On the plausibility of methane detections on Mars

As a potential biomarker, Martian methane has attracted attention through several reports of its detection over the last 20 years. However, the very existence of this gas has been continuously questioned, in particular because the observed lifetime should be several orders of magnitude shorter than the 300 years predicted by photochemical models. Although several fast removal processes have been hypothesized to explain the observations, none of them has met a large consensus.

It is in this context that the ESA-Roscomos ExoMars Trace Gas Orbiter (TGO) mission started its science operations in April 2018. ACS and NOMAD, two instruments onboard the TGO, have been collecting hundreds of highly sensitive measurements in solar occultation. No methane has been detected so far and an upper limit of 0.02 ppbv has been derived. The implications of this result on the methane problem on Mars will be addressed in this work.

This upper limit is a strong constraint on the background level and, in turn, on the potential emission scenarios making the reported methane detections consistent with the TGO results. While several model studies aimed at identifying them, we will here adopt a probabilistic approach to the problem in order to question the plausibility of those detections and estimate the lifetime required to make them plausible from a probabilistic standpoint.