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.
Highlights
The quantity, distribution, and behavior of methane in the atmosphere of Mars are of great interest to the planetary science community and the general public since the gas is recognized as a potential biosignature, due in part to its predominantly biological origin on Earth (Pachauri et al 2014)
A critical issue is to understand to what extent the nighttime measurements at Gale crater are representative of the methane flux over the whole day at Gale crater and other near-surface locations because day-night differences in the the Sample Analysis at Mars suite (TLS-Sample Analysis at Mars (SAM)) measurements have the potential to identify the mechanisms at play and to reduce the discrepancy between the Curiosity and ExoMars Trace Gas Orbiter (TGO) data sets (Moores et al 2019a)
Seasonal variation in the methane background levels suggested by the TLS-SAM background data (Webster et al 2018) has been reproduced to some extent by the one-dimensional numerical model of Moores et al (2019b) based on temperaturedependent emissions, and by Viúdez-Moreiras et al (2019) based on wind-dependent emissions
Summary
The quantity, distribution, and behavior of methane in the atmosphere of Mars are of great interest to the planetary science community and the general public since the gas is recognized as a potential biosignature, due in part to its predominantly biological origin on Earth (Pachauri et al 2014). The Tunable Laser Spectrometer (TLS-SAM) instrument of the SAM has reported occasional spikes in atmospheric methane (Webster et al 2015, 2018) up to 20 ppbv (Moores et al 2019a) detected above a low persistent background level.
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