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Mars whacked by object bigger than Pluto

'We've found the crater to prove it,' boffins claim

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New data from NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor backs a theory that the Red Planet's huge northern hemisphere Borealis basin was created by an impact 3.9bn years ago by a body some 1,900km (1,200 miles) in diameter, or larger than Pluto.

The impact theory was rolled out in 1984 to explain why Mars boasts "two strikingly different kinds of terrain in its northern and southern hemispheres". The former's surface lies roughly 6km (3.7 miles) lower than that of the latter, suggesting that the crust was blown away by a massive collision which subsequently "greatly influenced the planet's evolution", New Scientist explains. The basin is vast measuring 8,500km (5,300 miles) by 10,600km (6,600 miles) across and covering 40 per cent of the Martian surface.

The northern hemisphere basin is noticably smoother than the rugged southern hemisphere, and the difference in altitude between the two "meant that ancient outbursts of liquid water tended to flow from south to north". Furthermore, higher atmospheric pressure in the north provokes more surface-scouring winds, making it "likely that more dust has been blown from north to south than the other way".

However, the theory ran out of steam because the basin is visibly kidney-shaped, rather than circular or elliptical in common with other impact craters.

In fact, the Borealis basin is elliptical, something concealed by "giant volcanoes [which] formed along one part of the basin rim and created a huge region of high, rough terrain that obscures the basin's outlines", as NASA puts it.

The volcanic activity was a natural result of the thinner crust resulting from the planetary pile-up, and by using "a combination of gravity data, which tend to reveal underlying structure, with data on current surface elevations", Jeffrey Andrews-Hanna and Maria Zuber of MIT, along with Bruce Banerdt of NASA's Jet Propulsion Laboratory were able to "reconstruct a map of Mars elevations as they existed before the volcanoes erupted".

Banerdt said: "In addition to the elliptical boundary of the basin, there are signs of a possible second, outer ring - a typical characteristic of large impact basins."

While the new data is "convincing some experts who doubted the impact scenario", Andrews-Hanna cautioned: "We haven't proved the giant-impact hypothesis, but I think we've shifted the tide."

The researchers' findings are published in the latest issue of Nature. ®

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