image: Iron atoms form a rigid hexagonal close-packed (hcp) structure, with a subset of these atoms exhibiting collective motion along the [100] and [010] directions. Within this hcp iron lattice, interstitial light elements diffuse freely in a liquid-like manner, while substitutional light elements remain confined to their respective substitutional lattice sites. Consequently, the Earth’s inner core exists in a hybrid state of solid and liquid-like behavior.
Credit: Huang et al
Beneath Earth’s molten outer core lies a solid heart — the inner core, a sphere of iron light-element alloy compressed under more than 3.3 million atmospheres and heated to temperatures rivaling the Sun’s surface. For decades, scientists have puzzled over its strange behavior: though solid, it acts like a soft metal, slowing seismic shear waves and showing a Poisson’s ratio more akin to butter than steel. How could the planet’s solid center be both rigid and soft?
A landmark study published in National Science Review now offers a compelling answer. Researchers have discovered that Earth’s inner core isn’t a typical solid at all — it exists in a superionic state, where light elements flow like liquid through a solid iron lattice. This discovery redefines our understanding of the planet’s deepest interior.
The study, led by Prof. Youjun Zhang and Dr. Yuqian Huang from Sichuan University, along with Prof. Yu He from the Institute of Geochemistry, Chinese Academy of Sciences, shows that under extreme pressures and temperatures, iron–carbon alloys transform into a superionic phase. In this state, carbon atoms move rapidly through the crystal structure of solid iron, dramatically softening it.
“For the first time, we’ve experimentally shown that iron–carbon alloy under inner core conditions exhibits a remarkedly low shear velocity.” said Prof. Zhang. “In this state, carbon atoms become highly mobile, diffusing through the crystalline iron framework like children weaving through a square dance, while the iron itself remains solid and ordered. This so-called “superionic phase” dramatically reduces alloy’s rigidity.
From Theory to Experiment
While computer models had hinted at such a state in 2022, direct experimental proof remained elusive — until now. Using a dynamic shock compression platform, the team accelerated iron–carbon samples to speeds of 7 kilometers per second, creating pressures up to 140 gigapascals and temperatures near 2600 kelvin — conditions similar to those in the inner core.
By combining in-situ sound velocity measurements with advanced molecular dynamics simulations, the scientists observed a sharp drop in shear wave velocity and a spike in Poisson’s ratio — matching the “soft” seismic signals detected deep within Earth. On the atomic scale, the results revealed carbon atoms slipping freely through the iron lattice, weakening its rigidity without destroying its structure.
A Dynamic Core with Global Impact
The superionic model not only explains the core’s puzzling seismic properties but also opens new perspectives on Earth’s internal dynamics. The movement of light elements could help account for seismic anisotropy — variations in wave speeds depending on direction — and may even influence Earth’s magnetic field.
“Atomic diffusion within the inner core represents a previously overlooked energy source for the geodynamo,” said Dr. Huang. “In addition to heat and compositional convection, the fluid-like motion of light elements may help power Earth’s magnetic engine.”
The findings also settle long-standing debates about how light elements behave under extreme pressures. Previous studies focused on compounds or substitutional alloys, but this research highlights the importance of interstitial solid solutions — especially those involving carbon — in determining the core’s properties.
Rethinking the Planet’s Heart
According to Prof. Zhang, the discovery signals a shift in how scientists view Earth’s center. “We’re moving away from a static, rigid model of the inner core toward a dynamic one,” he explained.
Beyond Earth, the discovery of a superionic phase could also shed light on the magnetic and thermal evolution of other rocky planets and exoplanets. As Zhang notes, “Understanding this hidden state of matter brings us one step closer to unlocking the secrets of Earth-like planetary interiors.”
This work was funded by the National Natural Science Foundation of China, the Sichuan Science and Technology Program, and the CAS Youth Interdisciplinary Team.