Planet-piercing radar has revealed ancient impact craters beneath the surface of Mars. Radio beams from Europe’s Mars Express probe, in orbit around the Red Planet, penetrated the surface to expose its older face.

The echoes showed craters from asteroid impacts a few billion of years ago beneath today’s smooth, low plains of the martian northern hemisphere.

The discovery, revealed today in the journal Nature, will help scientists to understand how Mars and the Earth evolved.

X-ray-like images were produced with the spaceprobe’s Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument, MARSIS.

It helps solve the riddle of why Mars’s southern hemisphere is rough and heavily cratered but the north is smoother. Scientists believe the lowland craters were buried by volcanic lava and then sediment from ancient floods.

UK team member Professor Iwan Williams, of Queen Mary University, London, said yesterday: “These latest results show how MARSIS is able to do much more than just look for water a few kilometres below the surface. In mapping the subsurface of Mars the instrument has found numerous craters in an ancient surface that has since been covered by more recent debris.”

He added: “This discovery changes one of the outstanding problems regarding the martian surface. Instead of asking, ‘Why are the northern uplands young compared to the south’, we now need to ask, ‘Why did only the North get covered with dust or debris?’.”

The ESA image shows a projection of Mars radar results on a false-colour image of Mars.

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