re: Here is what Krasnopolsky has to say
Methane cannot originate from an extinct biosphere, as
in the case of “natural gas†on Earth, given the exceedingly low limits on organic matter set by the Viking landers and the dry recent history which has been extremely hostile to the macroscopic life needed to generate the gas. Therefore, methanogenesis by living subterranean organisms is the most likely explanation for this discovery.
From: http://www.cosis.net/abstracts/EGU04/06169/EGU04-A-06169.pdf
And he goes on to say that martian biota may be extremely scarce...
Interesting.
Cheers, BRFelix