I recently speculated that spacecraft both orbiting and sitting upon Mars may have already detected signs of life. In particular, some spacecraft have detected signs of methane:
In 2004 the European Space Agency probe Mars Express detected the presence of methane in the atmosphere of Mars. Methane can be produced geologically (and Mars is not short on volcanoes), or biologically. (Though media reports of that observation got a bit out of hand.) Either way, this is an important observation and research on the source of this methane is still ongoing.
The existence of methane is ambiguous: Though methane is produced biologically, as I wrote above, it's also produced geologically (and, in fact, the methane detected on Mars tends to be both localized and emanating from some of the more volcanic regions). It can also be delivered by comets. Given its ubiquity, methane may raise hopes, but in the end turn out to be a poor biomarker. Detecting life elsewhere will require multiple lines of evidence. That's where chemicals like hydrogen sulfide and methyl mercaptan (or methanethiol) come in. It was reported recently that the Curiosity Mars Rover will carry a Tunable Laser Spectrometer---an instrument that can detect gases like hydrogen sulfide and methyl mercaptan. Like methane, these chemicals can be created as metabolic byproducts, but like methane hydrogen sulfide is often associated with geological activity. Methyl mercaptan is more likely to be of a biological origin---the chemical is produced by the kind of microorganisms that live within the human digestive track. To be specific, it's one of the gases that gives human flatulence and halitosis their (ahem) aromatic qualities.
In summary one of the instruments on the Curiosity rover is, functionally, a Martian breathalyzer. Am I the only person who finds it comical that our grand, age-old quest to determine "Are we alone?" may find resolution with the bacterialogical equivalent of a "BRAAAAAAAAAAAP"?
