The presence of methane in the Martian atmosphere is a puzzle. Methane is broken down rapidly by sunlight and cannot last long in any atmosphere. A few simple calculations show that the lifetime of a CH4 molecule in martian climes is around 500 years. So methane ought not to exist in the Martian atmosphere at all unless it is being replaced on a regular basis.
So where could it have come from? Astrobiologists find this exciting because the methane in Earth’s atmosphere comes largely from cow farts (or more precisely the bacteria that live in their guts). The absence of cowpats on Mars rules out the presence of ruminants on the red planet but leaves open the possibility that another primary source could be responsible, such as bacteria .
But before they can consider the possibility of life on Mars, astrobiologists must rule out every other possibility. One of these is that clathrates near the Martian surface are constantly releasing small amounts of methane as temperatures and pressure near the surface change.
Now Caroline Thomas et amis at the Universite de Franche-Comte in France have worked out how likely that is and say there are two possibilities.
First, they say that clathrates could only exist near the surface of Mars if the atmosphere had once been methane rich (otherwise the clathrates could never have formed). Perhaps the atmosphere was once temporarily enriched by a comet impact.
Second, there has to be some other source of methane, perhaps biological.
So how to distinguish between these scenarios. The discovery of gray crystalline hematite deposits on the surface could be a proof of an early methane-rich martian atmosphere.
Otherwise a biological source is a real option. Let’s get those rovers a-huntin’.
Ref: arxiv.org/abs/0810.4359: Variability of the Methane Trapping in Martian Subsurface Clathrate Hydrates