New lab experiments reveal Earth’s inner core may be layered like an onion — with silicon and carbon enriching outer regions while the center remains iron-rich. This chemical stratification explains seismic anisotropy patterns previously unexplained.
For decades, scientists have struggled to explain why seismic waves travel at different speeds depending on direction through Earth’s inner core — a sphere roughly the size of the moon buried beneath our feet. Now, new high-pressure laboratory experiments suggest the answer lies not in deformation alone but in a chemically layered structure resembling an onion.
The inner core is thought to be mostly iron and nickel, yet seismic data consistently show it’s less dense than pure iron — implying lighter elements are present. Silicon, carbon, oxygen, sulfur, and even hydrogen have been proposed as candidates. But until now, no experiment had tested how these elements behave together under extreme conditions.
The breakthrough came from an international team led by Prof. Carmen Sanchez-Valle at the University of Münster. They studied an iron alloy containing 2 percent silicon and 0.4 percent carbon — a composition closely matching theoretical models for the inner core — using diamond anvil cells to simulate pressures up to 128 gigapascals and temperatures up to 1100 kelvin.
This experimental setup replicated conditions deep within Earth’s core. Using advanced X-ray diffraction methods at PETRA III at Deutsches Elektronen-Synchrotron in Hamburg, researchers tracked subtle changes in crystal structures as pressure increased.
Iron Alloys Under Extreme Conditions
At low pressure, the alloy formed a body-centered cubic structure. Above about 10 gigapascals, it transitioned into hexagonal close-packed form — a structure known to align easily under stress. This phase change completed near 27 gigapascals, mirroring conditions near the inner core boundary.
Crucially, when heated at higher pressures, a brief face-centered cubic phase appeared before vanishing again — revealing complex phase behavior under extreme conditions. By measuring lattice strain — how crystals deform — researchers quantified yield strength, which rose steadily with pressure to around 15 gigapascals at maximum load.
“Carbon made the difference,” said Sanchez-Valle. “When we compared iron–silicon alloys with and without carbon, the carbon-bearing alloy was consistently stronger.” This finding suggests carbon acts as a reinforcement agent, enabling the material to withstand immense stress while maintaining structural integrity.
Watching Crystals Change Shape
Researchers used elasto-visco-plastic self-consistent modeling to interpret diffraction pattern shifts — revealing that basal slip dominated deformation, though additional mechanisms were needed to fully match experimental data.
Temperature also played a key role. At constant pressure, strain values dropped significantly at elevated temperatures — indicating weakening under heat. However, even at high temperatures, this alloy outperformed pure iron and previous iron-silicon combinations.
Stronger Than Pure Iron
By extrapolating their measurements to true inner core conditions — pressures exceeding 330 gigapascals and temperatures above 5000 kelvin — researchers calculated viscosity estimates ranging between 10¹⁴ and 10¹⁸ pascal seconds. These values fall within the range predicted by geophysical models, confirming consistency with existing constraints.
Shear stresses estimated at 1,000 to 30,000 pascals align with thermal convection models — meaning convection alone could generate enough force to deform the material and produce lattice alignment responsible for seismic anisotropy.
A Layered Inner Core
The most groundbreaking insight emerged from combining experimental data with modeling: seismic anisotropy isn’t just a product of deformation — it stems from chemical layering.
As crystallization begins at the center and moves outward along Earth’s temperature gradient, early solids near the center likely form as iron-rich hexagonal close-packed solid solutions. As temperatures drop toward the boundary, more silicon and carbon become incorporated into the solid — while decreasing pressure favors higher concentrations of light elements.
This creates a natural gradient: the central region remains closer to pure iron and develops strong anisotropy, while outer layers become enriched in silicon and carbon — showing weaker anisotropy. “There have been several hypotheses for the origin of these anisotropies,” said Sanchez-Valle. “Unfortunately, there are very little experimental data on how such LPO might look like in Earth’s iron core.”
Why It Matters
Understanding the inner core’s structure helps refine models of Earth’s thermal history, magnetic field generation, and long-term evolution. The magnetic field — generated by convective motion in the outer core — relies on heat flowing from the inner core. If the inner core’s structure affects heat flow, it directly influences the stability of planetary protection against solar radiation.
Moreover, this research provides critical feedback for climate and planetary science models. For instance, if the inner core’s composition evolves over time due to cooling and crystallization, its impact on mantle dynamics — and thus surface tectonics — becomes increasingly important.
Practical Implications
While this discovery doesn’t offer immediate applications for everyday life, it advances fundamental understanding of planetary formation. Knowing how materials behave under extreme pressure allows scientists to better predict future changes in Earth’s core — potentially warning of magnetic field instability or shifts in plate tectonics.
It also offers clues to other terrestrial planets. Venus, Mars, and Mercury all have cores — some possibly layered — and studying Earth’s inner core can inform whether similar processes occur elsewhere.
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