極限実験が地球の内なる秘密を解き明かす

理化学研究所の研究者らは、極度の圧力下で純鉄の音速を測定し、地球の核の組成を明らかにし、他の物質の測定を改善できるようにしました。

科学者たちは、これまでの2倍の圧力で音速測定を行うことで、地球の核の組成をより正確に把握しました。

理化学研究所物質力学研究室のアルフレッド・Q・R・バロン氏が15年前に開始したプログラムに基づいて、研究者らはついに、地球の内核の静圧に相当する静圧における純鉄の音速を測定することができた。 これは理化学研究所で可能になりました[{” attribute=””>SPring-8 Center’s Quantum NanoDynamics Beamline: combining its world-leading spectrometer with new developments in x-ray optics and a new type of high-pressure cell, the team was able to perform measurements under stable conditions at pressures about double their previous record, and more than three times higher than any other facility in the world (Figure 1).

Figure 1: The high-resolution spectrometer at the RIKEN Quantum NanoDynamics Beamline (BL43LXU) of the RIKEN SPring-8 Center. The photos show the main spectrometer arm, sample positions, and two scientists for scale. Credit: © 2023 RIKEN SPring-8 Center

The work is interesting because it allows constraints to be placed on the composition of the Earth’s core. “We actually have amazingly little information about the center of the planet we live on,” says Baron. “Other work tracking the progress of seismological waves from earthquakes through the planet give us a model for the density and sound velocity as a function of depth—but that’s about it: the detailed composition remains unknown and the subject of debate, as the composition is important both to understand the present structure of the planet and also the evolution of the planet and even the Solar System.”

The most recent measurement, published in Nature Communications, was a collaboration between Baron’s Materials Dynamics Group at the RIKEN SPring-8 Center, Eiji Ohtani and Daijo Ikuta of Tohoku University, and researchers at Ehime University and the Japan Synchrotron Radiation Research Institute (SPring-8/JASRI). The team measured the sound velocity of pure iron as a function of pressure up to densities exceeding that of the Earth’s inner core, where the pressure exceeds 300 gigapascals, validating a linear relation known as Birch’s law.

Using the new results they propose a model where, in addition to iron and nickel, the Earth’s core has small amounts of silicon and sulfur.

“Further, and perhaps most important, the method can be extended to other materials allowing a new level of precision for measurements of sound velocities of materials at extreme pressures,” says Baron.

Reference: “Sound velocity of hexagonal close-packed iron to the Earth’s inner core pressure” by Daijo Ikuta, Eiji Ohtani, Hiroshi Fukui, Takeshi Sakai, Daisuke Ishikawa and Alfred Q. R. Baron, 25 November 2022, Nature Communications.
DOI: 10.1038/s41467-022-34789-2