Rocks picked up by the Apollo astronauts suggest that the basins which appear to form the Moon's "face" when seen from Earth resulted from a massive flurry of impacts by space rocks some four billion years ago.

But explaining this event, called the Late Heavy Bombardment, has been a matter of controversy.

If the dating is right, the lunar bombardment occurred around 600 million years after the Sun burst into light and the planets start to form, building up from clusters of primitive dust.

By that time, the Solar System should - in theory - have been a relatively calm place. Most of the Solar System's construction debris should have settled in stabilised orbits or been mopped up by the planets, sucked in by gravitational pull.

So how could this bombardment of the Moon have arisen?

The answer may lie in a novel theory that may also explain two other strange features of the Solar System.

Writing on Thursday in the British weekly journal Nature, scientists say the key is in the formation of the Solar System's two giant planets, Jupiter and Saturn, around 4.5 billion years ago.

If there was a time when Saturn completed exactly one orbit of the Sun for every two orbits made by Jupiter, the two planets would trigger a phenomenon called mean-motion resonance, they say.

Under this, one planet's gravitational perturbations affect the other, rather like two giant ships which pass close to another, causing waves that make both vessels bobble and slightly change course.

In the case of Jupiter and Saturn, the resonance eventually had an enormous knock-on effect.

It edged the two planets into wider orbits around the Sun, elongated and tilted their orbital planes and, in turn, eventually scattered the two outermost large planets, Uranus and Neptune.

When Neptune was flung outwards, it in turn scattered an orbiting cloud of rocky debris towards the Sun, some of which smacked into the Moon.

The planetary migration unfolded over hundreds of millions of years, and this explains why the Late Heavy Bombardment occurred relatively late in the history of the Solar System.

If this theory of planetary movement, backed by high-powered computer models, is right, it would also explain why the giant planets have stabilised into eccentric orbits around the Sun instead of neat circular ones.

And it would also provide the answer as to why their inclinations - the tilt of their orbital planes - are so much larger than those predicted by the conventional hypothesis of planetary formation.

Lead authors of the studies are Alessandro Morbidelli of France's Observatoire de la Cote d'Azur and Harold Levison of the Southwest Research Institute in Boulder, Colorado.

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