An international team of researchers has for the first time experimentally confirmed that Earth’s inner core possesses a hybrid physical state—between solid and liquid. The investigation was conducted by scientists from Sichuan University and the Institute of Geochemistry of the Chinese Academy of Sciences. This discovery clarifies long-standing seismic anomalies and could alter our understanding of the dynamics of the planet’s deep interior and the formation of its magnetic field.
Previously, it was believed that the inner core, encircled by the liquid outer layer, consisted of solid, compact iron. However, observations revealed that the core’s density is 3–5% lower than that of pure iron, suggesting the presence of light elements, such as carbon. Furthermore, seismic waves traverse the core at a velocity of 3.4–3.6 km/s, which is slower than expected for a dense iron body. A high Poisson’s ratio, comparable to rubber or solidified oil, also indicated unusual plasticity of the substance.
According to the researchers’ hypothesis, published in the journal National Science Review, the inner core is composed of iron-carbon alloys in a superionic state. Under conditions of extreme pressure and temperature, carbon atoms become mobile and move freely within the rigid iron crystal lattice, much like a fluid, while the iron framework maintains its solid structure.
To test this hypothesis, the scientists utilized a shock compression apparatus, accelerating alloy samples to a speed of 7 km/s under pressures up to 140 GPa and temperatures exceeding 2600 °C, simulating conditions near the upper layers of the inner core. Measurements of seismic wave velocity and molecular dynamics simulations demonstrated a sharp drop in shear wave speed and a jump in Poisson’s ratio upon the material transitioning to the superionic state.
Co-author Yu-jun Zhang explained that in this state, carbon atoms become extremely mobile, diffusing through the iron’s crystal structure and reducing the alloy’s stiffness. The superionic condition transforms the inner core’s model from static to more dynamic. It accounts for the unusual seismic features and may shed light on the planet’s internal movements, including the processes that generate Earth’s magnetic field.
The unimpeded movement of light elements within the core represents a previously unconsidered energy source for the geodynamo—the mechanism that generates the planet’s magnetic field. This data also facilitates a better comprehension of the evolution of magnetic fields in terrestrial planets, such as Mars.
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