Superionic matter might make up the inner core of Earth

A particular material that behaves like a mishmash of fluid and solid could be hidden somewhere down in the Earth.

Virtual experiences described in two studies suggest that the material in Earth's inner core, which includes iron and other, lighter elements, may be in a "superionic" state. That means that while the iron stays put, as in a solid, the lighter elements stream like a fluid.

The research gives a potential look at the inner workings of an enigmatic, inaccessible realm of the planet. According to conventional scientific wisdom, Earth's core consists of a fluid inner layer surrounding a solid inner core (SN: 1/28/19). However, past realizing that the inner core is wealthy in iron, scientists don't know exactly which other elements are present, and in what quantities.

"The inner core is truly challenging to scrutinize simply because it's so far below our feet," says geophysicist Hrvoje Tkalčić of Australian National University in Canberra.

Seismic waves stirred up by earthquakes can crash through the inner core, giving clues to what's inside. Yet, measurements of these waves have left researchers confounded. The speed of one kind of wave, called a shear wave, is lower than anticipated for solid iron or for many types of iron alloys — mixtures of iron with other materials. "That is a mystery about the inner core," says geophysicist Yu He of the Chinese Academy of Sciences in Guiyang.

In one new study, He and colleagues simulated a group of 64 iron atoms, along with various types of lighter elements — hydrogen, carbon and oxygen — under pressures and temperatures expected for the inner core. In a normal solid, atoms arrange themselves in a precise grid, clinging tightly to their positions. In a superionic material, some of the atoms arrange neatly, as in a solid, while others are fluid like nonconformists that slip directly through the solid lattice. In the simulation, the researchers found, the lighter elements moved about while the iron stayed in place.

That superionic status slowed shear waves, the researchers report February 9 in Nature, suggesting the weird phase of matter could explain the startling shear wave speed measured in the inner core.

Shear waves, also known as secondary or S waves, wiggle the Earth perpendicular to their course of travel, similar to the undulations that move along a leap rope that's squirmed up and down (SNS: 1/12/18). Other waves, called primary or P waves, compress and expand the Earth toward a path parallel to their travel, similar to an accordion being squeezed.

To really explain the inner core, scientists must find a combination of elements that keeps with all that scientists are familiar the inner core, including its S wave speed, P wave speed and its density. "You have to match all three things, otherwise it doesn't work," says mineral physicist John Brodholt of University College London.

In a study published in August 2021 in Earth and Planetary Science Letters, Brodholt and colleagues did just that. A simulation of iron, silicon and hydrogen atoms replicated the inner core's known characteristics. In the simulation, the material was also superionic: The iron and silicon stayed in position while the hydrogen streamed like a fluid.

Yet, Brodholt notes that their result is just one possible explanation for the inner core's properties. Brodholt and his colleagues have previously found other combinations of elements that could explain the inner core without going superionic, he says, leaving unresolved the question of what lurks in Earth's deepest depths.

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Another riddle of Earth's heart is the fact that the inner core's structure seems to change after some time. This has previously been deciphered as proof that the inner core rotates at an unexpected rate in comparison to the rest of the Earth. Yet, He and colleagues suggest that it could instead result from the motions of fluid like light elements swirling inside the inner core and changing the distribution of elements over the long haul. "This paper sort of offers an explanation for both of these phenomena" — the slow shear wave speed and the shifting structure — says Tkalčić, who was not associated with either new study.

One thing missing is laboratory experiments showing how these combinations of elements behave under inner core conditions, says geophysicist Daniele Antonangeli of Sorbonne University in Paris, who was not engaged with the new research. Such tests could help affirm whether the simulations are right.

Previous experiments have found proof that water ice can go superionic, perhaps under conditions tracked down inside Uranus or Neptune (SN: 2/5/18). In any case, researchers can't yet test the behavior of superionic materials under the conditions remembered to exist inside Earth's core. So scientists should continue to push the tests to further extremes, Antonangeli says. "The experimentalist that is inside me craves seeing experimental validation of this."