Methyl Mercaptan, the sulphur bearing molecule responsible for the stench of flatulence, is now being targeted as an indicator sign of life for future probes to Mars. The TLS (Tunable laser Spectrometer) on the next rover to Mars (Curiosity...in 2012), will be used to detect it and other compounds that maybe present on Mars. According to a paper in the journal of Planetary and Space Science, the gas should be detectable in concentrations as small as 100 parts per billion. If found, it will be almost conclusive proof of the existence of extant life being present in the soils of the planet.
http://www.newscientist.com/article/...e-on-mars.html