Giant crater explains strange shape of Mars

WASHINGTON (Reuters) - A giant crater made by an asteroid or comet explains why Mars is so lopsided, with a basin on one hemisphere and high terrain on the other, three separate teams of scientists said on Wednesday.

The impact gouged out a hole 5,200 miles across and 6,500 miles long -- the size of the combined areas of Asia, Europe and Australia, the researchers reported in the journal Nature.

It would be the largest impact yet found in the solar system.

The three studies describe the true size of the depression, sometimes called the Borealis Basin. Some of the edges have been erased by volcanic activity, they said.

It appears to have 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.

Jeffrey Andrews-Hanna and colleagues at the Massachusetts Institute of Technology, and Bruce Banerdt of NASA's Jet Propulsion Laboratory in California said the impact theory, originally proposed in 1984 by Mars expert Steven Squyres of Cornell University, 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 by a Mars-sized body," they noted.

Whatever made the Martian basin was huge, they added -- between 1,600 km (1,000 miles) and 2,700 km (1,600 miles) in diameter.

Mars is just 6,780 km (4,200 miles) in diameter.

In a second report, Margarita Marinova and colleagues at the California Institute of Technology say they 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," said Francis Nimmo of the University of California, Santa Cruz.

He said 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.

"We haven't proved the giant-impact hypothesis, but I think we've shifted the tide. The majority of the evidence is now in favor of the giant impact," Andrews-Hanna said in a statement.

(Reporting by Maggie Fox; Editing by Eric Walsh)