One of the mysteries about Mars is why its northern hemisphere consists largely of lowlands with few craters but its southern hemisphere is made up of heavily cratered highlands. On page 905, an international team confirms the existence of ancient craters below the smooth northern plains, which suggests the buried crust is close in age to the exposed crust in the southern hemisphere.

The data were collected by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument onboard the European Space Agency's Mars Express spacecraft.

MARSIS was developed by the Italian Space Agency and NASA. It was built by aerospace firm Alenia Spazio guided by the University of Rome 'La Sapienza' in partnership with the Jet Propulsion Laboratory in Pasadena, California, and the University of Iowa.

MARSIS sends low-frequency radar signals from space that bounce off features buried below the planet's surface. The joint programme helped make the most of limited resources, says Tom Watters, a senior scientist at the Smithsonian Institution's National Air and Space Museum in Washington DC. “It's a great way for the United States and Europe to partner on space exploration,” he says. “Budgets are always an issue and costs for these missions are always on the increase.”