Mercury's formation impact splattered Earth with material (Forwarded)

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EMBARGOED FOR 02:01 BST, WEDNESDAY, 5 APRIL 2006 (01:01 GMT,
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Ref.: PN 06/17 (NAM 10)

MERCURY'S FORMATION IMPACT SPLATTERED EARTH WITH MATERIAL

New computer simulations of Mercury's formation show the fate
of material blasted out into space when a large proto-planet
collided with a giant asteroid 4.5 billion years ago. The
simulations, which track the material over several million
years, shed light on why Mercury is denser than expected and
show that some of the ejected material would have found its way
to the Earth and Venus.

"Mercury is an unusually dense planet, which suggests that it
contains far more metal than would be expected for a planet of
its size. 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 reaccreted following the impact," said Dr Jonti
Horner, who is presenting results at the Royal Astronomical
Society's National Astronomy Meeting on 5th April.

To solve this problem, Dr Horner and his colleagues from the
University of Bern ran two sets of large-scale computer
simulations. The first examined the behaviour of the material
in both the proto-planet and the incoming projectile; these
simulations were among the most detailed to date, following a
huge number of particles and realistically modelling the behaviour
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.
Ejected particles were tracked until either they landed on a planet,
were thrown into interstellar space, or fell into the Sun. The
results allowed the group to work out how much material would have
fallen back onto Mercury and investigate other ways in which debris
is cleared up in the Solar System.

The group found that the fate of the debris depended on whereabouts
Mercury was hit, both in terms of its orbital position and in terms
of the angle of the collision.

Whilst purely gravitational theory suggested that a large fraction
of the debris would eventually fall back onto Mercury, the
simulations showed that it would take up to 4 million years for 50%
of the particles to land back on the planet and in this time many
would be carried away by solar radiation. This explains why Mercury
retained a much smaller proportion than expected of the material in
its outer layers.

The simulations also showed that some of the ejected material made
its way to Venus and the Earth. While this is only a small fraction,
it illustrates that material can be transferred between the inner
planets relatively easily. Given the amount of material that would
have been ejected in such a catastrophe, it is likely that there is
a reasonable amount (possibly as much as 16 million billion tonnes
[1.65x10^19 kg]) of proto-Mercury in the Earth.

IMAGES

[http://www.ras.org.uk/images/stories...formation1.jpg
(46KB)
http://www.ras.org.uk/images/stories...formation2.jpg
(65KB)
http://www.ras.org.uk/images/stories...formation3.jpg
(55KB)]

Caption: The images show the evolution of the impact over the first
three hours from the time of collision. The red colour on the plots
shows the 'core' material (iron), while the blue shows the lighter
mantle material (silicates, modelled as a rock type called dunite).
© Horner et Al 2006

NOTES FOR EDITORS

The 2006 RAS National Astronomy Meeting is hosted by the University
of Leicester. It is sponsored by the Royal Astronomical Society,
the UK Particle Physics and Astronomy Research Council (PPARC), the
University of Leicester and the National Space Centre, Leicester.

CONTACTS

Dr. Jonathan Horner
Physikalisches Institut
Space Research & Planetary Sciences
Sidlerstrasse 5, 3012 Bern, Switzerland
Tel: ++41 (0) 31 631 8546
Fax: ++41 (0) 31 631 4405
Web: http://www.phim.unibe.ch/~horner/

From Tuesday 4th - Wednesday 5th April, Dr Horner can be contacted
via the NAM press office (see above).