(This probably won’t make too much sense if you read it before the original post, "Methane (and thus life?) on Mars")
First, some more details on non-biological sources.
I asked for a little more info on methane coming out of Martian volcanoes from John Eiler, a geochemist at Caltech who’s thought a lot about these things, and he generously sent a full and very interesting reply. He says his best bet for the amount of methane associated with martian volcanism -- based on how oxidized martian rocks seem to be, and on the elemental ratios in the martian meteorites -- would be that lava at the surface, or magma injected into sub-surface intrusions, is probably significantly richer in methane on Mars than it is on earth: between 0.1% and 1% methane by volume, “with a less plausible upper bound of up to 10%”. This may mean that a fairly moderate amount of lava or magma giving up the gas dissolved in it could pump out the observed low level of methane. In that case, the methane would not mean life, but would mean active geothermal systems with lava/magma.
As I understand him, Eiler notes that water vapour, carbon dioxide, carbon monoxide and hydrogen would all be more abundant in the lava than methane, which means that if the methane comes from volcanism, there should be other shortlived gases present in the artmosphere as well, such as carbonyl sulfide, carbon monoxide and “various other sulfur gases and hydrocarbons”. Obviously these would provide new targets for spectrometers to seek out.
He also points out that there were other ways to get methane out of rock, without the benefit of bacteria. Water rich in carbonic acid (H2CO3 -- what you get when you dissolve carbon dioxide in water) running over or through iron-bearing minerals in which the iron isn’t fully oxidized may oxidize those minerals, giving off methane (and hydrogen) in the process. “These reactions,” Eiler says, “are believed to be a significant source of methane in gas seeps from the Canadian shield, parts of Indonesia, and seafloor hydrothermal vents, and it is quite plausible that they generate large amounts of methane in the martian crust.”
If this is actually going on without any bacteria to help it, and constitutes a source large enough to account for the methane that has been observed, then the methane would not be evidence of life. But it would be evidence for quite copious amounts of water in the martian crust today; that’s a pretty good consolation prize, it seems to me. (It would also show that what are called redox reactions are going on in the crust, and those are the sorts of reactions that life uses to get its energy)
It would, I imagine, be possible to tell the hypotheses apart if you were on Mars with a good mass spectrometer. If martian bacteria were anything like earthly bacteria, they would be choosy about carbon isotopes, using the lighter form of carbon rather than the heavier form. Reactions between acidic water and iron-bearing rocks, though, will not care about the weight of the carbon involved. So biogenic methane (methane made by living things) can be expected to weigh less than methane made by any other process. Beagle 2’s Gas Analysis Package would, I think, have been able to tell light methane from heavy methane. I don't know whether any instrument currently slated for Mars landing can do this with methane present only at about ten parts per billion. Maybe the TEGA gas analysis instrument on the 2007 Phoenix lander.
(It is obviously not very likely that, if the biological methane production process on Mars was due to some form of life completely unlike earthly methanogens, they would change the carbon isotope signature in the same way. But I think it’s a fair assumption that any living process would fractionate the methane to some extent, one way or another, and so produce methane with a different isotopic balance to that seen in the planet’s carbon dioxide reservoirs.)
A possible non-biological source suggested by another expert who's thought a lot about detecting life on Mars is that the methane might be due to sunlight breaking down carbon-bearing molecules that have been carried to the martian surface by meteoritic dust. The back of a friendly meteorite expert’s envelope suggests 3,000 tonnes of organics might be delivered to Mars every year in meteoritic dust, and the back of my (considerably less reliable, and likely to be wrong) envelope suggests that only 30 tonnes would have to be turned into methane every year to maintain the 10 parts per billion that seem to have been detected in the atmosphere. I have no idea whether that yield is plausible or not, or what other chemicals might be given off at the same time that we might have a chance of detecting. So far, this explanation seems the least exciting, since it keeps the whole process on or above the surface, and thus has no implications for heat or water below, let alone life. That clearly doesn't mean it's wrong.
Finally, here’s some information on earthly methanogens from phylogenomics wizard Jonathan Eisen of the Institute for Genomic Research.
All known methanogens so far are members of the archaea – the kingdom of single-celled microbes that look very much like the “prokaryotic” bacteria we’re all more familiar with, but which are genomically quite distinct. That said, Eisen warns that properties previously considered unique to archaea have now been found in prokaryotes, too, and says he thinks it “reasonably likely” that methanogenesis will eventually be found in bacteria. As I understand it, though, the presence of archaean traits such as the halorhodopsin genes in bacteria is normally taken as evidence of horizontal gene transfer between species (or in this case kingdoms), and most people (maybe not Jonathan) probably do think that methanogenesis arose originally among the archaea.
When people make family trees for the archaea, methanogens tend to turn up on a number of different branches, suggesting that the ability to make a living by making methane evolved on a number of separate occasions. However, according to Eisen, in some of the most recent trees they appear to be collecting together into one subgroup, which might mean it was something archaea were able to do from quite early on.
As to whether archaea on earth might find Mars at all hospitable, Eisen says that, as far as he knows, no one has yet identified methanogens adapted to very cold conditions or to high salt concentrations, though he wouldn’t be surprised to find some cold-adapted methanogens. Some acid tolerant methanogens have recently been described, and acid tolerance might well be handy on Mars.
That’s about it for the methanogens, as of now.
I hope to have more on various aspects of this tomorrow.