A strong, repeatable seasonal cycle in the background methane mixing ratio has been observed at the Gale Crater landing site of the Mars Science Laboratory rover with the Tunable Laser Spectrometer of the Sample Analysis at Mars instrument. However, as of yet, no physical process has been proposed that can explain both the timing and amplitude of the observations. Here we show that a one-dimensional numerical model considering adsorption onto and diffusion through the regolith can reproduce the variation, including a phase lag, if the regolith is impregnated with methane from a prior plume or supplied from below by microseepage. Combining the model results with geological constraints, we estimate that the amount of microseepage at Gale is at most 3 × 10−5 tonnes km−2 yr−1. Gale’s unique dynamical environment makes such seeps easier to detect in surface sampling measurements. Over most of the Martian surface, atmospheric mixing is stronger or atmospheric transport more effective, and we expect the amplitude of the seasonal cycle to be smaller for the same strength of seep.
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The code used to generate the figures shown in this paper and in the Supplementary Information are available at www.yorku.ca/jmoores/MarsMethane1D.tar.gz and there are no restrictions on availability.
All data used in this study, REMS and SAM-TLS values, are available on the Planetary Data System (https://pds.nasa.gov). For methane data, the reader is further directed to ref. 4 where reduced tabulated values are available. All other data used in the production of this paper are listed in the text, and the figures were generated from this data using the code described under Code availability.
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J.E.M. and C.L.S. acknowledge funding for this work provided by the Canadian Space Agency’s Mars Science Laboratory Participating Scientist Program.
The authors declare no competing interests.
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Moores, J.E., Gough, R.V., Martinez, G.M. et al. Methane seasonal cycle at Gale Crater on Mars consistent with regolith adsorption and diffusion. Nat. Geosci. 12, 321–325 (2019). https://doi.org/10.1038/s41561-019-0313-y