Webster and 43 co-authors reported in 2018 that variable levels of methane in the martian atmosphere have eluded explanation partly because the measurements are not repeatable in time or location. The authors reported in situ measurements at Gale crater made over a 5-year period by the Tunable Laser Spectrometer on the Curiosity rover. The background levels of methane have a mean value 0.41 ± 0.16 parts per billion by volume (ppbv) (95% confidence interval) and exhibit a strong, repeatable seasonal variation (0.24 to 0.65 ppbv).This variation is greater than that predicted from either ultraviolet degradation of impact-delivered organics on the surface or from the annual surface pressure cycle.The large seasonal variation in the background and occurrences of higher temporary spikes (~7 ppbv) are consistent with small localized sources of methane released from martian surface or subsurface reservoirs. With ancient atmospheric pressures of several hundred millibars, large amounts of methane may be stored in the cold martian subsurface as clathrates in a stability zone several times thicker than that of Earth.
Although the seasonal signature of the TLS-SAM measurements is not consistent with direct clathrate release, clathrates may provide a source of surface microseepage (diffuse exhalations without any specific morphological structure that may vent from outcropping of rocks or river or lake beds). On Mars, such seepage would occur preferentially through permeable pathways, such as faults, fractures, or in breaches in sealing lithologies; this would not require identifiable geomorphological structures on the surface.Weak microseepage exhalations could explain background and plume methane anomalies observed on Mars, perhaps near the dichotomy boundary and at Gale crater, where there is fractured sedimentary rock. Microseepage flux may vary over time, depending on variations of gas pressures along the subsurface migration pathway or on seasonal changes in the soil, or even where microbial activity may consume methane.
Regardless of the subsurface origin, methane that finds its way to surface layers over long time periods may be expected to show seasonal variation. The authors considered a process that retains methane at the surface temporarily before releasing it through a process linked to the surface temperature. That process could be adsorption on a surface with a high surface area–to-volume ratio, such as dust or soil. Although mineral dust cannot serve as a methane sink, it can moderate the release. Adopting an energy barrier of ~20 to 35 kJ/mol—which is somewhat higher than that reported for the physical adsorption of methane into clays, zeolites, and Mars analog soil — the authors found that large seasonal variations are expected.
Plausible correlations of the background methane values with atmospheric water vapor and with surface temperatures point to physical or chemical surface (or dust) processes, or microseepage release. The amplitude of the seasonal cycle indicates that there remain unknown atmospheric or surface processes occurring in present-day Mars.
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