A new investigation suggests that the inner core is not uniform, but rather stratified into layers with different chemical compositions, with the deepest region having little silicon and carbon.
Earth’s inner core, an iron-rich sphere buried more than 5,000 kilometers below the surface, may be much more complex than previously thought.
A new published in Nature Communications suggests that the planet’s solid core is composed of chemical layerswith varying concentrations of light elements such as silicon and carbon, which shape the way seismic waves propagate through it.
For decades, scientists have used seismic waves generated by earthquakes to probe Earth’s inaccessible interior. As these waves cross the inner core, they behave in a peculiar way: their speed varies according to the direction of propagation, a phenomenon known as seismic anisotropy. Although several theories have attempted to explain this effect, no single model has been able to fully explain the observed patterns.
Researchers at the University of Münster in Germany set out to unravel this enigma by examining how iron, which is the dominant component of the inner core, behaves when mixed with silicon and carbon in extreme conditions. It is believed that these elements are present in the nucleus and may play a fundamental role in forming its structure.
To simulate the conditions of the inner core, the team subjected tiny samples of iron-silicon-carbon alloys to intense pressure and temperatures reaching 820 degrees Celsius. Using X-ray diffraction, they analyzed how crystals within these alloys aligned when deformed, a property known as preferred network orientation (LPO). As explained by , this alignment influences the way in which sound waves, including seismic waves, propagate through solid materials.
Previous studies had examined pure iron, but data on iron mixed with silicon and carbon was limited. New experiments revealed that the addition of these lighter elements significantly alters the crystal structure of iron. Compared to pure iron, the alloys exhibited different lattice arrangements that would transmit seismic waves at varying speeds, closely corresponding to anomalies detected in the outer regions of Earth’s inner core.
The findings support the idea that the inner core is not uniform, but rather stratified into layers with different chemical compositions. According to the researchers, the deepest central region of the inner core may contain relatively little silicon and carbon, resulting in strong seismic anisotropy. Moving outward, increasing the concentration of these lighter elements could reduce anisotropy, creating a layered, onion-like structure.
This stratification may have formed as the nucleus crystallized gradually over geological time, with silicon and carbon becoming unevenly distributed during solidification. Such chemical stratification offers a convincing explanation for the depth-dependent seismic patterns observed by geophysicists.
