Abstract
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.
Highlights
The search for methane on Mars has followed a long and tortuous path towards what could potentially be considered a major discovery for Mars’ exploration
Relying on solar occultation spectra collsetrcettecdhiinngtvhiebrmatiido-ninbfraanrdedoframnegtehabnraecckeentitnregd tahreouνn3 da3sy0m00mcemtr−ic1 (3.3 μm), we describe a dataset encompassing the April 2018 to November 2020 period, completing the dataset used in K19 that only covered a quarter of MY34, essentially probing the dusty conditions of the 2018 global dust storm (Guzewich et al 2019)
We processed almost a Mars year and a half of data produced by the Atmospheric Chemistry Suite (ACS) instrument in the spectral range encompassing the main mid-infrared absorption band of methane, yielding no methane detection and delivering an annual mean upper limit of 20 pptv, thereby refining the first report of K19 that specified an upper limit of 50 pptv for the Ls 163–241◦ period of MY34
Summary
The search for methane on Mars has followed a long and tortuous path towards what could potentially be considered a major discovery for Mars’ exploration. Olsen et al (2020), using the ACS, showed that the Q- and R-branch ranges of methane are affected by ozone continuum absorption These two findings, besides their implication for previous and debated methane detection attempts on Mars (Webster et al 2020), have led to the implementation of these so far unaccounted species as additional gaseous parameters to provide a better fit. The retrieval itself, which is applied to a narrow (
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