MAKING METHANE ON MARS

Mars keeps surprising the experts. Consider methane, for example. On Earth
some 90 percent of this gas comes from biological sources -- cows,
termites, rice plants -- so why is it also being made on (presumably)
sterile Mars? Geological processes such as volcanism contribute hardly any
methane at all.

A new study of Martian atmospheric chemistry by Sushil K. Atreya
(University of Michigan) implies that methane production might be quite
active on the Red Planet and that scientists cannot rule out a biological
source. Atreya examined possible abiogenic sources of Martian methane....

http://SkyandTelescope.com/news/article_1585_1.asp