Earth’s inner core may be layered like an onion

Seismologists have long observed that compressional earthquake waves behave unevenly as they pass through Earth’s inner core. Waves traveling parallel to Earth’s rotation axis propagate about 3 to 4% faster than those moving in the equatorial direction, indicating that the inner core is anisotropic rather than uniform.

This anisotropy is not constant with depth. Seismic models show that the outer parts of the inner core are comparatively weakly anisotropic, with velocity differences of about 2%, while the central region exhibits much stronger anisotropy reaching up to 6%.

Such depth-dependent behavior has been difficult to reconcile with simple models of a solid iron sphere. Explanations based solely on temperature, pressure, or crystal alignment in pure iron have struggled to reproduce the full range of observed seismic patterns.

The new study addresses this discrepancy by introducing chemical stratification as a controlling factor, proposing that compositional changes with depth directly influence how seismic waves propagate.

Iron dominates, but lighter elements change everything

Earth’s core is composed predominantly of iron, but it is not chemically pure. Small amounts of lighter elements such as silicon, carbon, and oxygen are required to explain the density deficit inferred from seismic observations.

The outer core is liquid, but the inner core is solid and believed to crystallize as iron alloys incorporating these lighter elements. Even small variations in alloy composition can significantly affect mechanical strength and elastic properties.

Previous experimental work has focused mainly on pure iron or iron-nickel systems, leaving the combined effects of multiple light elements largely unexplored. This gap has limited the ability to link laboratory data directly to seismic observations.

By investigating iron alloys containing both silicon and carbon, the new experiments target compositions that are increasingly supported by geochemical and cosmochemical constraints on core formation.

Recreating deep Earth conditions at PETRA III

The experiments were carried out by an international team led by scientists from the University of Münster, in collaboration with researchers from the Deutsches Elektronen-Synchrotron and the European Synchrotron Radiation Facility, as well as the University of Lille.

Using the PETRA III synchrotron light source at DESY in Hamburg, the team simulated deep Earth conditions by compressing iron, silicon, and carbon alloys inside diamond anvil cells. These devices use two opposing diamond tips to generate extreme pressures while allowing X-rays to probe the sample.

In the experiments, the samples were first compressed, then heated to temperatures exceeding 820°C (1 510°F), and subsequently compressed further to pressures approaching one million times atmospheric pressure, equivalent to about 100 GPa.

Although still below true inner core conditions, these pressures and temperatures are sufficient to study deformation mechanisms and crystal alignment, which can then be extrapolated to greater depths using theoretical models.

Crystal alignment emerges under extreme stress

X-ray diffraction measurements revealed that the initially polycrystalline iron alloy developed a clear lattice preferred orientation as compression increased. This means that individual crystals rotated and aligned in specific directions, rather than remaining randomly oriented.

The alignment was decoded using radial X-ray diffraction, with the X-ray beam oriented perpendicular to the compression axis. This geometry allows scientists to track how lattice planes deform under non-hydrostatic stress.

First author Efim Kolesnikov explained that the diffraction patterns were analyzed after the experiment to derive key plastic properties of the material, specifically yield strength and viscosity. These parameters describe how the alloy resists deformation and flows under long-term stress.

The radial diffraction technique used in the study has been extensively developed at the Extreme Conditions beamline P02.2, making it possible to extract both elastic and plastic information from tiny samples under extreme conditions.

From laboratory data to seismic velocities

The experimentally derived plastic properties were incorporated into theoretical deformation models to extrapolate the behavior of the iron silicon carbon alloy to inner core pressures and temperatures.

Using these models, the researchers calculated compressional sound velocities for the alloy and compared them with those expected for pure iron under the same conditions. The results showed a marked reduction in seismic anisotropy when silicon and carbon were present.

At inner core conditions, the modeled anisotropy of the alloy was approximately 2%, closely matching seismic observations for the outer regions of the inner core. Pure iron, by contrast, is predicted to produce much stronger anisotropy.

Team leader Ilya Kupenko noted that this difference points to a compositional gradient, with iron content increasing toward the center of the core, producing stronger anisotropy at greater depths.

An onion-like structure beneath our feet

The findings support a model in which Earth’s inner core is chemically stratified rather than uniform. In this scenario, the innermost inner core is relatively iron-rich, while the outer layers are enriched in silicon and carbon.

Such stratification could naturally arise during the crystallization of the inner core, which begins at the center and progresses outward as Earth cools. Light elements become increasingly incorporated into the solid phase over time.

This layered structure would produce exactly the kind of depth-dependent seismic anisotropy observed in global seismic datasets, with stronger anisotropy at the center and weaker anisotropy toward the inner core boundary.

The results also align with recent seismic models that suggest radial gradients in sound velocity, reinforcing the idea that chemical layering plays a fundamental role in shaping Earth’s deepest interior.

Why this changes our view of the deep Earth

By directly linking laboratory deformation experiments with seismic observations, the study provides a physically grounded explanation for one of the most persistent mysteries in Earth science.

The work demonstrates that even small amounts of light elements can substantially alter the mechanical and seismic properties of core materials, challenging models that rely on pure iron behavior alone.

Understanding the inner core’s structure has implications beyond seismology. Core viscosity and deformation influence how the inner and outer core interact, which in turn affects the long term behavior of Earth’s magnetic field.

Together, the findings suggest that Earth’s inner core is not a simple solid sphere, but a chemically layered structure whose composition records the planet’s thermal and chemical evolution over billions of years.

References:

¹ Depth-dependent anisotropy in the Earth’s inner core linked to chemical stratification – Kolesnikov, E., Li, X., Müller, S.C. et al – Nature Communications – December 8, 2025 – https://doi.org/10.1038/s41467-025-67067-y – OPEN ACCESS

² An onion core: Researchers find hints of a multilayered centre of the Earth – DESY – December 10, 2025

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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