In 2004, three different research groups independently announced the discovery of significant concentrations of methane in the atmosphere of Mars. But the predicted lifetime of methane in the Martian atmosphere has been calculated at just 200 days (Source : Nature, 460(7256):720-3 ) - so it appears that the gas must be being constantly generated by as yet unknown mechanism(s) - possibly geochemical processes, volcanic or hydrothermal activity, or living organisms.
It's speculated that living organisms could generate the methans either as a waste product, or by decomposition. The current NASA Perseverance mission will attempt to sample the Martian soil in search of evidence.
Further reading: Methane on Mars (The Economist)
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