Signatures of life   Mars may have lost much of its atmosphere during asteroid impacts early in its history. The Beagle 2 lander will look for signatures of life on Mars, whether long-dead or still-living, by measuring the ratio of two different types of carbon in the rocks. Biological processes on Earth favour the lighter isotope of carbon, carbon-12, over the heavier carbon-13. Hence, a high carbon-12 to carbon-13 ratio is taken as evidence of life and has been found in rocks up to 4 billion years old, even where geological processing has occurred. The hope is that the same occurred on Mars.   Controversy rages over whether this structure, found inside the martian meteorite ALH 84001, is a fossilised bacteria. Last update: 10 October 2005