Scientists found that the inner core's structure changes as it rotates. It deforms at its border, potentially accumulating more material in some areas and less in others -- almost like creating hills and valleys.
For decades, scientists have tried to understand how Earth's inner core operates compared to the rest of the planet. Now, a team of researchers have found compelling new evidence that it's changing shape right under our feet.
Like a planet inside another planet, Earth's inner core appears to move to its own beat. A team of scientists previously found that it has been spinning at slightly different speeds than the rest of solid Earth. But now, the same team found that the inner core's structure is also changing as it rotates. It deforms at its border, the new study says, potentially accumulating more material in some areas and less in others -- almost like creating hills and valleys.
The inner core helps generate the Earth's protective magnetic field and is even linked to the length of our days, but scientists are doubtful these shape changes alone will have noticeable effects on our planet. Still, the finding adds to our understanding of how the inner core behaves and is changing with time.
"This is kind of the first time we've seen the evidence for this kind of motion," said John Vidale, lead author of the study published Monday in the journal Nature Geoscience. "The surface of the inner core is moving around in ways we hadn't detected and still don't understand very well."
Earth is divided into several layers: crust, mantle, outer core, inner core and even a recently suspected innermost inner core. In the latest study, scientists studied the region where the solid iron outer core meets the liquid iron inner core -- or the inner-outer core boundary layer.
The boundary region lies 3,000 miles below our surface and is thought to be softer than a solid, but not quite a liquid. That's because the boundary is at the melting point of iron. When materials are closer to their melting point, they become softer.
"Even though that inner core part is really solid, [this boundary] is really soft," said Guanning Pang, a co-author and geophysicist at Cornell University. "Maybe as soft as jelly."
Skip to end of carousel
Hidden Planet
(Enisaurus for The Washington Post)
Welcome to The Washington Post's Hidden Planet column, which explores the wondrous, unexpected and sometimes funny science of our planet and beyond. Read more.
End of carousel
Because the core can't be visited or viewed, scientists use earthquake signals to learn about the core's movement. Seismic signals can travel from one side of Earth to the other, passing through the core on its journey. The key is to look at similar earthquakes that have struck the same spots over years. If the wave signals are different across the years, they can receive indirect information about the core.
"They're basically exactly the same earthquakes that hit years apart -- the same place, the same fault," Vidale said. "We look to see if the waveforms have changed to the seismograms of the waves that went through the inner core."
It's a common technique that scientists use to learn more about our core's speed, motion and shape. Last year, the team analyzed more than 100 seismic signals from North America to the South Sandwich Islands in the south Atlantic Ocean. They found that the inner core was rotating faster and then slower than the rest of Earth in recent decades, measuring how long it takes for the inner core to do a full revolution.
Building on that finding, the new study took another look at the seismic signals between these two regions. But this time, they looked at the signals when the inner core was in the same position. They found it was mostly equivalent but with slight variations. They determined the changes are likely tied to physical deformations at the inner core's surface.
The most likely cause for the deformation, Vidale said, are forces from the outer core. He said the circulation of the liquid outer core could be pushing and pulling the softer boundary layer. They don't know how much it's deforming or how much it's moving, but "we finally see evidence that there is some movement that's detectable."
Vidale was surprised that it could be seen. Most changes from the core are minute and are only noticeable across long geologic time periods. But these changes were noticeable over the span of a few years in the dataset.
While Earth's core can have an effect on the length of our day, these changes to the inner core's shape won't affect the planet in ways noticeable to humans.
The findings just deepen "this fascination with the inner core," Vidale said.