In the first published study to track methane on Mars, researchers have concluded that life is the only plausible source of the gas. The putative martians are hiding in a few isolated spots and the rest of the planet is totally sterile, they say. Teams at conferences have already discussed finding martian methane. But Vladimir Krasnopolsky, an atmospheric scientist from the Catholic University of America in Washington DC, says that his study, to be published shortly in the peer-reviewed journal Icarus, is the first hard evidence for methane on the planet.