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. ®
Mars
An asteroid the size of Pluto that slammed into the Northern hemisphere of Mars created the Borealis basin, based on the latest survey of NASA's Mars Reconnaissance Orbiter and Mars Global Surveyor.

The report released by the magazine ''Nature'' explains the 1984 observation of the unusual shape of Mars terrain in the northern hemisphere against the southern hemisphere.

An asteroid with a body of 1,900 kilometers (1,200 miles) smashed into the planet some 3.9 billion years ago, creating the vast Borealis basin (measuring 8,500km 5,300 miles by 10,600km 6,600 miles across), which occupies nearly 40 per cent of the Martian surface.

The new data has convinced a lot of scientists, but the theory will need further proof before being accepted by the wider scientific community.