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A Stringent Upper Limit of 20 pptv for Methane on Mars and Constraints on Its Dispersion Outside Gale Crater

Abstract : 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 a geophysical origin. In this report we complement and improve on the previously reported detection attempts by the Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter (TGO) (Korablev et al., 2019). 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. Here we use 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- upper limits of around 10 pptv (20 pptv at 2-), with an annual mean of the smallest upper limits of 20 pptv. They are three-to-four times lower than previously reported TGO upper limits of 50-60 pptv (Korablev et al., 2019; Knutsen et al., 2020). Upper limits are controlled by the amount of dust in the atmosphere, which impairs detection around the equator and during the southern spring and summer seasons. Observations performed near Gale crater yielded 1- upper limits of up to four times less (~65 pptv) than the background values measured by SAM TLS at the Curiosity rover during the corresponding seasons (Webster et al., 2018). These results, supported by mesoscale modeling, make reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity even more difficult (Montmessin et al., 2021). Together with the low upper limits obtained recently by SAM TLS during daytime (Webster et al., 2021), our data place stronger constraints on the putative sources of methane and the physical or chemical mechanism of methane destruction needed to satisfy all measurements. OK, AT, AF, DB acknowledge RSF funding under grant 20-42-0903. References Knutsen, E. W., Villanueva, G. L., Liuzzi, G., et al. 2020, Icarus, 357, 114266 Korablev, O., Vandaele, A. C., Montmessin, F., et al. 2019, Nature, 568, 517 Montmessin, F., Korablev, O. I., Trokhimovskiy, A., et al. 2021, A&A 650, A140 Webster, C. R., Mahaffy, P. R., Atreya, S. K., et al. 2018, Science, 360, 1093 Webster, C. R., Mahaffy, P. R., Pla-Garcia, J., et al. 2021, A&A 650, A166
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Submitted on : Saturday, August 13, 2022 - 6:43:46 PM
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Oleg Korablev, Franck Montmessin, A. Yu. Trokhimovskiy, Franck Lefèvre, A. Fedorova, et al.. A Stringent Upper Limit of 20 pptv for Methane on Mars and Constraints on Its Dispersion Outside Gale Crater. AGU Fall Meeting 2021, Dec 2021, Chicago & Online, Unknown Region. ⟨insu-03751138⟩



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