Science.-The ‘solid’ core of the Earth can contain hard and soft iron

07-10-2021 Earthquake locations (red) and corresponding seismic stations (yellow pins). RESEARCH AND TECHNOLOGY POLICY BUTLER AND TSUBOI (2021).


Seismic waves suggest that the Earth’s “solid” inner core contains a range of liquid, soft and hard structures that vary in the upper 300 kilometers of the inner core.

More than 5,000 kilometers below the Earth’s surface is the inner core, a ball-shaped mass of mainly iron that is responsible for the Earth’s magnetic field. In the 1950s, researchers suggested that the inner core was solid, in contrast to the region of liquid metal that surrounds it.

No human or machine has ever been to this region. Depth, pressure and temperature make the interior of the Earth inaccessible. So Butler, a researcher at the Institute of Geophysics and Planetology at the University of Hawaii at Manoa, and co-author Seiji Tsuboi, a research scientist at the Japan Agency for Marine-Terrestrial Science and Technology, relied on the only means available to probe the Earth further. internal: the waves produced by earthquakes.

“Illuminated by earthquakes in the crust and upper mantle, and observed by seismic observatories on the Earth’s surface, seismology offers the only direct way to investigate the inner core and its processes,” Butler said.

As seismic waves move through various layers of the Earth, their speed changes and they can be reflected or refracted depending on the minerals, temperature, and density of that layer.

To infer characteristics of the inner core, Butler and Tsuboi used data from seismometers directly opposite to the location where an earthquake occurred. Using Japan’s Earth Simulator supercomputer, they evaluated five pairings to broadly cover the inner core region: Tonga-Algeria, Indonesia-Brazil, and three between Chile-China.

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“In stark contrast to the homogeneous soft iron alloys considered in all terrestrial models of the inner core since the 1970s, our models suggest that there are adjacent regions of hard, soft and liquid or soft iron alloys in the upper 300 kilometers of the inner core, “Butler said. “This imposes new limitations on the composition, thermal history and evolution of the Earth.

The study of the inner core and the discovery of its heterogeneous structure provide important new information about the dynamics at the boundary between the inner and outer core, which impact the Earth’s generation magnetic field.

“Knowing this boundary condition from seismology may allow for better predictive models of the geomagnetic field that protects and protects life on our planet,” Butler said.

The researchers plan to model the structure of the inner core in more detail using the Earth Simulator and compare how that structure compares to various features of the Earth’s geomagnetic field. The research was published in Physics of the Earth and Planetary Interiors.

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