Wide variety of possible sources

Methane can indeed come from a wide variety of sources: transformation of a rock, olivine; ultraviolet radiation degradation of organic molecules from meteorites; production by the impact of comets; salting out from clathrates, frozen gases trapped beneath the surface, or other gases adsorbed on the regolith, dust left on the surface by the impact of meteorites; erosion of basalt containing methane inclusions; geothermal production. But none of these explanations are totally consistent with the observations.