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Two-Layer Convection in Outer Liquid Core of Earth (Press Release)

Release Date
11 Nov, 2011
  • BL10XU (High Pressure Research)
-Significant effect causing variation of magnetic field of earth

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

Scientists carried out a high-pressure phase transition experiment and found that iron monoxide (FeO), an important component of the outer liquid core of the earth, is stable with a crystal structure (phase transition) that has not been conventionally known to exist at certain temperatures and pressures (240 GPa or higher and 4,000 K or higher) in the outer core of the earth. This study was carried out by Kei Hirose (Visiting Principal Scientist, also a professor at Tokyo Institute of Technology) and Haruka Ozawa (Technical Scientist) of the Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (President, Yasuhiro Kato), and Futoshi Takahashi (Assistant Professor) of the Graduate School of Engineering, Tokyo Institute of Technology (President, Kenichi Iga) and their colleagues, in cooperation with the scientists of Japan Synchrotron Radiation Research Institute (President, Tetsuhisa Shirakawa). The result was examined by a numerical simulation to elucidate the convection state in the outer core. As a result, the possibility of two-layer convection in the outer core, which was previously considered to occur in a single layer, owing to the suppression of convection by the newly found phase transition of FeO, was demonstrated for the first time in the world. This phenomenon is caused by the convection of metals in a liquid phase and should greatly contribute to clarifying the variation of the magnetic field of the earth, which serves as a barrier protecting us from solar wind and cosmic rays arriving from space.

The achievements of this study were published in the American scientific journal Science on 11 November 2011.

Publication:
"Phase Transition of FeO and Stratification in Earth's Outer Core"
Haruka Ozawa1,2, Futoshi Takahashi2, Kei Hirose1,2, Yasuo Ohishi3, Naohisa Hirao3
1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro
3Japan Synchrotron Radiation Research Institute

1. Background
At the center of the earth, there is a core mainly composed of iron with a diameter of 3,500 km. The metallic core is divided into two regions at a diameter of 5,150 km: a liquid outer core and a solid inner core (Fig.1). The magnetic field of the earth is induced by the convection of the liquid metal in the outer core. Molten iron constitutes the outer core and contains approximately 30% FeO. Conventionally, it has been considered that the convection of the outer core occurs in a single layer from the observation of seismic waves. However, the effect of changes in the crystal structure of components due to changes in conditions such as temperature and pressure has not been taken into consideration in conventional studies, and the mechanism has not been clarified in detail. In this study, the change in the crystal structure of FeO in the outer core was examined to investigate the convection state in the outer core.

2. Achievements
The research group examined the changes in the crystal structure of FeO, a component of the outer core, under conditions of 227 GPa at 3,770 K to 324 GPa at 4,180 K, which are similar to those in the outer core, using the high-pressure research beamline (BL10XU) at SPring-8. It was found that the sodium chloride (NaCl)-type (B1) structure transformed to a cesium chloride (CsCl)-type (B2) structure at 240 GHz at 4,000 K, similar to the conditions of the middle of the outer core (Fig.3). This is the world's first demonstration of the existence of a CsCl-type FeO structure.

This change in the crystal structure suppresses the convection and may change the convection state in the outer core. The result was examined by a numerical simulation to elucidate the convection state in the outer core. It was found that two-layer convection, instead of single-layer convection as conventionally thought, occurs in the outer core because the convection is suppressed at the depth where the FeO transforms to a CsCl-type structure (Fig.4).

3. Significance of the study and future prospects
The convection in the outer core has been conventionally considered to occur in one layer. However, the possibility of two-layer convection was demonstrated by considering the phase transition of FeO found in this study. The magnetic field of the earth is induced by the convection in the outer core. The earth's geomagnetism has reversed every 700,000 years throughout the history of the earth. The reversal may be caused by the instability of the two-layer convection.

<<Figures>>

Fig. 1 Cross section of the earth
Fig. 1 Cross section of the earth

Fig. 2 Three-dimensional structure of UTX bound to histone H3
Fig. 2 Laser-heated diamond-anvil cell (A, B)

A sample sandwiched between two opposing diamond culets (C) is irradiated with lasers under a high pressure to realize the temperature and pressure simulating those of the earth's interior in a laboratory.

Fig. 3 Change in crystal structure of FeO under high pressure and high temperature (state diagram)
Fig. 3 Change in crystal structure of FeO under high pressure and high temperature (state diagram)

The red area indicates the range of pressures and temperatures examined in this experiment. B1, NaCl-type structure; B2, CsCl-type structure; B8, NiAs-type structure; rB1, distorted NaCl-type structure. Dotted line, Fei and Mao (1994); Melting curve, Fischer and Campbell (2010).

Fig. 4 Change in crystal structure of FeO under high pressure and high temperature (state diagram)
Fig. 4 Cross section of meridian plane showing the convection in the outer core

The arrows show the flow of convection. The left figure shows the case in which there is no phase transition in the outer core. In the right figure, the phase transition of the outer liquid core induces two-layer convection.


For more information, please contact:
　Dr. Kei Hirose
　　E-mail：mail