The remains of the largest asteroid impact crater known anywhere in the solar system have been identified on Mars, explaining the origin of the lowland basin that dominates the planet's northern hemisphere.

The vast depression, which covers an area about the size of Asia, Australia and Europe put together, was carved out about four billion years ago — very soon after the formation of the solar system — when Mars was struck by an object with a diameter of between half and two thirds the size of the Moon, research has suggested.

The collision gave rise to the Borealis basin, which covers 40 per cent of the planet's surface and measures aproximately 5,300 by 6,600 miles (8,500km by 10,600km).

The next largest impact craters known to man are the Hellas basin in Mars's southern hemisphere, and the South Pole-Aitken basin on the Moon.

A bigger impact occurred when Earth was struck soon after its formation, producing clouds of debris that formed the Moon, but it was so catastrophic that there is no evidence left.

The findings, published in the journal Nature by an international team of scientists, promise to explain one of Mars's most mysterious qualities — the way in which its highland and lowland areas have different geology.

In the flat, low-lying plains of the northern Borealis basin, the crust is about 15 miles thinner than in the upland regions on the rest of the planet.

The impact hypothesis was first proposed in 1984 by Steve Squyres, of Cornell University, who now leads Nasa's Mars Rover team, but it had largely fallen from favour with planetary scientists because of a lack of evidence.

It has now been revived by a new computer-assisted analysis of the Borealis basin, which shows its underlying shape to be elliptical and consistent with formation as an impact crater.

Jeffrey Andrews-Hanna, of the Massachusetts Institute of Technology, a leader of the study team, said: “We haven't proved the giant-impact hypothesis, but I think we've shifted the tide. The majority of the evidence is now in favour of the giant impact.”

Francis Nimmo, of the University of California Santa Cruz, another team member, said: “It's a very old idea, but nobody had done the numerical calculations to see what would happen when a big asteroid hits Mars. The impact would have to be big enough to blast the crust off half of the planet, but not so big that it melts everything.”

One of the main problems with the impact hypothesis was that the basin is not roughly circular, as are most impact craters. Craters can also be elliptical if an object strikes at an angle of about 45 degrees, but the basin did not look this shape from orbit, either.

New analysis of Martian geology, using data from spacecraft such as Mars Global Surveyor and Mars Reconnaissance Orbiter, has now shown that the basin is actually elliptical, but that this shape has been partially obscured by volcanic activity along one part of the rim. The object must have struck at an angle of between 30 and 60 degrees, the analysis suggests.

“An impact is really the only mechanism that can produce these large-scale elliptical depressions, these large holes in the ground,” said Dr Andrews-Hanna. The research adds another incident to a growing list of impacts that shaped the planets. “The early solar system was a very dangerous place to be a planet,” added Dr Andrews-Hanna.

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