On Earth, methane is something of a villain—a powerful greenhouse gas that is far more effective than carbon dioxide at trapping heat. But it is also a marker of life that emerges by and large from biological processes, such as the digestive systems of cows and termites. So when methane was identified on Mars in 2003 and 2004, efforts were rekindled to seek out life there, either historical or extant.

via Methane on Mars: Is Something Organic Brewing on the Red Planet?: Scientific American.