Researchers are discovering an older, craggier face of Mars buried beneath the surface, using a pioneering sounding radar from the European Space Agency's Mars Express spacecraft.

The radar echoes, captured by the MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) instrument, strongly suggest there are ancient impact craters buried beneath the lowland smooth plains of the Martian northern hemisphere.

MARSIS was provided to the European Mars mission by NASA and the Italian Space Agency. The MARSIS instrument transmits radio waves that pass through the Martian surface and are reflected by the subsurface structures which have a contrast in electrical properties with the materials that buried them.

The project is the first-ever exploration of a planet by sounding radar. "It's almost like having X-ray vision. Besides finding previously unknown impact basins, we've also confirmed that some of the subtle, roughly circular topographic depressions in the lowlands are related to impact features" said Thomas R. Watters, MARSIS team member of the Smithsonian National Air and Space Museum's Center for Earth and Planetary Studies.

Understanding how Mars evolved explains evolution of early Earth. Signs of the forces that modeled Earth a few billion years ago are more evident on Mars because, on Earth, many of them have been obliterated during Earth's more active resurfacing by tectonic activity.

The difference in elevation and apparent age between the northern lowlands and southern highlands of Mars - known as the dichotomy - puzzles the scientists about the geologic evolution of the planet.

Almost the whole southern hemisphere is craggy, with heavily cratered highlands, while most of the northern hemisphere is smoother and lower in elevation. Since the impacts with asteroids that cause craters can happen anywhere on a planet, the areas with fewer craters are interpreted as younger surfaces where geological processes have erased the impact scars.

The abundance of buried craters that the radar has detected beneath Mars' smooth northern plains points to the fact that the underlying crust of the northern hemisphere is extremely old, "perhaps as ancient as the heavily cratered highland crust in the southern hemisphere." That crust was buried first by vast amounts of volcanic lava and then by sediments carried by episodic flood waters and wind.

Photo credit: NASA