Methane has been spotted in the atmosphere of Mars by several researchers, reigniting speculations about the possibility of life on the red planet. On Earth, methane is a common by-product of the metabolism of single-celled organisms. So its presence in the martian atmosphere could be a sign of bacteria still living on the planet. But that isn’t the only possible explanation, says Vittorio Formisano of the Institute of Physics of Interplanetary Space in Rome, who helped to confirm the finding. The methane could be produced by purely geological processes, such as volcanic activity.