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By Lewis Page

Scientists say they may finally have cracked a long-standing boffinry conundrum – the mystery of why it is that the Moon rocks brought back by the Apollo astronauts of the 1960s and '70s are magnetic. The Moon, unlike the Earth, has no global magnetic field – a compass would not work on the lunar surface – and so its rocks shouldn't be magnetised. But they are.

Now, however, a crack team of boffins believe they may have unravelled the secrets of the remote lunar past and hit upon the cause of the mysterious Moon magnetisation. In essence, they believe that in the past the moon's liquid core may have been driven to spin and churn as Earth's does, generating a magnetic field in a similar fashion. The difference would have been that instead of being driven by heat from the inner core, as in the case of Earth, the smaller lunar core would have been set spinning by motion in the solid mantle covering it.

“This is a very different way of powering a dynamo ... [it] involves physical stirring, like stirring a bowl with a giant spoon,” says Christina Dwyer, grad student at the University of California and one of the team that solved the selenean puzzle.

The Moon is even today slowly receding outward from planet Earth, and has been doing so for a long time. Earlier in its history it was much closer to the mother planet, and tidal effects on the lunar mantle would have caused it to stir up the Moon's core and so generate a magnetic field. This in turn would have magnetised surface rocks, accounting for their condition when the Apollo astronauts found them.

But eventually, after a billion years or so, the Moon lost its magnetism.

“The further out the Moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off,” says Dwyer.

The scientists' research is published in hefty boffinry mag Nature. ®