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If Beagle 2 eventually does land on Mars, what signs of extraterrestrial life will it find? Little green men are out but Mars and other planets may well harbour micro-organisms, bacteria or other relatively simple life forms.

According to Monica Grady, Mars is in many ways like Earth. Both planets are rocky and were formed from the same source as the rest of the solar system.

There are signs of former lakes and rivers on Mars's surface and, even though the planet's atmosphere is now very thin, there is evidence to suggest it was once thicker.

"We don't know when the water disappeared," explains Grady. "But there may well have been an atmosphere, a clement temperature, presumably methane as the planet cooled, and carbon dioxide. It was probably warmer and wetter than now, possibly with all the building blocks of life. On Earth the earliest fossils are from 3.5 million years ago. We think Mars had water then, so why should life not have formed there too?" Although today's thin atmosphere almost certainly precludes organic material on Mars's surface, life may still survive underground, perhaps living off the rocks themselves, but any remaining life is almost certainly primitive.