A giant crater made by an asteroid or comet is the reason Mars is so lopsided, scientists said today.

The impact gouged out a hole 5,200 miles across and 6,500 miles long, leaving a basin covering 40 per cent of the red planet, researchers reported in the journal Nature.

The depression is the size of the combined areas of Asia, Europe and Australia, which makes it by far the largest crater in the solar system.

In 1984 scientists proposed an impact had caused the two-faced appearance of Mars with two strikingly different kinds of terrain in its northern and southern hemispheres.

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©Reuters
An artist's impression of a huge asteroid impact that would explain Mars' lopsided shape


This fell into disfavor because the 'Borealis Basin' didn't seem to fit the expected round shape

However, the latest three studies said some of the basin's edges have been erased by volcanic activity.

"We haven't proved the giant-impact hypothesis, but I think we've shifted the tide," said Jeffrey Andrews-Hanna, a researcher at the Massachusetts Institute of Technology (MIT).

It appears the crater held an ocean in the early days of the planet, before Mars lost so much of its atmosphere and the water either sublimated away or froze beneath the surface.

NASA's Phoenix Mars Lander last week scraped through the dry red dust covering the planet's surface to reveal what appears to be white ice underneath.

Mr Andrews-Hanna and MIT colleagues said the impact theory best explains the crater.

When the solar system was just maturing 4 billion years ago, big objects often smashed into one another. The formation of the Earth's Moon is attributed to a giant impact on the Earth.

In a second report, Margarita Marinova and colleagues at the California Institute of Technology made three-dimensional simulations of the impact.

'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. We showed that you really can form the dichotomy that way,' they said.

The shock waves from the impact would have traveled through the planet and disrupted the crust on the other side, causing changes in the magnetic field.

In a third report, Nimmo and colleagues said such magnetic anomalies have been measured in Mars' southern hemisphere.