A new pressure gauge has been identified using advanced X-rays at RIKEN’s SPring-8 centre. The old scale was found to overestimate pressure by 20% at levels found in the Earth’s core. The implications of this new scale are wide-ranging, with the inner core revealed to contain twice as much light matter as previously thought.

Researchers have unveiled a more precise pressure gauge using advanced X-rays, revealing that the Earth’s inner core contains twice as much light matter as previously estimated. Their methodology provides an easier path to future pressure measurements.

In a research published on September 8 in the journal Advancement of sciencea team of scientists has identified a new measure of pressure, which is crucial to understanding the formation of the Earth.

Use of X-rays from RIKEN’s uniquely powerful spectrometer Spring-8 They avoided some of the large approximations of previous work, and discovered that the previous measure overestimated the pressure by more than 20% at 230 gigapascals (2.3 million atmospheres), the pressure reached in the Earth’s core. This is similar to someone running a marathon that they thought was 26 miles (42 kilometers) long, but found out they only ran 21 miles (34 kilometers). Although 20% may seem like a modest correction, it has significant implications.

Implications for the formation of the Earth

An accurate pressure gauge is crucial to understanding the Earth’s composition. In particular, the fundamental composition is hotly debated because it is important for understanding our planet today, and for understanding the evolution of the solar system in the distant past.

While it is generally accepted that the core is composed mostly of iron, evidence from tracking the propagation of seismic waves from earthquakes suggests that the core also contains lighter material.

When the new scale was used to interpret the seismic model, the team found that the amount of light matter in the inner core was about twice what was previously expected, and indeed the total mass of light matter in the entire core was perhaps five times greater. Or more, the Earth’s crust – the layer on which we live.

Research Methodology

In the new work, the team, led by Alfred Q. R. Baron of the RIKEN SPring-8 Center, Daigo Ikuta and Eiji Ohtani of Tohoku University, used inelastic X-ray scattering (IXS) to measure the speed of sound of a rhenium sample under pressure. A small sample of rhenium (<0.000000001 gram = 1 ng) is placed under extreme pressure by crushing it between two diamond crystals in a diamond anvil cell (DAC).

The cell was placed in the large IXS spectrometer in the BL43LXU (Figure 2) and small shifts (~1 ppm) in the scattered X-ray energy from the rhenium were carefully measured, allowing the researchers to determine the speed of sound for the rhenium. .

They determined both the compressive/longitudinal and shear/transverse speeds of sound, and the density of rhenium. This allowed the researchers to determine the stress to which the rhenium was exposed.

Rhenium density as an indicator of pressure

The new study provides a direct relationship between rhenium density and pressure. “The density of rhenium at high pressure is easy and quick to measure, and there are many facilities around the world where such measurements can be made,” Baron says. However, measuring the speed of sound is more difficult and, under these pressures, is probably only practically possible using the RIKEN spectrometer in the BL43LXU at SPring-8.

The team has done the heavy lifting so that other scientists can now use a much easier-to-measure density to determine pressure.

“When we used our new measure to interpret the behavior of metallic iron under high pressure and compared it to a seismic model of Earth, we found that the light material hidden in the inner core may be about twice as much as previously expected,” say Ikuta, Ohtani, and Baron. Similar changes, perhaps even larger in magnitude, can be expected when considering the structure of other planets. Our work also suggests a re-evaluation of the pressure dependence of almost all material properties measured at pressures similar to or greater than that of the Earth’s core.

Reference: “Detecting the density deficit in the Earth’s core by multi-megabar elemental barometer” by Daigo Ikuta, Eiji Ohtani, Hiroshi Fukui, Tatsuya Sakamaki, Rolf Heide, Daisuke Ishikawa, and Alfred Q. R. Barron, 8 September 2023, Advancement of science.
doi: 10.1126/sciadv.adh8706

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