New calculations reveal the crater known as the Borealis basin measures 8,500 kilometres across and 10,600 kilometres long -- the size of Asia, Europe and Australia combined. It's believed to be four times bigger than the current titleholder, the South Pole-Aitken basin on Earth's moon.

The idea of an ancient impact was first advanced by Steve Squyres of Cornell University and Don Wilhelms of the U.S. Geological Survey in the 1980s. Squyres, currently the lead scientist for the twin Mars rovers, had always hoped other scientists would "pick that ball up and run with it."

"The shape is really one of the key pieces of evidence that it was probably formed in a giant impact," said MIT postdoctoral researcher Jeffrey Andrews-Hanna, whose original "gut feeling" favoured the other theory.