Researchers have made a groundbreaking discovery regarding Earth’s inner core, revealing that it exists in a superionic state. This new phase allows carbon atoms to flow freely through a solid iron lattice, fundamentally altering our understanding of the planet’s deepest layers. The findings, published on December 10, 2025, in the National Science Review, suggest that this unusual state contributes to the core’s softness and may play a role in generating Earth’s magnetic field.
Beneath the molten outer core lies the inner core, a compact sphere composed of a dense iron and light-element alloy. It is subjected to extreme pressures exceeding 3.3 million atmospheres and temperatures comparable to the surface of the Sun. For decades, scientists have grappled with the paradox of the inner core’s solid yet pliable characteristics, which slow seismic waves and exhibit a Poisson’s ratio similar to that of butter rather than steel.
Revolutionizing Earth Science
The research team, led by Prof. Youjun Zhang and Dr. Yuqian Huang from Sichuan University, alongside Prof. Yu He from the Institute of Geochemistry, Chinese Academy of Sciences, has provided a compelling explanation for this anomaly. They report that under the extreme conditions present in the inner core, iron-carbon alloys transition into a superionic phase. Here, carbon atoms move rapidly through a stable iron framework, significantly reducing the alloy’s stiffness.
The researchers noted, “For the first time, we’ve experimentally shown that iron-carbon alloy under inner core conditions exhibits a remarkably low shear velocity,” said Prof. Zhang.
This superionic state not only accounts for long-standing seismic anomalies but also reshapes our interpretation of Earth’s internal processes. The mobility of light elements like carbon may influence seismic anisotropy, which refers to directional variations in seismic wave speeds, and could be crucial in sustaining Earth’s magnetic field.
Experimental Validation and Implications
Prior computer simulations in 2022 hinted at the inner core’s potential superionic nature, but experimental confirmation had remained elusive. The research team utilized a dynamic shock compression platform to propel iron-carbon samples to speeds of 7 kilometers per second, achieving pressures of up to 140 gigapascals and temperatures nearing 2,600 Kelvin. This experimental setup accurately mimicked the conditions found in the inner core.
By combining in-situ sound velocity measurements with advanced molecular dynamics simulations, the researchers demonstrated a significant decrease in shear wave speed and an increase in Poisson’s ratio. These results are consistent with the unexpectedly soft seismic characteristics recorded within the Earth.
On an atomic level, the data revealed that carbon atoms can move freely through the iron’s structured lattice, weakening it without destroying its integrity. This discovery shifts the prevailing view of the inner core from a static model to a more dynamic one.
According to Dr. Huang, “Atomic diffusion within the inner core represents a previously overlooked energy source for the geodynamo. In addition to heat and compositional convection, the fluid-like motion of light elements may help power Earth’s magnetic engine.”
This research was supported by the National Natural Science Foundation of China, the Sichuan Science and Technology Program, and the CAS Youth Interdisciplinary Team. The implications of identifying a superionic phase extend beyond Earth, potentially improving our understanding of magnetic and thermal evolution in other rocky planets and exoplanets.
As Prof. Zhang summarizes, “Understanding this hidden state of matter brings us one step closer to unlocking the secrets of Earth-like planetary interiors.”
