Earth’s inner core may be layered like an onion

Deep inside Earth, researchers see signs that the solid inner core may be arranged in multiple chemical layers far below.

High-pressure experiments on iron alloys, squeezed at Germany’s PETRA III synchrotron, reproduce puzzling differences in earthquake wave speeds measured around the globe.

Waves change in the inner core

The work was led by Professor Carmen Sanchez-Valle, whose research at the University of Münster probes chemistry deep inside planets.

Earth’s inner core sits about 3,200 miles beneath our feet. It’s a solid metal sphere wrapped in liquid iron.

Compressional earthquake waves, called P waves, speed up by about three to four percent along polar rather than equatorial paths. 

Scientists call that pattern seismic anisotropy, a directional difference in wave speed depending on which way vibrations travel through solid metal.

How scientists model the inner core

To probe that possibility, the team created tiny samples of iron mixed with small amounts of silicon and carbon, mimicking core alloys.

Those lighter elements probably share space with iron in the core because pure iron alone would make Earth too dense overall.

Earlier computer models and high-pressure tests showed these elements change iron’s behavior, but realistic inner core mixtures remained untested until now.

The new samples narrow that gap by targeting compositions close to values predicted for Earth’s deepest metal from earlier geophysical studies.

Mimicking inner core pressure

The team loaded their alloys into a device called a diamond anvil cell, a press that squeezes samples between two opposing diamonds.

This setup squeezed the metal to pressures millions of times greater than atmospheric pressure, similar to conditions near Earth’s inner core.

The researchers then heated the compressed alloys to about 820 degrees Celsius, roughly 1,500 degrees Fahrenheit, to encourage deformation.

At the PETRA III beamline in Hamburg, focused X-ray beams probed the samples while they were being squeezed and heated.

Crystal pattern clues

As the alloys deformed, their microscopic crystals began lining up in preferred directions, creating lattice-preferred orientation – a subtle pattern of shared crystal directions.

The team used radial X-ray diffraction, a technique that records how crystal planes rearrange around the compression direction, to decode that orientation.

From the diffraction images, the researchers extracted how easily the alloy deforms. They quantified yield strength, the stress needed to make the metal flow plastically.

“We were able to decode the LPO via X-ray diffraction perpendicular to the compression axis”, said Dr. Efim Kolesnikov, the study’s first author.

Texture changes waves

Knowing how the crystals aligned under stress, the researchers modeled how sound waves would travel through the alloy under pressure.

They calculated compressional wave speeds along several directions, and then compared those values with earlier measurements for pure iron under the same extreme conditions.

The alloy containing silicon and carbon showed a larger contrast between directions, meaning its seismic anisotropy was stronger than that of pure iron.

That result implied that even modest amounts of light elements could significantly change how the inner core guides earthquake waves over time.

Chemical layers of the inner core

Earth’s inner core is probably not chemically uniform, with lighter elements more abundant near the top and iron dominating deeper regions.

Using their deformation data, the team explored how a gradual increase in iron content with depth would change wave speeds along different directions.

The results showed that such layering could reproduce the observed difference between the outer and inner parts of the core’s anisotropy. 

The result is an inner core made of nested shells with slightly different chemistry – described as onion-like layering in this model.

Implications of the layering

A chemically layered inner core would hint that solid metal did not freeze out all at once but evolved over billions of years.

Areas richer in silicon and carbon near the top might preserve an earlier stage of solidification, while deeper zones record more recent growth.

Such layering could also reflect how heat escapes from the liquid outer core, channeling energy unevenly into the mantle above over long intervals.

Those thermal patterns would influence how quickly the core cools and how long it can keep powering the planet’s global magnetic field.

Regions of the inner core

Seismologists have argued for years about whether the inner core contains separate regions, including proposals for an innermost inner core and hemispherical differences. 

Earlier studies showed that P waves travel about three percent faster along Earth’s spin axis, suggesting some large-scale ordering inside the metal.

Some teams matched that pattern with models of aligned iron crystals alone, while others proposed chemical variations, partial melting, or differing grain sizes.

The new experiments strengthen the chemical layering idea by tying specific alloy properties to realistic anisotropy patterns, without ruling out other structural features.

Big questions remain

Despite the progress, the experiments cover only one mixture of iron, silicon, and carbon, while the real inner core probably hosts other elements.

Future work will need to test how ingredients like oxygen, hydrogen, and sulfur alter crystal textures and sound speeds under similar extreme conditions.

Seismologists also continue to refine global earthquake catalogs, searching for subtle changes in wave paths that could test the new layered core predictions.

“There have been several hypotheses for the origin of these anisotropies,” said Professor Sanchez-Valle.

Now, with these new findings, the idea of a chemically layered inner core takes on fresh significance.

The study is published in the journal Nature Communications.

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