Researchers have found that Earth's inner core exists in a superionic state, where carbon atoms move freely through a solid iron lattice, explaining its unexpectedly soft behavior. This discovery, confirmed through experiments simulating core conditions, resolves long-standing seismic puzzles. The findings suggest the core's dynamics may also support the planet's magnetic field.
Earth's inner core, a dense sphere of iron and light elements under extreme pressure exceeding 3.3 million atmospheres and temperatures around 2600 kelvin, has long puzzled scientists. Despite being solid, it exhibits properties like a softened metal, with seismic shear waves slowing and a Poisson's ratio resembling butter rather than steel.
A study published in National Science Review provides a breakthrough explanation. Led by Prof. Youjun Zhang and Dr. Yuqian Huang from Sichuan University, along with Prof. Yu He from the Chinese Academy of Sciences' Institute of Geochemistry, the team demonstrates that iron-carbon alloys in the inner core enter a superionic phase. In this state, carbon atoms diffuse rapidly through the stable iron framework, akin to liquid motion within a solid structure, which significantly reduces the alloy's rigidity.
"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. "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."
The evidence came from dynamic shock compression experiments, accelerating samples to 7 kilometers per second to reach 140 gigapascals and near-core temperatures. Combined with molecular dynamics simulations and in-situ sound velocity measurements, the results showed a sharp drop in shear wave speed and a rise in Poisson's ratio, matching observed seismic data.
This superionic model accounts for seismic anisotropy—variations in wave speeds by direction—and offers new insights into the geodynamo. The motion of light elements could provide an additional energy source for 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 research shifts views from a static inner core to a dynamic one, with implications for understanding rocky planets and exoplanets. It was funded by the National Natural Science Foundation of China and other programs.