According to some geologists, the atmospheric methane of Mars can have a simple explanation: the olivine of the rocks.
Biologists at Dartmouth College (USA) estimate that the annual transformation of eighty thousand tons of olive tree would be enough to generate the amount of methane calculated by NASA. And it seems that this amount of olive is very possible.
The origin of Martian methane has been quite controversial — some question the presence of methane — since it has been speculated that it was due to the activity of bacteria.