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Can Curiosity and TGO be Both Correct About Methane on Mars?

Presentation #300.09 in the session Martian Ice, Climate, and Habitability (Oral Presentation)

Published onOct 23, 2023
Can Curiosity and TGO be Both Correct About Methane on Mars?

After twenty years of observations, it is still an open question whether methane really exists in the atmosphere of Mars. Recent measurements made by the Tunable Laser Spectrometer on the Curiosity rover have seemingly confirmed the presence of methane in the sub-ppbv to ppbv range near the surface, but these results are again challenged by two instruments on the ExoMars Trace Gas Orbiter (TGO), both of which have reported non-detections in the mid- to high-altitudes. As TGO is unable to probe the near-surface atmosphere, it is still possible that the results from both missions are trustworthy, but then we will need to explain the large gradient in methane concentration from the site of Curiosity to the altitudes above 5 km.

Traditional 1D thinking (which assumes that methane concentration is mostly homogeneous in the horizontal direction) has failed to reconcile the discrepancy between Curiosity and TGO, given that the lifetime of methane predicted by known chemistry far exceeds the mixing timescale in the atmosphere of Mars (which would lead to homogeneous mixing across the whole atmosphere). But 3D thinking (which considers horizontal inhomogeneity) has provided a novel solution—methane could be released into the atmosphere from point sources, which would significantly elevate the methane concentration near the point(s) of release, and the Curiosity rover, by accident, landed near such a point source and thus can detect anomalously strong methane signals.

We use an inverse Lagrangian transport model to test if this proposed solution could reconcile the discrepancy between Curiosity and TGO, and if yes, what could be inferred about the location(s) of the point source(s). We find that a point source in northwestern Gale crater is indeed able to explain the discrepancy. We also find that such a point source can explain the observed diurnal and seasonal variabilities of near-surface methane concentration. But we also find that such a point source is too close to Curiosity—we must have been extremely lucky when we chose the Curiosity landing site. To mitigate this “coincidence problem”, new chemical reactions or physical processes that remove methane from the atmosphere are warranted. From our modeling, we find that such chemical reactions must occur in the atmosphere (in the gas phase or on the surface of floating particles), rather than on the surface of Mars. Lastly, we design a strategy for future missions to deploy a sensor network that is aimed at constraining the location(s) of the methane point source(s) down to a spatial scale of a few kilometers within Gale crater.

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