Earth’s Inner Core May Be Layered Like an Onion, Study Suggests

Seismic waves travelling through Earth’s inner core have disclosed a great deal about the planet’s iron-rich centre: it changes shape, can reverse its rotation, has an unusual texture and contains an uncommon state of matter.

A fresh study, designed to account for anomalous measurements, now indicates that Earth’s core could be arranged in layers, much like an onion.

Seismic anisotropy in Earth’s inner core

Scientists in Germany set out to examine seismic anisotropy: changes in the speed of seismic waves moving through Earth when they reach the inner core, according to the direction in which they travel.

"There have been several hypotheses for the origin of these anisotropies," says mineralogist Carmen Sanchez-Valle, from the University of Münster.

"We set out to study the combined effect of silicon and carbon on the deformation behavior of iron."

To investigate this, the team examined how these important inner-core elements may interact at immense pressures and temperatures reaching 820 °C (1508 °F).

Using X-ray diffraction, they searched for a characteristic known as lattice-preferred orientation (LPO), which refers to the way crystals in solids become aligned through thermal patterns.

Until now, researchers had limited substantial evidence showing what iron’s LPO might look like after silicon and carbon are added to create alloys.

LPO can alter how sound waves pass through metals such as iron, and scientists have proposed that it could account for seismic anisotropy. In this study, the process was examined at the smallest scales, with the alloys sealed, compressed and heated inside extremely small containers.

"The diffraction patterns were analyzed after the experiment to derive plastic properties – specifically, yield strength and viscosity – of the iron-silicon-carbon alloys, which were further modeled through theory to extrapolate them to inner core conditions," explains Sanchez-Valle.

Silicon and carbon may form layered core regions

The findings showed that, relative to pure iron, adding silicon and carbon changed the arrangement of the iron alloy’s crystal lattice.

The resulting variations in seismic-wave velocity would correspond with the anomalies detected in the outer region of the inner core.

This provides further support for the idea that Earth’s inner core is made up of multiple layers-an extraordinary scientific achievement when examining material more than 5,000 kilometres (3,107 miles) below us, beneath rock and liquid metal.

The researchers suggest that the inner core’s central region may contain little silicon and carbon, producing pronounced seismic anisotropy, "while the increasing concentration of light alloying elements towards the outer layers of the inner core results in reduced anisotropy."

Geologists continue to advance their understanding of the complex structures below Earth’s surface, largely by tracking seismic-wave travel and reproducing inner- and outer-core conditions in laboratories.

This detailed research requires scientists to identify inconsistencies, devise potential explanations and test them-a process the team behind this study successfully carried out.

"The depth-dependent anisotropy pattern observed in the Earth's inner core may result from chemical stratification of silicon and carbon following core crystallization," conclude the researchers.

The research was published in Nature Communications.