Have scientists caught the scent of life on Mars? Observations from a European probe circling the planet and a telescope on Earth have detected a wisp of methane in its thin atmosphere. On Earth, most methane, aka marsh gas, comes from living things, such as the microbe-rich goop in swamps. Don’t get in a lather yet. On Mars, the source could well be nonbiological, such as water interacting with hot, volcanic rock under the surface. But even that could raise hopes of Mars life: Heat-loving microbes teem on and within Earth’s undersea volcanic vents. “If this is right, it is very exciting,” exclaimed James Kasting, an atmospheric chemist at Penn State University, as word spread last week at a meeting on astrobiology–the search for alien life–at NASA’s Ames Research Center south of San Francisco.