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Sound-speed measurements shed light on Earth’s interior


exploring the behaviour of materials at high pressures using a diamond anvil cell
The stepped bevel diamond anvil: Scanning electron microscope (SEM) image of the stepped bevel diamond anvil after machining by a dual focused ion beam. From Nature Communications: Creative Commons Attribution 4.0 International License (CC BY 4.0)

Researchers have measured the speed of sound in pure iron under pressures similar to those that exist at the Earth’s inner core boundary. The result suggests that this region of the core may be enriched in silicon and sulphur.

“It may be surprising, but we do not have much information about the centre of the planet we live on,” says team member Alfred Baron of the RIKEN SPring-8 Center in Japan. “One can dig down a few kilometres, and volcanoes and plate tectonics can bring up material from depth of a few hundred km, but what lies below, down to the centre of the Earth some 6000 km beneath our feet, is not well understood.”

Our current picture of Earth’s interior suggests that the planet’s outer core (located around 3000 km down) is mostly liquid iron, with an inner core of solid iron underneath. This information is obtained by tracking seismic waves from earthquakes as they propagate through the planet, yielding data on the density and the speed of sound, and specifically the compressional and shear wave velocities (vp and vs respectively). However, the values thus measured do not agree exactly with what is expected for pure iron according to the Preliminary Reference Earth model (PREM), explains Baron. Hence, there must be something else – possibly something less dense – present in the core.

“What that material is, and how much of it there may be, are active areas of investigation as they have implications for understanding the present properties of the Earth and the evolution of the solar system as a whole,” he says.

Improved version of a diamond anvil cell

An alternative means of exploring the behaviour of materials at high pressures is to use a diamond anvil cell (DAC). However, even with this tool, it takes considerable skill to achieve pressures comparable to those in the Earth’s core.

In the latest work, researchers led by Daijo Ikuta and Eiji Ohtani used an improved version of a DAC known as a stepped-bevel anvil, combined with inelastic X-ray scattering and X-ray diffraction measurements. The X-ray scattering technique allows researchers to observe atomic motions in materials using X-rays and is the only method for measuring the sound velocity in metals directly under extreme static compression in a DAC. The researchers made these measurements at RIKEN’s facility for inelastic X-ray scattering, the Quantum NanoDynamics Beamline at Spring-8 in Hyogo Prefecture.

These measurements revealed that at a pressure of 310-327 GPa – the highest static pressure ever achieved in studies using inelastic X-ray scattering and in-situ X-ray diffraction techniques – the density of hexagonal-closed-packed-iron is 13.87 g/cm3. The researchers also found that vp and vs of the inner core are around 4% and 36% slower, respectively, than the corresponding velocities of pure iron at inner-core pressures. “These density and sound velocity values can be explained by the addition of around 3% silicon and 3% sulphur (by weight) to the iron in the inner core, as may occur by a selective enrichment of material due to inner core growth from the outer core,” Baron tells Physics World.

The results are detailed in Nature Communications.


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