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

Abstract

We report new measurements of atmospheric methane by the Curiosity rover’s Tunable Laser Spectrometer that is part of the Sample Analysis at Mars suite (TLS-SAM), finding nondetections during two daytime measurements of average value 0.05 ± 0.22 ppbv (95% confidence interval CI). These are in marked contrast with nighttime background levels of 0.52 ± 0.10 (95% CI) from four measurements taken during the same season of northern summer. This large day-night difference suggests that methane accumulates while contained near the surface at night, but drops below TLS-SAM detection limits during the day, consistent with the daytime nondetection by instruments on board the ExoMars Trace Gas Orbiter. With no evidence for methane production by the rover itself, we propose that the source is one of planetary micro-seepage. Dynamical modeling indicates that such methane release is contained within the collapsed planetary boundary layer (PBL) at night due to a combination of nocturnal inversion and convergent downslope flow winds that confine the methane inside the crater close to the point where it is released. The methane abundance is then diluted during the day through increased vertical mixing associated with a higher altitude PBL and divergent upslope flow that advects methane out of the crater region. We also report detection of a large spike of methane in June 2019 with a mean in situ value over a two-hour ingest of 20.5 ± 4 ppbv (95% CI). If near-surface production is occurring widely across Mars, it must be accompanied by a fast methane destruction or sequestration mechanism, or both.

Mars Science Laboratory

“…Analyses of atmospheric samples and water released from clay minerals indicate the preferential loss of light isotopes, consistent with atmospheric escape (Webster et al 2013;Mahaffy et al 2015). Atmospheric methane is found to vary diurnally and seasonally, indicating active production, likely from subsurface seepage (Webster et al 2021). Meteorological observations over multiple Mars years have revealed the influence of the crater on wind, pressure, and atmospheric dust (Rafkin et al 2016).…”

From Atmospheric Evolution to the Search of Species of Astrobiological Interest in the Solar System—Case Studies Using the Planetary Spectrum Generator

“…There are reports of both positive and non-detections of methane on Mars by, respectively, observations by the MEx and ExoMars space probes [25,26]. The discrepancy between these has generated much debate [27][28][29][30][31][32].…”

Constraints on Emission Source Locations of Methane Detected by Mars Science Laboratory

“…Regarding the near-surface abundance levels detected by MSL, each emission source should satisfy sustainable midnight values of ∼0.4 ppbv (Webster et al, 2018). Recently, Webster et al (2021) reported the first enrichment observations performed during the daytime. Two daytime measurements close to midday of average value 0.05 ppbv ±0.22 ppbv (95% confidence interval) were reported between the southern winter solstice and the equinox (L s = 121° and 134°).…”

“…In this way, the nighttime abundance of methane from a surface source, restricted from vertical mixing by the stable nighttime PBL, could be much higher than in the well-mixed daytime. Recent enrichment observations suggest that the values during daytime are below the detection limit of TGO (Webster et al, 2021). Viúdez-Moreiras, Arvidson, et al (2020) suggested that the methane abundance detected by MSL could be significantly influenced by local meteorology and/or that methane source emissions could be highly dependent on location, areocentric solar longitude (L s ), and local time.…”