chemistry may also play a role
Oliver Morton in his Mianly Martian blog has interesting comment from John Eiler, geochemist at Caltech on other non-bilogical sources (see blog titled More on Methane). With C, H, Fe2+ in the rocks and that weird Martian regolith, strange oxidation reduction reactions activated by intense UV or other radiation could also be involved. That Martian regolith (not soil as that implies organic material) has been a chemical puzzle since the first Viking and I don't see much enlighten yet, just more puzzles (eg blueberries jarosite & Fe2+ in same rock, etc).