There are tantalising clues that water once flowed on Mars. Pictures sent back by space probes show gullies and canyons that seem to have been carved by rivers and lakes. This water may now be trapped under the surface, possibly in huge quantities.
Mars Express will search for this water using a ground-penetrating radar, known as MARSIS, that can probe up to 3000 metres below the surface.
If water exists, so too may life. Some scientists claim that rocks from Mars that have fallen to Earth contain evidence of primitive microbes.