A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater

International audienceReports on the detection of methane in the Martian atmosphere have motivated numerous studies aiming to confirm orexplain its presence on a planet where it might imply a biogenic or more likely a geophysical origin. 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 inlength, and was significantly hindered by a dusty period that impaired detection performances. We unveil 640 solar occultation measurements gathering 1.44 Martian years worth of data produced by the ACS. No methane was detected. Probing the clear northern summer season allowed us to reach 1-σupper limits of around 10pptv (20 pptv at 2-σ), with an annual mean of the smallest upper limits of 20 pptv. Upper limits are controlled by the amount ofdust in the atmosphere, which impairs detection performance around the equator and during the southern spring and summer seasons.Observations performed near Gale crater yielded 1-σupper limits of up to four times less than the background values measured bythe Curiosity rover during the corresponding seasons. Reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity is even moredifficult in light of this annual survey performed by ACS. Stronger constraints are placed on the physical and chemical mechanismcapable of explaining why the mean of the best overall upper limits of ACS is ten times below the smallest methane abundancesmeasured by Curiosity