Making mercury.

A theory about the early solar system suggests that Mercury arose when a giant asteroid struck a large planet 4.5 billion years ago, leaving behind what would become the solar system's innermost planet. New computer simulations indicate that some of the debris from the collision would have found its way to Earth and Venus. The simulations also account for Mercury's abundance of heavy elements.

Jonti Horner of the University of Bern in Switzerland and his colleagues found that the proposed asteroid collision would have ejected into space the lower-density, outer layers of the giant planet. Using simulations, the team then tracked the fate of the debris over several million years.

The findings reveal that the pressure exerted by sunlight would have scattered most of the ejected material before it had a chance to fall back to the planet. That would explain Mercury's high density, Horner says.

The simulation indicates that a small fraction--16 quadrillion tons--of the debris from Mercury, struck its neighbors Venus and Earth and bored into their interiors, he says.

Horner presented the findings at the Royal Astronomical Society's National Astronomy Meeting on April 5 in Leicester, England.--R.C.