Page last updated at 20:00 GMT, Wednesday, 25 June 2008 21:00 UK

Mars' two-faced riddle 'solved'

Artist's impression of Mars impact (Jeff Andrews-Hanna)
The impact struck the northern half of the Red Planet

The puzzle of why the northern and southern hemispheres of Mars look so different may now have been solved.

Mars' crust is thicker in the southern hemisphere, and magnetic anomalies are found in the south but not the north.

New studies in Nature magazine suggest that a massive space rock smashing into the planet could have created an abrupt disparity between the two halves.

This asteroid would have been close to the size of Earth's moon and hit Mars' northern regions, scientists say.

According to one group of researchers, the rock struck with an energy equivalent to a quadrillion atomic bombs like the one dropped on Nagasaki in 1945.

Mars' northern hemisphere is an enormous lowland basin which might once have held a mighty ocean.

It's a very old idea, but nobody had done the numerical calculations
Francis Nimmo, UCSC

The southern hemisphere, on the other hand, comprises rugged, crater-pitted highlands with an altitude up to 8,000m (26,000ft) greater than the north.

The new research suggests Mars bears the largest impact scar known anywhere in the Solar System.

It challenges an alternative theory which proposes that the "two faces" of Mars are the result of enormous volcanic disruption 3.8 billion years ago.

Deep impact

The scientists on the latest work used data from two Mars-orbiting spacecraft: Reconnaissance Orbiter and Global Surveyor.

Researchers led by Francis Nimmo at the University of California, Santa Cruz, US, carried out computer simulations to show that an impact with particular conditions could produce the present-day appearance of Mars.

These conditions indicate a space rock about one-half to two-thirds the size of Earth's Moon, striking the Red Planet at an angle of 30 to 60 degrees. This would have produced an elliptical crater.

"It's a very old idea, but nobody had done the numerical calculations to see what would happen when a big asteroid hits Mars," said the associate professor of Earth and planetary sciences at UCSC.

Mars Global map (Nasa)
Elevations in the Martian south are higher than in the north

The team's findings are corroborated by another study in Nature led by Oded Aharonson, associate professor of planetary science at the California Institute of Technology (Caltech), also US.

"The dichotomy is arguably the oldest feature on Mars," Dr Aharonson explained. The feature arose more than four billion years ago, before the rest of the planet's complex geological history was superimposed."

This was about the same time that a much bigger object slammed into the Earth, throwing material into orbit around our infant planet. This material is thought to have coalesced to form the Moon.

Indeed, the coincidence in timing of the formation of our Moon and the Mars dichotomy is probably not coincidental at all.

Tail end

"It happened probably right at the end of the formation of the four terrestrial planets - Mercury, Venus, Earth and Mars," said Craig Agnor, a co-author on the Francis Nimmo study.

He told BBC News: "We think the planets formed out of a disc of rocks. As the rocks collide, you get bigger rocks and so on. Eventually, you end up with four planets and a lot of rocks - of various sizes.

"In terms of the process of the planets sweeping up the last bits of debris, this could have been one of the last big bits of debris."

Shock waves from the impact would travel through the planet and disrupt the crust on the other side, causing changes in the magnetic field recorded there.

The predicted changes are consistent with observations of magnetic anomalies in the southern hemisphere, according to Dr Nimmo.

In a third study published in Nature, Jeffrey Andrews-Hanna and Maria Zuber of the Massachusetts Institute of Technology (MIT) in Cambridge, and Bruce Banerdt of Nasa's Jet Propulsion Laboratory (JPL) in California, provide impact evidence from gravitational and topographical signatures on Mars.

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