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 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.
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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.
In some sense, diamond anvil cells offer an easy route to replicating extreme conditions. But the data is not always easy to interpret.
‘That’s what we’re really fascinated about, the formation of the earth.’
Prof. Dan Frost, University of Bayreuth, Germany
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.
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.
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.
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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