BBC News Online: Sci/Tech

How the moon was made

A huge impact created the Moon

The result is a happy one for scientists, because older simulations did not fit too well with observed reality.

Robin Canup of the Southwest Research Institute, Boulder, Colorado, US and Erik Asphaug of the University of California, Santa Cruz, were able to use much more powerful computers than their predecessors.

Easier job

Now they have a much easier job matching up the model to the current orbits and compositions of the Earth and its satellite.

As Jay Melosh of the University of Arizona explains in the journal Nature, the key has been not just adding raw computer power, but refining the simulation.

"It has taken squadrons of physicists in the United States, Russia and elsewhere nearly 50 years to come up with computers and three-dimensional computer codes that can adequately treat the effects of impacts and explosions under relatively simple conditions in which self-gravity is not important.

"Adding self-gravity to these codes therefore posed a formidable challenge," he writes.

Three dimensions

The new model is the highest-resolution computer model so far of the birth of the Moon.

It takes into account in three dimensions both the thermodynamic effects of another planet hitting the Earth and the gravitational interactions between all the pieces which were dislodged.

Previous simulations have left scientists conjecturing a much bigger impacting planet, a much smaller Earth or even more than one collision.

They pointed, too, to an earlier collision, when the Earth was less fully formed.

Had this really taken place, the Moon would have probably been much more iron-rich than it really is.

Details of the research are published in the journal Nature.