Institute of the Russian Academy of Sciences (IKI), Russian Academy of Sciences [Moscow] (RAS), Department of Physics [Oxford], University of Oxford [Oxford], Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL), Belgian Institute for Space Aeronomy / Institut d'Aéronomie Spatiale de Belgique (BIRA-IASB), Instituto Nacional de Técnica Aeroespacial (INTA), and Southwest Research Institute [Boulder] (SwRI)
Context. Reports on the detection of methane in the Martian atmosphere have motivated numerous studies aiming to confirm or explain its presence on a planet where it might imply a biogenic or more likely a geophysical origin. Aims. Our intent is to complement and improve on the previously reported detection attempts by the Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter (TGO). This latter study reported the results of a campaign that was a few months in length, and was significantly hindered by a dusty period that impaired detection performances. Methods. We unveil 640 solar occultation measurements gathering 1.44 Martian years worth of data produced by the ACS. Results. No methane was detected. Probing the clear northern summer season allowed us to reach 1σ upper limits of around 10 pptv (20 pptv at 2σ), with an annual mean of the smallest upper limits of 20 pptv. Upper limits are controlled by the amount of dust in the atmosphere, which impairs detection performance around the equator and during the southern spring and summer seasons. Observations performed near Gale crater yielded 1σ upper limits of up to four times less than the background values measured by the Curiosity rover during the corresponding seasons. Conclusions. Reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity is even more difficult in light of this annual survey performed by ACS. Stronger constraints are placed on the physical and chemical mechanism capable of explaining why the mean of the best overall upper limits of ACS is ten times below the smallest methane abundances measured by Curiosity. The ACS investigation was developed by the Space Research Institute (IKI) in Moscow, and the Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS) in Guyancourt. The investigation was funded by Roscosmos, the National Centre for Space Studies of France (CNES) and RSF (Russian Science Foundation 20-42-0903). This work was funded by CNES, the Agence Nationale de la Recherche (ANR, PRCI, CE31 AAPG2019, MCUBE project), the Natural Sciences and Engineering Research Council of Canada (NSERC) (PDF–516895–2018), the UK Space Agency (ST/T002069/1), the UK Science and Technology Facilities Council (ST/R001502/1, ST/P001572/1). All spectral fitting was performed by F.M. The interpretation of the results was done by F.M. and O.K. The preparation of ACS spectra is done at LATMOS by L.B. and at IKI by A.T. Ancillary data are produced in LATMOS by G.L. and in IKI by A.P. Input and aid on spectral fitting were given by K.O. and A.T. The ACS instrument was designed, developed, and operated by A.P., A.S., A.T., F.M., and O.K. Peerreview