Jens Asbjørn Bøgen
A new scientific investigation is reshaping long-held assumptions about the deep interior of the planet.
For decades, researchers have known that Earth’s inner core behaves strangely, acting both solid and oddly soft at the same time.
Now, fresh experimental evidence is offering a coherent explanation for this long-standing puzzle.
Previously, seismologists have observed that shear waves slow dramatically as they pass through the inner core, despite the region’s extreme pressure and heat. Its mechanical response has appeared more like a pliable substance than the iron-rich sphere scientists expected to find.
But according to a study published in National Science Review, the answer may lie in an exotic physical state.
A superionic phase
Researchers from Sichuan University and the Chinese Academy of Sciences report that the inner core does not behave like a standard solid but instead enters a superionic phase in which light elements move as freely as a liquid within a rigid iron scaffold.
The team, led by Prof. Youjun Zhang and Dr. Yuqian Huang with Prof. Yu He, concludes that iron-carbon alloys exposed to inner-core pressures allow carbon atoms to glide rapidly through the iron lattice.
This mobility dramatically weakens the material’s rigidity while leaving the overall structure intact.
Unusual atomic motion
According to Science Daily, the researchers say their experiments provide clear evidence of this process.
“For the first time, we’ve experimentally shown that iron-carbon alloy under inner core conditions exhibits a remarkably low shear velocity,” Prof. Zhang said.
He compared the movement of carbon atoms to “children weaving through a square dance,” slipping between iron atoms that remain locked in a crystalline pattern.
Although earlier computer models in 2022 hinted at such superionic behaviour, verifying it had proved difficult. Using a dynamic shock compression system, the team blasted iron-carbon samples to around 7 km per second, mimicking pressures of up to 140 gigapascals and temperatures close to 2600 kelvin.
Their measurements showed seismic properties matching those observed deep underground, including a sharp rise in Poisson’s ratio and a steep drop in shear wave speed.
Molecular simulations revealed atoms moving freely inside the lattice without causing structural collapse.
Shifting scientific models
These findings could revise how scientists understand the planet’s internal machinery. The authors argue that the diffusion of light elements may help explain seismic anisotropy and could contribute energy to the geodynamo that sustains Earth’s magnetic field.
Dr. Huang said, “Atomic diffusion within the inner core represents a previously overlooked energy source for the geodynamo.”
The study also refines debates over how light elements behave under extreme pressure, pointing to the importance of interstitial solid solutions rather than compounds alone.
Prof. Zhang said the results mark a step away from viewing the inner core as a rigid mass. Instead, he described it as a dynamic zone shaped by the motion of lighter elements.
The same mechanism, he added, may influence the evolution of rocky planets beyond Earth.
Sources: National Science Review, Science Daily
