Earth's inner core is a strange mess of superionic fluids

Earth’s inner core may be made of fluid light elements swirling around amid a solid lattice of iron, according to a computer simulation study.

A team of physicists recently modeled how alloys of iron and lighter elements like hydrogen, carbon, and oxygen would behave at the extremely high temperatures and pressures at the center of our planet. Under those conditions, the iron part of the mixture remained solid, but the lighter elements became what’s called a superionic fluid: a state more fluid than a solid, but not quite as a fluid as a liquid.

According to the paper physicist Yu He, of the Chinese Academy of Sciences, and his colleagues published in February in the journal Nature, this combination of solid iron and superionic hydrogen, carbon, or oxygen could explain a few very perplexing things seismologists have observed about our planet’s inner depths.

What’s New — He and his colleagues’ simulations suggest that under the conditions in the inner core, a simple alloy of iron and carbon could turn into something with a much more complex structure. And that, in turn, could explain a few very perplexing things seismologists have observed about our planet’s inner depths.

In computer models based on quantum mechanics, He and his colleagues simulated what would happen to molten mixtures of iron and different combinations of hydrogen, carbon, and oxygen at temperatures around 5,200 degrees Celsius and pressures of about 3.6 million atmospheres. That’s what Earth’s inner core — which starts about 5,300 kilometers below our feet — is like.

And it turns out that under these conditions, iron becomes a solid; its atoms stay in one place, vibrating quietly. But atoms of lighter elements, like the ones He and his colleagues say are most likely to be part of the mix of molten stuff in Earth’s core, bounce around more, flowing almost — but not quite — like a liquid. The result is a solid framework of iron atoms, with a fluid mix of lighter atoms floating in the spaces between them.

When He and his colleagues simulated how seismic waves would move through their model of Earth’s inner core, the results looked very similar to real-world seismic data. That told the researchers that their model was a plausible explanation for what happens in our planet’s core. It also suggested that superionic fluids in Earth’s core could explain why seismic waves move through the inner core the way they do.

Here's the Background — During an earthquake, some seismic waves, called surface waves, travel along the Earth’s crust; others, called body waves, pass right through the planet’s interior. Measuring how quickly body waves pass through the planet’s layers, and how their properties change along the way, can reveal information about our planet’s structure. Body waves, for example, are how we know that Earth’s core has two layers, and that the outer layer is mostly liquid iron.

Measuring seismic waves passing through Earth’s core has also told scientists that it’s made of something slightly less dense than pure iron. Lighter elements like hydrogen, carbon, and oxygen are probably part of the mix, forming a combination called an alloy. Those elements are likely suspects because they’re three of the four most common elements in our Solar System, and they’ve been part of Earth’s interior since the planet formed.

However, seismologists have noticed that certain seismic waves passing through Earth’s inner core slow down much more than they should if they’re passing through a solid ball of a uniformly-mixed iron alloy. That means something else must be happening in the inner core.

“Seismological observations suggest that the structure of the inner core is complicated and difficult to understand,” wrote He and his colleagues in their paper, but they say their recent simulations may point to the answer to what they call “the long-standing seismic puzzles of the inner core.”

Why It Matters — He and his colleagues say their model could explain why seismic waves slow down as they pass through Earth’s inner core, as well as why seismic waves pass through Earth’s center about 3 percent faster from pole to pole than through the equator.

Pressure waves, such as sound waves and seismic waves, travel faster through denser materials. Superionic fluids are less dense than solids; their atoms are further apart and move around more freely. That means seismic waves won’t travel through a superionic fluid as quickly as through a solid, which could explain why seismic waves slow down on their way through Earth’s inner core.

Meanwhile, while the iron atoms in the inner core are solid and stationary, superionic lighter elements are in constant motion. Earth’s interior is hot, but some areas are hotter than others. That generates a constant churning motion called convection, because hot material tends to rise while cooler material tends to sink. Convection in Earth’s mantle causes tectonic plates to move around, producing the earthquakes whose seismic waves help us understand the core.

In the inner core, if He and his colleagues are right, convection would keep the superionic fluid moving, flowing among the solid latticework of iron atoms. That constant flow might mean that some parts of the inner core might contain more solid bits than others, or more fluid. And that could explain why seismic waves move just a little bit faster from pole to pole.

“This phenomenon may provide an alternative interpretation of the different travel times of similar earthquakes,” wrote He and his colleagues.

A better understanding of how superionic fluids work in Earth's core could help us understand other worlds, too. Some planetary scientists have also suggested that superionic fluids could exist in the interiors of ice giants like Neptune, Uranus, and exoplanets of similar sizes.

What’s Next — Earth has a magnetic field because the liquid iron of the outer core also moves around thanks to convection, and that movement creates a giant electromagnet at the center of our planet. He and his colleagues suggest that Earth’s magnetic field may also affect how superionic carbon, hydrogen, and oxygen move around in the inner core.

On the other hand, deep inside the ice giants Neptune and Uranus, convecting superionic fluids might actually influence the planets’ magnetic fields.

It’s going to take further research to understand whether there’s a connection between magnetic fields and superionic fluids in Earth’s inner core, but the results could tell us something new about the dynamo that powers our planet.