SPring-8, the large synchrotron radiation facility

Release Date

15 Oct, 2010

Tokyo Institute of Technology
Japan Agency of Marine-Earth Science and Technology
Japan Synchrotron Radiation Research Institute

<1. Background and history>
　The center of the earth consists of a metallic core with a radius of 3,500 km. The core is divided into an inner solid core (inner core) and an outer liquid core (outer core); that is, the inner core, with a radius of 1,200 km (the radius of the moon is approximately 1,700 km), makes up the deepest part of the earth. On the basis of previous research results, it is widely considered that the inner core consists primarily of iron and approximately 5% of nickel.

　The inner core shows strong seismic anisotropy, namely, significant variations in seismic velocity and rate of attenuation depending on the direction of propagation. Although such anisotropy contains important information about the growth of the inner core and the dynamics (movement) inside the inner core, it is necessary to determine the crystal structure of the inner core material (iron) to understand the anisotropy.

　The interior of the earth exists under high-pressure and -temperature conditions. The inner core at the center of the earth is considered to be under the ultrahigh pressure of 330-364 GPa and the ultrahigh temperature of 5,000 K (Kelvin, absolute temperature) or higher (the temperature is uncertain and ranges from 5,000 K to 6,000 K). No one had ever succeeded in realizing the conditions of such ultrahigh pressure and temperature in a laboratory until very recently. While researchers have attempted to determine the crystal structure of iron under high pressure since around 1950, no experiment under conditions similar to those in the inner core had ever been performed. On the basis of experiments under low pressure and theoretical calculations, various structures such as the hexagonal close-packed structure, body-centered cubic structure, face-centered cubic structure (refer to Fig.1 for these structures), orthorhombic structure and double hexagonal close-packed structure have been suggested for the crystal structure of iron in the inner core, which has been the cause of considerable controversy.

　This research group has developed technologies related to the generation of ultrahigh-pressure and -temperature conditions using a device called a diamond cell (Fig.2). Very recently, they have succeeded in generating ultrahigh-pressure and -temperature conditions similar to those at the center of the earth (press release on 5 April 2010). In this study, they succeeded in clarifying the crystal structure of iron under such conditions using the technologies they developed.

<2. Achievements>
　To examine the changes in the crystal structure of metallic iron, experiments under pressures up to 377 GPa and temperatures up to 5,700 K were performed using the high-brilliance X-rays at the High Pressure Research Beamline (BL10XU) of SPring-8. It was clarified that a dense structure, the hexagonal close-packed structure, is stable under the ultrahigh-pressure and -temperature conditions in the inner core (refer to the phase diagram in Fig.3), and that crystals of iron must be preferentially aligned so that the c-axis (the longitudinal side of the yellow box in the crystal structure shown on the left in Fig.1) is parallel to the earth’s axis of rotation to explain the strong seismic anisotropy (significant variations in seismic velocity and rate of attenuation depending on the direction of propagation) observed in the inner core.

<3. Future development>
　This study revealed the alignment of iron crystals in the inner core. Further clarification of the mechanism of alignment of metallic crystals will contribute to the clarification of their growth (the crystallization of liquid iron in the outer core) and the internal dynamics (the transfer of solid iron, which is more readily crystallized in the low-temperature area, to the high-temperature area) of the inner core.

　Future studies should examine other properties of the earth's core using the experimental technologies used in this study. The clarification of the density, viscosity, electrical conductivity and thermal conductivity of liquid iron will help clarify the chemical composition of the outer core (the original materials and the formation mechanism of the earth) and the formation mechanism of the earth's magnetic field.

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