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New light shed on how the Earth was formed

27 September 2017

Artists impression of a planet forming

The late-stage building blocks of planetary formation (planetessimals and proto-planets) and the extensive volatile degassing that took place.
(Credit: Oxford University)

UK research may help to explain why the Earth is formed differently to other planets in our solar system.

A research team, based at Oxford University and funded by the Science and Technology Facilities Council (STFC), studied the processes behind the formation of the planets in our solar system, trying to find out why Earth has such a short supply of elements like lead, zinc and copper compared to the other planets, but an overabundance of indium.

Scientists already know that the solar system began as a disc of dust and gas surrounding the centrally-growing sun. The gas condensed to solids which accumulated into larger rocky bodies like asteroids and mini-planets. Over a period of 100 million years these mini-planets collided with one another and gradually accumulated into the planets we see today, including the Earth.

Although it is widely understood that the Earth was formed gradually, from much smaller bodies, many of the processes involved in shaping our growing planet are less clear.

The conventional explanation as to why our planet has a relatively short supply of some elements is that the Earth grew without them, and small amounts were added later from asteroids – but this does not explain the overabundance of other elements like indium.

Postgraduate student Ashley Norris and Bernard Wood, Professor of Mineralogy at Oxford’s Department of Earth Sciences, set out to unpick this mystery. In a particular series of experiments they melted rocks at 1300°C in oxygen-poor conditions and determined how the different elements evaporated from the molten lava.

During the experiments each of the elements of interest evaporated by different amounts. The analyses revealed that the evaporation measured in the molten lava experiments align very closely with the pattern of depletion observed in the Earth. In particular, the amount of indium left agrees exactly with its observed abundance in the Earth - its abundance, turns out not to be an anomaly.

Professor Wood said: ‘Our experiments indicate that the pattern of volatile element depletion in the Earth was established by reaction between molten rock and an oxygen-poor atmosphere. These reactions may have occurred on the early-formed planetesimals which were accreted to Earth or possibly during the giant impact which formed the moon and which is believed to have caused large-scale melting of our planet.’

Having focused their original experiments on 13 key elements, the team are now in the process of looking at how other elements, such as chlorine and iodine, behave under the same conditions.

For more information, please see - Oxford University website: The volatile processes that shaped Earth.

Science and Technology Facilities Council Switchboard: 01793 442000