As Mars Express sends back the best ever data about the chemicals present in the martian atmosphere, rumours abound that scientists are beginning to detect signs of life on the red planet. What signs of life are scientists looking for on Mars? The smells of digestion. Life, as we know it, depends on chemicals built up from carbon and nitrogen, and whenever those chemicals break down they release gases like methane (CH4) and ammonia (NH3). Some bacteria on Earth get their energy by reacting carbon dioxide with hydrogen to make methane and water. Such ‘methanogenic’ bacteria are prime candidates for life on Mars, because they do not need sunlight or oxygen to survive.