Saw this on the Unmannedspaceflight.com site:

Volcanoes ruled out for Martian methane.
10:26 04 November 2005
NewScientist.com news service
Jeff Hecht
...
"Earth’s volcanoes release methane, but they generate thousands of times more sulphur dioxide, which has an even shorter atmospheric lifetime than methane – just two years. So finding sulphur dioxide would mean volcanic vents on Mars were actively releasing the gas, and presumably methane as well.
However, high-resolution measurements revealed no trace of Martian sulphur dioxide. From that, Krasnopolsky calculated that Mars’ atmosphere could contain no more than one part per billion of the gas. This in turn means that Martian volcanoes can be releasing no more than one seven-hundredth as much sulphur dioxide as terrestrial volcanoes – and far too little methane to sustain the levels recorded in the Martian atmosphere.
Biological origins
"I can't rule out a geological source of methane" because there are many conceivable mechanisms, Krasnopolsky told New Scientist, but ”this makes biological methane more plausible".
Others would not go so far. The sulphur dioxide measurements seem convincing, says Jim Lyons, a planetary scientist at the University of California at Los Angeles, US. But while the lack of sulphur dioxide may ruled out the presence of methane-spewing volcanoes, it leaves open other possibilities. Perhaps heat diffusing from buried magma is driving reactions between underground moisture and minerals, producing methane but no sulphur dioxide."
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http://www.newscientistspace.com/article/dn8256-volcanoes-ruled-out-for-martian-methane.html

- Bob Clark