28 Jun 07

The Earth's metallic core contains a few per cent of silicon, according to new research by Oxford University scientists.

The surprising discovery, reported in this week's Nature, could help confirm a theory first put forward in 1952 that the Earth's core was too light to be made of pure iron and must contain several per cent of an unknown lighter element - or elements - of lower atomic weight.

The findings suggest that silicon was already incorporated in the Earth's core before the Moon formed. They also mean that it is likely that the Earth's core was formed under very high pressures and temperatures under conditions different from those associated with the formation of other planets, such as Mars.

Postdoctoral researcher Bastian Georg and Professor Alex Halliday of Oxford University's Department of Earth Sciences carried out the research, which involved comparing the relative proportion of different isotopes of silicon on Earth with bodies from elsewhere in the solar system. This was done by dissolving meteorites and comparing their isotopic compositions with those of rocks from Earth, using a particularly powerful mass spectrometer at the ETH in Zurich. What they found was that, overall, rocks from Earth have a greater proportion of heavier silicon isotopes compared to rocks from other planets and astronomical objects.

'The most likely explanation for these findings is that, on Earth, silicon is divided between silicon dissolved in the metal core and the silicate mantle and crust that makes up the majority of the Earth,' said Professor Halliday. 'Preliminary evidence suggests that very high pressure, high temperature conditions during the formation of the early Earth could have caused this to occur.'

It may be that the relatively large mass of the Earth is the key to its unusually violent birth. The Earth is eight times larger than Mars and this could have caused higher pressure conditions than those found during the formation of other planets. This extra mass would also have caused larger amounts of energy to be released as its gravity attracted, and then caused it to collide with, other planets and planetary embryos.

Professor Halliday said: 'We also found that the Moon has the same silicon isotopic 'fingerprint' as the Earth. Because of its small size this cannot have been caused by high-pressure core formation. Instead it is good evidence for the theory that the collision between the Earth and another planet that created the Moon was so energetic that atoms of the disk from which the Moon formed mixed with those from the silicate Earth in a rock vapour atmosphere. This means that the silicon in the silicate Earth must have already had a heavy isotopic composition before the Moon formed.'

A report of the research, entitled 'Silicon in the Earth's core', is published in Nature on 28 June. The research was carried out by Bastian Georg, working with Professor Alex Halliday from Oxford University's Department of Earth Sciences, in collaboration with Edwin Schauble from the University of California, Los Angeles and Ben Reynolds at the ETH in Zurich. It was supported by grants from the Swiss National Science Foundation, the UK's Science and Technology Facilities Council and the USA's National Science Foundation.