Thirty days to create a moon

November 10, 2000

It may have taken God six days to create the Earth, but scientists have calculated that it took five times as long to make the Moon.

The formation of our planet's sole natural satellite, from the vast disk of debris out of which most theoreticians believe it emerged, has been modelled by Japanese scientists.

Their computer simulations provide the most detailed glimpse yet of the aftermath of the Earth's most traumatic experience - its collision with another planet.

This impact is believed to have occurred early in the history of the solar system, some 4 billion years ago. Its immense violence would have thrown out enormous quantities of pulverised rock, that would have condensed into a vast disk of particles marshalled by the Earth's gravity.

Eiichiro Kokubo and colleagues at the University of Tokyo, used high resolution N-body simulations - a technique that models the behaviour of a system made up of a large number of interacting bodies - to propose what may have happened next. They drew on orbital data and measures of the Moon's composition.

"Our paper is the first to clearly show that the timescale of lunar formation, from an impact-generated disk of debris, is in the order of a month," Kokubo said.

At the start, the Earth would have been girded by a spinning disk of particles between two and four times the mass of the Moon.

As bits of rock debris collided and formed into clumps, this disk would have begun to shrink. Within the Roche limit - the Earth's "exclusion" zone in which its gravitational tidal forces break other celestial bodies apart - the clumps would have been stretched into spiral, arm-like structures, reaching out into space like seats on a fairground carousel.

The gravitational torque would have pushed particles out along the spiral arms. However, where the tips of the arms reached the Roche limit, beyond the destructive influence of the Earth, the particles would have collapsed into moonlets under the force of their own gravity.

These moonlets would have clumped together to form a lunar seed, sweeping around its orbit beyond the Roche limit, mopping up the particles pouring away from the spiral arms.

As it grew, this nascent moon would have begun to exert its own gravitational pull on the disk, capturing its outer limits and pushing the rest back into the Earth.

In all, the Moon would have picked up between 10 and 55 per cent of the material that started out in the impact disk. All this in 30 days. The findings will appear in solar system studies journal Icarus .

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