Scientists at University of Bern, Switzerland, produced the simulations, which depict the fate of material blasted out into space when a large proto-planet collided with a giant asteroid about 4.5 billion years ago, in the early years of the solar system.

"Mercury is an unusually dense planet, which suggests that it contains far more metal than would be expected for a planet of its size,� said team leader Jonti Horner, who presented the research at a meeting of the Royal Astronomical Society.

"We think that Mercury was created from a larger parent body that was involved in a catastrophic collision, but until these simulations we were not sure why so little of the planet's outer layers were re-accreted following the impact."

To solve the problem, the team ran two sets of large-scale computer simulations. The first examined the behavior of the material in both the proto-planet and the incoming asteroid. The simulations were among the most detailed to date, following a huge number of particles and realistically modeling the behavior of different materials inside the two bodies.

At the end of the first simulations, a dense Mercury-like body remained, along with a large swathe of rapidly escaping debris. The trajectories of the ejected particles were then fed in to a second set of simulations that followed the motion of the debris for several million years.

A second simulation tracked the ejected particles until they landed on a planet, were thrown into interstellar space, or fell into the Sun. The results revealed how much material would have fallen back onto Mercury and allowed the researchers to investigate ways that debris is cleared within the solar system.

The group found that the fate of the debris depended on where Mercury was hit, in terms of its orbital position and the angle of the collision. Prevailing gravitational theory suggested a large fraction of the debris eventually would fall back onto the planet, but the simulations showed it would take up to 4-million years for 50 percent of the ejecta to return to Mercury, enough time for much of it to be carried away by solar radiation.

This explains why Mercury retained a much smaller proportion than expected of the material in its outer layers, Horner explained. He said the simulations also showed a small fraction of the ejected material made its way to Venus and Earth - a finding that illustrates how easily material can be transferred among the inner planets.

Given the amount of material that would have been ejected in such a catastrophe, Horner said, Earth could contain as much as 16 quadrillion tons of proto-Mercury particles.

Related Links
RAS 2006
Royal Astronomical Society