Comparing MSL Curiosity Rover TLS‐SAM Methane Measurements With Mars Regional Atmospheric Modeling System Atmospheric Transport Experiments

Abstract

The detection of methane at Gale crater by the Tunable Laser Spectrometer–Sample Analysis at Mars instrument aboard the Curiosity rover has garnered significant attention because of the implications for the presence of Martian organisms (Webster et al., 2015, https://doi.org/10.1126/science.1261713). Methane's photochemical lifetime is several centuries unless there is a fast, as‐yet‐unknown destruction mechanism (Lefèvre and Forget, 2009, https://doi.org/10.1038/nature08228). This is much longer than the atmospheric mixing time scale, and thus, the gas should be well‐mixed except when near a source or shortly after a release. Although most measurements report low background levels of ~0.4 parts per billion by volume, observed spikes of several parts per billion by volume or greater and a subsequent return to the background level are intriguing (Webster et al., 2015, https://doi.org/10.1126/science.1261713). The Mars Regional Atmospheric Modeling System is used to simulate, via passive tracers, the transport and mixing of methane released inside and outside of the crater from instantaneous and steady state releases, and to test whether the results are consistent with in situ observations made by the Mars Curiosity rover. The simulations indicate that the mixing time scale for air within the crater is approximately 1 sol. The timing of methane measurements within the crater is also important, because modeled methane abundance varies by ~1 order of magnitude over a diurnal cycle under all the scenarios considered. While the observed low background levels can be reproduced by the model under some circumstances, it is difficult to reconcile the measured peaks with the modeled transport and mixing. For periods of high methane abundance lasting longer than a few hours there must be a continuous release of methane inside the crater to counteract mixing, or there must be a large, methane‐rich air mass continually transported into the crater. The few scenarios that can produce peaks are problematic, because they would result in background methane values above what is observed.

Day-night differences in Mars methane suggest nighttime containment at Gale crater

“…The daytime absence of methane recorded by TLS-SAM, and the new additional nighttime values reported here, are a critical update to the Curiosity data set to constrain the possible mechanisms of methane production and removal on Mars and are a step toward reconciling the apparent differences between this data set and that of the TGO. Building upon the models of Moores et al (2019b,a), Viúdez-Moreiras et al (2020), Pla-Garcia et al (2019, and of Etiope & Oehler (2019), the TLS-SAM data provide evidence for methane production from a near-surface source, most likely from continuous micro-seepage, that is temporarily contained in the near-surface atmosphere during the night due to the low planetary boundary layer, reduced atmospheric mixing, and horizontal transport. Increased atmospheric dynamics during the daytime mixes the gas accumulated in the night into the global atmosphere where it is diluted to the very low levels constrained by the TGO nondetection.…”

“…The methane abundance tends to increase overnight when the PBL is shallow and the downslope, convergent circulations locally confine the surface release. The methane abundance tends to decrease during the day when the PBL grows (Moores et al 2019b) and divergent, upslope circulations transport methane away from the release area (Pla-Garcia et al 2019). Animations of the diurnally varying circulation and methane abundance from MRAMS are provided in the appendix.…”

New Insights into the Search for Life on Mars

“…TLS detected also episodically increases ('spike') of Martian methane [49]: for example on June 16, 2013 and on early January 2014 readings averaged ten time the background level (6-8 ppbv). The largest concentration of methane detected in situ by the Curiosity reached a spike to 21 ppbv, on June 20, 2019, dropping quickly over a few days (Figure 27).…”

A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater

“…In particular, the fact that Gale could be the only place on Mars where methane is released is a hypothesis that is difficult, if not impossible, to justify. In addition, the simultaneous existence of spikes and a background of methane is ruled out by the atmospheric circulation configuration predicted for Gale (Pla-Garcia et al 2019).…”

“…Since then, many teams have searched for methane using ground-based observatories as well as orbital and landed assets, basing their analysis on mid-and thermal-infrared wavelengths where methane possesses strong and distinctive absorption signatures. An annotated list of these attempts can be found in Zahnle et al (2011), Knutsen et al (2021), and in tabulated form in Pla-Garcia et al (2019). Hitchcock & Lovelock (1967) pointed out that the composition of the Martian atmosphere could provide clues as to the possible presence of life on the planet.…”