Simulations give Mercury insight

By Jonathan Amos
BBC News science reporter, in Leicester

The planet is denser than expected for a planet of its size

Computer simulations shed new light on why Mercury is a very dense planet.

The innermost world has long been thought to be rich in iron, suggesting its lighter, outer layers were probably lost in a huge collision.

Modelling work by scientists from the University of Bern, Switzerland, strongly supports this theory.

It shows how much of the material that was thrown out in the catastrophe was never accreted back on to Mercury, leaving it with a large, metal core.

"A lot of the debris from the collision did fall back on to Mercury and some of it is still floating around, but the majority of it would likely have been cleared away in 5-10 million years by gravitational and other forces," said Bern's Dr Jonti Horner.

The researcher was presenting his university's findings here at the UK National Astronomy Meeting in Leicester.

Scientists think some very big collisions have shaped the Solar System we know today.

The Earth and the Moon, Pluto and its moon, and even Uranus's tilt - all are probably the result of huge impacts billions of years ago.

Mercury's present state has been put down to a collision between a large projectile about half the size of the current planet and a proto-world about twice the size of the world we know today.

The simulations plot what happens to the ejected material In detail

Modelling at Bern has already shown how a glancing blow on the proto-world could strip off its outer layers to leave an object that was dominated by heavy metal-rich core material.

The puzzle has been why this body did not then re-accrete most of the ejected debris as it continued to circle the Sun.

But Horner and colleagues think they have solved this problem.

By studying the paths of the ejected wreckage in detailed simulations, the team can demonstrate that Mercury cannot gather up the material fast enough before a range mechanisms - the forces of gravity and the pressure of sunlight - clear its path of fragments.

"It simply takes too long for Mercury to grab the material back," said Dr Horner.

Mercury's composition today is roughly 70% metal (iron and nickel) and 30% silicates.