Last month, the Curiosity team announced that they had not detected any methane on Mars. This was something of a disappointment, as earlier studies had suggested, somewhat controversially, that methane was present at detectable levels- and on, Earth, anyway, the vast majority of methane is produced biologically. Furthermore, given Mars’ atmosphere, methane is not stable over long periods of time- so for it to be there, something has to be actively producing it. While there are geologically processes that can generate methane, there’s a fairly simple way to determine if the methane in question is biological in origin, by measuring the ratio of isotopes of carbon in the methane (biological systems prefer lighter isotopes, because, in plain English, they weigh less and take less energy to move around). As a matter of fact, Curiosity had equipment on board that could make this particular measurement, and, needless to say, the negative result has been a bit anticlimactic.
However, this doesn’t rule out life on Mars. In retrospect, methanogenesis seems like an unlikely energy source for a Martian microbe to use- it involves a very complex metabolic pathway, with lots of different enzymes and cofactors. And while you can generate methane from CO2, most of the methanogenesis on Earth has its origins in the decay of organic matter- something there’s probably not going to be a whole lot of on Mars, given how oxidizing the chemical environment is.
Furthermore, there are lots of other, greater sources of energy- Mars is chock full of sulfur and iron compounds, which can make terrific “food” for chemolithoautrophic microbes. Methanogenesis pales somewhat in comparison.
While, again, it was a bit of a disappointment, there’s still much that has yet to be revealed about our neighboring planet.