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The presence and concentration of methane (CH4) in the martian atmosphere is of paramount importance to planetary scientists and exobiologists because it could be the signal of present or past microbial life. Alternatively, it could be related to nonbiological processes including present or past volcanism or hydrothermal activity.

The discrepancy between the CH4 detection from measurements by Curiosity on the surface in Gale Crater and the CH4 non-detection by the ExoMars Trace Gas Orbiter (TGO) from orbit has led to a vibrant debate in the scientific community. Two recent articles published in AGU journals (Luo et al., 2021 and Viúdez-Moreiras et al., 2021) start from the premise that both measurements are correct and use innovative approaches to characterize the potential source regions of the CH4 emissions and resulting implications that would reconcile the different data sets. They converge on a surprising result.

How and when has methane been observed on Mars?

Earth-based and orbital observations of methane in the Martian atmosphere have been reported since 1999 and they are at odds since the beginning. They show highly variable and inconsistent methane concentrations, with global averages ranging from 5 to 33 ppbv (Pla-García et al., 2019). Over the last years, Curiosity has observed in Gale a background level of ~0.41 ppbv with a few episodic spikes up to ~21 ppbv (Webster et al., 2021). Intriguingly, concurrent TGO observations of the atmosphere in the vicinity of Gale resulted in no detection, with estimated global (planet-average) upper limits as small as 0.02 ppbv (Montmessin et al., 2021).

Curiosity methane signals as of January 2020 versus Mars season and local time. Measurements labeled with indices 1 to 7 represent “methane spikes” (values above 5 ppbv), while the 29 data points below 5 ppbv are regarded as the background level. Direct-ingest measurements are shown in circles, while squares are used for enrichment measurements. Colors show the local time of methane ingestions. Note that TGO observations in Gale resulted in no CH4 detection. Adapted from Luo et al. (2021)

How do these two studies try to reconcile the seemingly contradictory measurements?

To pinpoint the possible source areas of CH4 emissions, the study by Viúdez-Moreiras et al. (2021) uses atmospheric dispersion modeling while Luo et al. (2021) uses back trajectory analysis.

Based on prevailing modeled winds, Viúdez-Moreiras et al. (2021) map out downstream emission regions by predicting how much methane released at a given area and time can reach Curiosity (Eulerian approach), while Luo et al. (2021) map out upstream emission regions by transporting backwards in time the amount of methane measured by Curiosity (Lagrangian approach).

In both cases, Curiosity measurements provide the ground truth, and the upper limits of TGO measurements provide the boundary condition for the maximum global average CH4 concentration.

How have these investigations helped to clarify the Martian methane mystery?

The seemingly contradictory observations used as constraints and combined with a rigorous scientific logic lead the authors of the two studies to infer that only very small CH4 emissions coming from a source located in the northwestern rim of Gale Crater, i.e., from very close to the rover, can lead to a detection by Curiosity and non-detection by TGO. The result also excludes globally all other sources of methane as such sources would generate a global background level larger than the upper limit (0.02 ppbv) observed by TGO.

The Martian scientists are therefore left with the following alternative scenarios, all of which have difficulties: (1) Curiosity has landed on or right next to the only CH4 source on the planet, which is extremely unlikely, (2) there is something wrong with the measurements, but this possibility has been investigated in detail and ruled out (Webster et al., 2018, 2021), 3) CH4 lifetime in the Martian atmosphere is for some unknown reason much shorter than predicted by known chemistry, including the possibility of an unknown fast removal mechanism, which should not affect significantly other species, since the known chemistry explains well their behavior. The possible scenarios are nailed down but the mystery thickens.

(Left) Map showing the contribution from a putative emission site to the methane mole fraction measured by Curiosity (white star) for Spike 2. The strongest footprint lies over the entire northwester crater floor (red color). A similar behavior is found for other Spikes. Adapted from Luo et al. (2021). (Right) Map of maximum methane mixing ratio (ppbv) above 10 km during sunrise/sunset (corresponding to what TGO would observe) at Ls = 0° as function of the location (latitude and longitude) of the emission source. The thick blue line indicates the upper limit imposed by TGO, and thus only the north-northwestern slopes of Aeolis Mons, where MSL is located, would most likely produce methane mixing ratios consistent with Curiosity and TGO. A similar result is obtained at Ls = 90°. Adapted from Viúdez-Moreiras et al. (2021)

—Germán Martínez (gmartinez@lpi.usra.edu; 0000-0001-5885-236X), Associate Editor, JGR: Planets; Anni Määttänen ( 0000-0002-7326-8492), Editor, JGR: Planets; and David Baratoux ( 0000-0002-1785-5262), Editor, Earth and Space Science

Citation: Martínez, G., A. Määttänen, and D. Baratoux (2022), The mystery of methane on Mars thickens, Eos, 103, https://doi.org/10.1029/2022EO225010. Published on 15 February 2022.
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