IF LIFE exists on Titan, Saturn's biggest moon, we could soon know about i. The chemical signature of microbial life could be hidden in readings taken by the European Space Agency's Huygens probe when it landed on Titan in January. Titan's atmosphere is about 5 per cent methane, and Chris McKay of NASA thinks that some of it could be coming from methanogens, or methane-producing microbes. Now he and Heather Smith in Strasbourg, France, have worked out the likely diet of such organisms on Titan. They think the microbes would breathe hydrogen rather than oxygen, and eat organic molecules drifting down from the upper atmosphere. “The microbes might breathe hydrogen rather than oxygen, and eat molecules drifting down from the upper atmosphere” McKay and Smith calculate that if methanogens are thriving on Titan, their breathing would deplete hydrogen levels near the surface to one-thousandth that of the rest of the atmosphere. Detecting this difference would be striking evidence for life, because no known non-biological process on Titan could affect hydrogen concentrations as much.