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Earth's 'solid' inner core may not be so solid after all! Ball-shaped mass 3,200 miles beneath the surface contains both mushy and hard iron, study claims

 No human or machine has ever been 3,200 miles beneath Earth's surface because the depth, pressure and temperature make it inaccessible.

But scientists have long believed that our planet's inner core was solid, in contrast to the liquid metal region surrounding it.

Now that's been brought into question by a new study that claims the ball-shaped mass, which is responsible for Earth's magnetic field, contains both mushy and hard iron.

Scientists have long believed that our planet's inner core was solid. Now that's been brought into question by a new study that claims the ball-shaped mass contains both mushy and hard iron. Earthquake waves (pictured) were used as the basis for the research

Scientists have long believed that our planet's inner core was solid. Now that's been brought into question by a new study that claims the ball-shaped mass contains both mushy and hard iron. Earthquake waves (pictured) were used as the basis for the research


The research has been led by Rhett Butler, a geophysicist at the University of Hawaii, who suggests that Earth's 'solid' inner core is, in fact, made up of a range of liquid, soft, and hard structures which vary across the top 150 miles of the mass.

Earth's interior is layered like an onion. The iron-nickel inner core is 745 miles in radius, or about three-quarters the size of the moon and is surrounded by a fluid outer core of molten iron and nickel about 1,500 miles thick.

The outer core is surrounded by a mantle of hot rock 1,800 miles thick and overlain by a thin, cool, rocky crust at the surface. 

Because the inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth — earthquake waves. 

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

As seismic waves move through various layers of Earth, their speed changes and they may reflect or refract depending on the minerals, temperature and density of that layer. 

To better understand the features of the Earth's inner core, Butler and his co-author Seiji Tsuboi, a research scientist at the Japan Agency for Marine-Earth Science and Technology, used data from seismometers directly opposite the location where an earthquake was generated. 

They used Japan's Earth Simulator supercomputer to assess five pairings to broadly cover the inner core region: Tonga and Algeria, Indonesia and Brazil, and three between Chile and China. 


A cut-away of Earth's interior shows the inner core (red) and liquid iron outer core (orange). Seismic waves travel through the Earth's inner core faster between the north and south poles (blue arrows) than across the equator (green arrow)

A cut-away of Earth's interior shows the inner core (red) and liquid iron outer core (orange). Seismic waves travel through the Earth's inner core faster between the north and south poles (blue arrows) than across the equator (green arrow)

Because the Earth's inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth — earthquake waves (stock image)

Because the Earth's inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth — earthquake waves (stock image)

'In stark contrast to the homogeneous, soft iron alloys considered in all Earth models of the inner core since the 1970's, our models suggest there are adjacent regions of hard, soft, and liquid or mushy iron alloys in the top 150 miles of the inner core,' said Butler. 

'This puts new constraints upon the composition, thermal history, and evolution of Earth.' 

The researchers said this discovery of the inner core's diverse structure could offer important new information about the dynamics at the boundary between the inner and outer core, which impact the Earth's magnetic field.

'Knowledge of this boundary condition from seismology may enable better, predictive models of the geomagnetic field which shields and protects life on our planet,' said Butler.

The researchers now plan to model the inner core structure in more detail using the Earth Simulator supercomputer so they can see how it compares with various characteristics of Earth's geomagnetic field.

The research has been published in the journal Science Direct.

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