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For decades, scientists have wondered why the terrain in Mars’ northern and southern hemispheres are distinctly different: smooth in the north, and rough and cratered in the south. Some thought the difference was caused by an ancient collision, while others thought it a result of internal geological processes.

Jeffrey Andrews-Hanna, a postdoctoral researcher at the Massachusetts Institute of Technology, now says that data from two spacecraft orbiting Mars point to an impact. He suggests that Mars’ Borealis Basin, a giant feature located in the northern hemisphere and covering about 40 percent of the planet, is actually an impact crater. If so, at 5,300 miles (8,500 km) wide, it would be the largest known in the solar system.

The basin’s shape is not readily apparent today because geological changes in the 3.9 billion years since the impact have changed the landscape of Mars. For instance, lava-spewing volcanoes that formed along the basin’s ridge have built up mountains that cover up one side of the basin.

Mars Reconnaissance Orbiter and Mars Global Surveyor recorded the strength of the planet’s gravity over the basin, and mapped its surface elevations. Combined, these data helped planetary scientsists deduce what the region looked like before the volcanoes formed: a giant ellipse.

Andrews-Hanna calculates that to create such a giant crater, the object crashing into Mars had to be about 1,200 miles (1,920 km) wide — almost as big as Pluto. RJ