The inner core may contain concentric layers of iron alloyed with light elements, a new Nature Communications study suggests. Laboratory experiments on iron–silicon–carbon alloys at about one million atmospheres and ~1,500°F were used to measure yield strength and viscosity. Modeling indicates that a compositional and mechanical gradient — more iron at greater depths — can explain why seismic waves travel 3–4% faster along Earth’s rotation axis than across the equator, with anisotropy increasing toward the center.
Earth’s Inner Core May Be Layered Like An Onion, New Study Finds
Lead image: Johan Swanepoel / Shutterstock
Earth is often compared to an onion because it has nested layers — atmosphere, crust, mantle and a deep interior. A new study in Nature Communications suggests that the onion analogy may extend all the way to the inner core: the very center of our planet could be composed of concentric zones with different compositions and mechanical properties.
Unusual Seismic Behavior
Seismologists have long noted an oddity deep inside Earth. Beginning roughly 3,200 miles beneath the surface, earthquake-triggered seismic waves travel about 3–4% faster when they move parallel to Earth’s rotation axis than when they travel through the equatorial plane. This directional dependence of wave speed — called anisotropy — becomes stronger with depth inside the inner core.
Laboratory Simulations of Core Material
Led by University of Münster geochemist Carmen Sanchez-Valle, the research team synthesized core-like materials to match the inner core’s density. Because pure iron–nickel is slightly too dense, the researchers created alloys of iron with modest amounts of silicon and carbon to represent likely light-element additions in the core.
To reproduce core conditions, the team used the Extreme Conditions Science beamline PO2.2. Samples were squeezed between two flattened diamond anvils, heated to about 1,500°F (≈820°C) and compressed to pressures on the order of one million atmospheres. After each run, the researchers analyzed X-ray diffraction patterns to derive the alloys’ plastic properties — notably yield strength and viscosity — which indicate how readily the materials deform.
Modeling Seismic Waves and a Layered Core
Co-author Efim Kolesnikov and colleagues modeled how seismic waves would propagate through the compressed iron–silicon–carbon alloys versus through pure iron. The results show that gradients in composition and mechanical properties — specifically an increasing fraction of iron with depth and varying amounts of silicon and carbon — can reproduce the observed anisotropy and its increase with depth.
“The diffraction patterns were analyzed after the experiment to derive plastic properties — specifically, yield strength and viscosity — of the iron–silicon–carbon alloys,” said Kolesnikov, describing how the team assessed deformation under core-like conditions.
Implications
If the inner core is indeed stratified into concentric layers with differing light-element content and mechanical behavior, this onion-like structure provides a plausible physical explanation for the long-observed seismic anomalies. The finding also highlights how much more there is to learn about Earth’s deep interior, even after 4.5 billion years of planetary evolution.
Source: Sanchez-Valle et al., Nature Communications (study reported by Nautilus).
