If the atmosphere of Mars contains methane, various possibilities have been proposed for where the methane could come from and how it could disappear.
Potential non-biological sources for methane on Mars include comets, degradation of interplanetary dust particles by ultraviolet light, and interaction between water and rock. A potential biological source would be microbes, if microbes have ever lived on Mars. Potential sinks for removing methane from the atmosphere are photochemistry in the atmosphere and loss of methane to the surface.
Source: mars.jpl.nasa.gov 02 Nov 2012