October 15, 2010
Tokyo Institute of Technology
Japan Agency of Marine-Earth Science and Technology
Japan Synchrotron Radiation Research Institute
Clarification of Material of Earth’s Core
- Experiments under Ultrahigh-Pressure and -Temperature Conditions Similar to
Those in the Inner Core
Kei Hirose, Professor, and Shigehiko Tateno, specially appointed Assistant Professor, of Tokyo
Institute of Technology, and Yoshiyuki Tatsumi, Program Director, of the Japan Agency of
Marine-Earth Science and Technology, in collaboration with Yasuo Ohishi, a senior scientist, of
the Japan Synchrotron Radiation Research Institute, have found that the material of the inner core
(solid core) at the deepest part of the earth is iron with a hexagonal close-packed structure. Metallic
iron was examined under ultrahigh-pressure and -temperature conditions, similar to those in the
earth's inner core, using the ultrahigh-pressure and -temperature generation technologies they
developed. As a result of the examination of the changes in the crystal structure of the metallic iron
using the high-brilliance X-rays of SPring-8, they found for the first time that the hexagonal
close-packed structure is stable under those conditions. The clarification of the crystal structure
enables the interpretation of seismic observations that had previously been difficult. The
understanding of the formation and evolution of the earth’s core will be markedly improved in the
future
These achievements were published in the American scientific journal Science on 15 October
2010.
● 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.
● 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.
● Future developmen
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.
Fig. 1 Crystal structures of iron suggested in the past
Ye llow boxes indicate the unit cells (the minimal units of a repetitive structure).
Fig. 2 Diamond anvil for generating ultrahigh pressure
The specimen is sandwiched between two diamond anvils and pressurized to 300 GPa or
higher.
Fig. 3 Changes in the crystal structure of iron under
high pressure and temperature
(phase diagram)
Geotherm: temperature profile of earth's interior; hcp: hexagonal close-packed structure;
fcc: face-centered cubic structure; bcc: body-centered cubic structure; Liq.: liquid phase
For more information, please contact:
Technology)
E-mail：
Dr. Yoshiyuki Tatsumi
Technology)
Prof. Kei Hirose (Tokyo Institute of
(Japan Agency of Marine-Earth Science and
E-mail：
Dr. Yasuo Ohishi (JASRI)
E-mail：