Signatures of life
Where there's water, there could be life. "Meteorites from Mars that have landed on Earth show clear evidence that conditions appropriate to life did exist on the planet, including in the recent past," said Colin Pillinger, Consortium Leader for the Beagle 2 lander at the Open University, Milton Keynes, UK. "However, features in the meteorites which have been described as nanofossils are highly controversial. Unfortunately, we cannot be sure that organic matter found in the meteorites is the remnant of organisms that lived on Mars and not due to contamination on Earth. We need to repeat the experiments on rocks that never left the Red Planet."
The Beagle 2 lander would have looked for signatures of life on Mars, whether long-dead or still-living, by measuring the ratio of two different types of carbon in the rock. 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 4000 million years old, even where geological processing has occurred.
On Earth, some life that is still active produces another signature - methane. The simplest biological sources, such as those associated with peat bogs, rice fields and ruminant animals, continuously supply fresh gas to replace that destroyed by oxidation.
Methane also has a very short lifetime on Mars because of the oxidising nature of the atmosphere, so its presence would indicate a replenishing source, which may be life, even if it is buried beneath the surface. If this methane exists, the Mars Express orbiter's PFS intrument will be able to detect this gas in the atmosphere.
The only previous landers to look directly for evidence of life on Mars were NASA's Vikings in 1976. However, Mars's harsh, oxidising atmosphere would almost certainly have destroyed any such evidence on the surface.