ENVIRONMENT
What’s at the centre of the
earth?
05 January 2016
by Jon Cartwright
Scientists are using diamonds to uncover what makes up the earth’s core. Image credit: Flickr / gnuckx
New experiments that crush material between two diamonds to simulate the extraordinarily high
temperatures and pressures found in the earth's interior are providing answers to the age-old
questions of what our planet is made of, and where its ingredients came from.
‘We are measuring things that were not measured before,’ says geophysicist Prof. James Badro of the
Paris Institute of Earth Physics, France.
Prof. Badro has been working with colleagues on the
DECORE project, backed by the EU’s European
Research Council (ERC), to understand the
composition of the earth’s core. The core is thought to
be mostly iron, yet analyses of seismic waves suggest
that it is not dense enough to be iron on its own.
The best way to understand the precise composition is
to replicate the conditions of the core in the lab. That
means temperatures of thousands of degrees and
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pressures of about 100 gigapascals – over a million
times the pressure of our atmosphere.
To mimic such conditions, Prof. Bardo and colleagues used a diamond anvil cell – a device that
crushes a sample between two diamond tips – which they heated with a laser. They began with two
samples in the cell: one made of silicates, like the earth’s mantle, and one made of iron.
(Article continues after the image)
Diamond anvil cells help researchers simulate temperature and pressure conditions of the earth’s core. Image courtesy of S.
Merkel, Univ. Lille, France
Once the samples were crushed and heated to core-like conditions, the researchers used a
combination of two techniques – ion-beam microscopy and electron microscopy – to deduce the
composition of the resultant alloy.
They found that some of the silicate’s elements – vanadium, chromium, nickel and cobalt – seeped into
the iron in large quantities, whereas others – silicon and oxygen – only went in a bit. The density of the
resultant alloy was just right to explain the type of seismic waves received from the earth’s actual core,
suggesting that the composition of the alloy was the same as the core.
Data interpretation
In some sense, diamond anvil cells offer an easy route to replicating extreme conditions. But the data
is not always easy to interpret.
Professor Dan Frost of the University of Bayreuth in
Germany says that, in particular, the exact
pressure that has been applied to the sample during
the experiment is hard to pinpoint. This is important
because it is the parameter that corresponds to
depth beneath the earth’s surface.
‘That’s what we’re
really fascinated
about, the formation
of the earth.’
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Although basic physics says that pressure is the
force applied per unit area, the quantity is hard to
calculate because some of the force supplied by
the diamond tips always ‘leaks’ back into the
retaining gasket.
Prof. Dan Frost, University
of Bayreuth, Germany
As part of another ERC-funded project called DEEP, Prof. Frost and others have pioneered a method to
record reliable pressures inside diamond anvil cells. They measured two different parameters – the
sample volume and its compressibility – which can be entered into an equation to solve for pressure.
Making these measurements required apparatus that could simultaneously perform two analytical
techniques. ‘We had to put it together from bits – it was a custom job,’ said Prof. Frost, who recently
won a EUR 2.5 million Leibniz Prize from the German Research Foundation for his work.
As a result, Prof. Frost and colleagues managed to draw up a table of how the volume of a sample
relates to temperature and pressure. In the future, then, all researchers need to do to find the accurate
pressure is to measure the sample volume.
Mantle
The work has borne fruit already. Having done studies of minerals at various pressures, corresponding
to various depths, the researchers believe that the earth’s lower mantle should have a similar chemical
composition to the upper mantle.
That result throws the origin of the earth into a different light. Some scientists had thought that the
earth was at least partly made from meteorites which most likely originate from the asteroid belt, and
since these did not have the exact composition of the upper mantle, the assumption was that the
missing constituents had sunk into the lower mantle.
If the two regions of mantle have the same composition, says Prof. Frost, that theory goes out the
window – it could be that the earth was made from matter elsewhere in the early solar system.
‘That’s what we’re really fascinated about, the formation of the earth,’ he adds. ‘Tracking down where all
those building blocks come from.’
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480-81. Www.nature.com. Nature Publishing Group. Web. 21 Dec. 2015. Hata, Chisato. "World's First Realization of Ultrahigh
Pressure and Ultrahigh Temperature at the Earth's Center - Finally Reaching the Earth's Core." Spring 8, n.d. Web. 21 Dec. 2015.
Hogan, Marianne. "The Earth's Inside." The University of Colorado, n.d. Web. 21 Dec. 2015.
More info
Prof. James Badro
DECORE
Prof. Dan Frost
DEEP
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