Was the pressure inside the Earth's core overestimated? - RIKEN and others establish a new theory.

Publication Date: September 25, 2023, 18:50

On September 22, a joint announcement was made by the RIKEN Institute, Tohoku University, and the Japan Synchrotron Radiation Research Institute (JASRI). They have established a new absolute pressure scale (equation of state) related to the Earth's interior. The research revealed that previous studies had overestimated the pressure within the Earth's core by more than 20%.

The research was conducted by an international team led by Alfred Baron, Group Director of the RIKEN SPring-8 Center, Daijo Ikuta, a specially appointed researcher at the Graduate School of Science, Tohoku University (at the time of the research), and Professor Emeritus Eiji Ohtani. The details of the study were published in the open-access journal "Science Advances," published by the American Association for the Advancement of Science.

Pressure in high-pressure experiments is calculated using a "pressure scale," an equation of state that shows the relationship between the density and pressure of a standard substance. The conventional pressure scale, which has been commonly used, is a corrected version based on the assumption and extrapolation of the "Rankine-Hugoniot adiabatic curve," an equation of the relationship between pressure and density. Although many pressure scales have been proposed, there is a maximum error of 40% between them due to the assumptions and extrapolations in the correction method, indicating a significant uncertainty. Therefore, an absolute (primary) pressure scale that does not use assumptions and extrapolations has been eagerly awaited.

The absolute pressure scale can be realized if the longitudinal wave velocity, transverse wave velocity, and density of a substance can be independently measured under high pressure. However, while the density measurement has been established by the powder X-ray diffraction method, the longitudinal wave velocity under ultra-high pressure could only be measured up to about 1.5 million atmospheres, and the transverse wave velocity could not be measured at all. Therefore, the research team improved the inelastic X-ray scattering method and examined whether it was possible to measure the transverse wave velocity while measuring the longitudinal wave velocity at higher pressure.

The reason why it was difficult to measure transverse waves conventionally is that the inelastic scattering intensity is weak compared to longitudinal waves. Therefore, measurements were attempted by combining a high-intensity and small-diameter synchrotron X-ray beam at the large synchrotron radiation facility "SPring-8," a diamond anvil high-pressure generator, and the inelastic X-ray scattering method and powder X-ray diffraction method. In addition, a special device called a "solar screen" was used to improve the X-ray optics system and reduce noise other than the signal from the sample to the limit. As a result, it was successful in measuring the inelastic scattering signal from the transverse waves, which had been buried in noise, under ultra-high pressure conditions exceeding the Earth's core-mantle boundary (1.35 million atmospheres) up to 2.3 million atmospheres in the core interior.

An example of the measurement of inelastic scattering from rhenium at 12.3 million atmospheres. Although it is a weak signal of about 0.025 counts per second, it is said that the inelastic X-ray scattering signal due to the transverse waves (red peak) from rhenium can be clearly separated from the longitudinal waves (blue peak) of rhenium and the signal from the diamond, which is a high-pressure generator (yellow and green peaks). (Source: Tohoku University Press Release PDF)

With the ability to measure three physical properties, it became possible to create an absolute pressure scale even under ultra-high pressure equivalent to the Earth's core. The research team used these three physical properties to create a compression curve of rhenium under ultra-high pressure using the absolute pressure scale obtained from metallic rhenium and compared it with the same compression curve of previous research. It was found that the conventional pressure scale significantly overestimated the pressure at pressures exceeding the core-mantle boundary of 1.35 million atmospheres. The difference increased with increasing pressure, and it was revealed that it had been overestimated by more than 20% at 2.3 million atmospheres.

Comparison of the compression curves of metallic rhenium evaluated by the absolute pressure scale and the conventional pressure scale. The black squares and lines are the relationship between pressure and density (compression curve) under each measurement condition of rhenium, evaluated by the absolute pressure scale constructed in this study. Red is shock compression experiment, yellow is theoretical research, green is ruby-helium-tungsten scale, and blue is gold scale, which are the compression curves of previous research. The temperature is room temperature in all cases. (Source: Tohoku University Press Release PDF)

According to the "Preliminary Reference Earth Model" (PREM), the major regions inside the Earth are the upper mantle (and mantle transition layer), lower mantle, outer core, and inner core. The upper mantle is composed of olivine, the lower mantle is composed of minerals such as bridgmanite and ferropericlase, the outer core is a liquid iron alloy, and the inner core is a solid iron alloy, with light substances such as silicon and sulfur included in part of the inner core.

(Left) Correlation between seismic wave velocity and density with respect to depth from the Earth's surface. (Right) Schematic diagram of the Earth's internal structure. Distribution of P-wave (longitudinal wave) and S-wave (transverse wave) propagation speeds and density distribution with respect to depth from the Earth's surface based on seismic wave velocity observations. According to PREM, the Earth's interior is largely divided into four layers: the upper mantle (and mantle transition layer), lower mantle, outer core (liquid iron alloy), and inner core (solid iron alloy). (Source: Tohoku University Press Release PDF)

Based on the conventional pressure scale, the estimated density difference between metallic iron and PREM was about 4%, while the density difference between metallic iron and PREM based on the absolute pressure scale was about 8%, equivalent to twice the density difference. Even under the conditions of the outer core, the density difference between metallic iron and PREM based on the absolute pressure scale is 30% to 50% larger than the previous estimates. Based on these results, when the amount of light substances contained in the Earth's core was calculated, it was suggested that the core contains an amount of light substances equivalent to more than five times the Earth's crust, which is an important finding that forces changes to the conventional discussion on the Earth's internal structure.

Comparison of the density of metallic iron under the conditions of the Earth's core boundary reevaluated by the absolute pressure scale and PREM. The red squares are the density of metallic iron under the temperature and pressure conditions of the Earth's core (estimated to be 3.3 million atmospheres at the core boundary, 3.65 million atmospheres at the Earth's center, and 6000K in temperature) reevaluated by this absolute pressure scale. The gray circles are the density of the core according to PREM obtained from seismic wave observations, and the difference in density with the iron density in this study is about 8%. The blue triangles are the density of metallic iron under the same conditions according to previous research based on the conventional pressure scale, and the density difference with PREM is about 4%. There is a gap equivalent to twice the density difference between this study and previous research, suggesting the possibility that the Earth's core contains twice the amount of light substances estimated so far. (Source: Tohoku University Press Release PDF)

According to the research team, the impact of the absolute pressure scale determined this time is not limited to the Earth's core, and it is also forcing changes in the study of the internal structure of giant planets in the solar system and exoplanets under greater pressure than the Earth's core. It is a result that prompts reconsideration of the behavior of all substances under ultra-high pressure dealt with in condensed matter physics, chemistry, and materials science, and it is said to have a major impact on the entire field of high-pressure science.

In the future, the research team is considering improving the accuracy of the absolute pressure scale determined this time and extending the pressure range to which the scale can be applied to the interior of exoplanets under higher pressure than the Earth's core. In addition, they are planning to reevaluate the internal structure of the Earth's core and exoplanets in more detail using the absolute pressure scale.

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