Earth’s inner core may not be a conventional solid at all, but a superionic material where light elements drift like liquid through a rigid iron lattice. New experiments show that this unusual state dramatically softens the core, matching seismic clues that have puzzled scientists for decades.
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Beneath Earth’s molten outer core lies a solid central region, the inner core, a compact sphere made of an iron light-element alloy pressed by more than 3.3 million atmospheres and heated to temperatures comparable to the Sun’s surface.
For many years, researchers have struggled to explain its unusual behavior: although it is solid, it behaves like an unexpectedly soft metal, slowing seismic shear waves and displaying a Poisson’s ratio closer to butter than to steel. This has raised a long-standing question about how the planet’s solid center can appear both firm and surprisingly pliable.
A major study published in National Science Review now provides a strong explanation. Scientists have found that Earth’s inner core is not behaving like an ordinary solid at all; instead, it occupies a superionic state, where light elements move through a rigid iron lattice with liquid-like mobility. This finding reshapes scientific views of what is happening deep within the planet.
The research team, led by Prof. Youjun Zhang and Dr. Yuqian Huang of Sichuan University and Prof. Yu He of the Institute of Geochemistry, Chinese Academy of Sciences, demonstrated that iron-carbon alloys shift into a superionic phase when subjected to intense pressure and heat. In this form, carbon atoms travel quickly through the crystal framework of solid iron, greatly reducing its stiffness.
“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.
Credit: Huang et al.
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.
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.”
The Life of Earth
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