“Superionic” material could make up Earth’s inner core


Image courtesy of Wikimedia Commons

Avanti Khare, Sci-Tech Editor

Two new studies may be challenging the conventional wisdom that the Earth’s outer core is a liquid shell for the compact, dense inner core. Beyond knowing that the inner core is rich in iron, scientists do not know exactly what other elements are present in its composition, and in what quantities. According to Science News, two new computer simulations suggest that the material in Earth’s inner core may be in a “superionic” state. This means that while the iron stays put as a solid, the lighter elements flow around it like a liquid.

In one new study, geophysicist Yu He of the Chinese Academy of Sciences in Guiyang and his colleagues simulated a group of 64 iron atoms and various types of lighter elements under the pressures and temperatures expected of the inner core. The elements used in this study to complement the iron were hydrogen, carbon, and oxygen. In a traditional solid material, the atoms that make up the substance are arranged in an orderly grid and do not move substantially from their positions. However in a superionic material, some of the atoms arrange themselves neatly like a traditional solid, while others move as freely as the atoms of liquids and slip through the holes in the lattice structure of the solid. The researchers found that the lighter elements flowed freely while the iron stayed in place.

A second study published in August 2021 in Earth and Planetary Science Letters lended credibility to a strange change in measurement of seismic waves through the Earth’s inner core. There are two major kinds of seismic waves that were studied. Shear (S) waves shake the Earth perpendicular to the direction of their travel. Primary (P) waves compress and expand the Earth in a direction parallel to their travel. This study showed that a simulation of iron, silicon, and hydrogen atoms reproduced the inner core’s known characteristics, including its S-wave velocity, P-wave velocity, and its density. In this simulation, the material was also superionic, but the iron and silicon stayed in a fixed shape while the hydrogen flowed like a liquid.

Another question facing scientists about the Earth’s core is the fact that its structure seems to change over time. He and colleagues suggest that these changes could result form the motions of the liquid-like light elements swirling around inside the inner core and changing the distribution of elements over time.