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Deep inside the Earth is a solid metal ball that spins independently of our rotating planet, like a top swirling inside a larger top, shrouded in mystery.
This inner core has attracted researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and its motion—the rate and direction of rotation—has been the focus of a decades-long debate. A growing body of evidence suggests that the spin of the core has changed dramatically in recent years, but scientists remain divided about exactly what’s happening—and what it means.
Part of the problem is that the deep interior of the Earth cannot be observed or directly sampled. Seismologists have obtained information about the movement of the inner core by examining how the waves from large earthquakes that ping the region behave. Variations between waves of similar strength that passed through the core at different times allowed scientists to measure changes in the position of the inner core and calculate its rotation.
“Differential rotation of the inner core was proposed as a phenomenon in the 1970s and 1980s, but it was not until the 1990s that seismological evidence was published,” said Dr. Lauren Waszek, Associate Professor of Physical Sciences at James Cook. University in Australia.
But scientists have argued about how to interpret the findings, “primarily because of the challenge of observing the inner core in detail, because of its remoteness and the limited data available,” Waszek said. As a result, “studies that followed over the next years and decades disagreed on the speed of the rotation as well as its direction with respect to the mantle,” she added. Some analyzes have even suggested that the core does not rotate at all.
One promising model proposed in 2023 described an inner core that in the past rotated faster than the Earth itself, but now rotated more slowly. Scientists reported that for a moment the rotation of the core matched that of the Earth. Then it slowed even more until the core moved backwards relative to the layers of fluid around it.
At the time, some experts cautioned that more data were needed to strengthen this conclusion, and now another team of scientists has produced compelling new evidence for this hypothesis about the rotation rate of the inner core. The research, published June 12 in the journal Nature , not only confirms the core slowing, but supports the 2023 proposal that this core slowing is part of a decades-long pattern of slowing and speeding up.
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Scientists study the inner core to find out how the deep interior of the Earth was formed and how the activity is connected across all the subsurface layers of the planet.
The new findings also confirm that rotation rate changes follow a 70-year cycle, said study co-author Dr. John Vidale, Dean’s Professor of Earth Sciences at the University of Southern California’s Dornsife College of Letters, Arts and Sciences.
“We’ve been arguing about this for 20 years and I think this is the end of it,” Vidale said. “I think we’ve ended the debate about whether the inner core is moving and what its pattern has been over the past few decades.”
But not everyone is convinced the matter is settled, and how the slowing of the inner core might affect our planet is still an open question – although some experts say Earth’s magnetic field could come into play.
A solid metallic inner core is buried about 3,220 miles (5,180 kilometers) deep inside the Earth and is surrounded by an outer core of liquid metal. The inner core is made mostly of iron and nickel and is estimated to be as hot as the sun’s surface—about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
Earth’s magnetic field tugs at this solid ball of hot metal, making it spin. At the same time, gravity and the flow of the liquid outer core and mantle pull on the core. Over many decades, the push and pull of these forces cause changes in the core’s rotation rate, Vidale said.
The cracking of metal-rich fluid in the outer core generates electrical currents that power Earth’s magnetic field, which protects our planet from the sun’s deadly radiation. Although the inner core’s direct effect on the magnetic field is unknown, scientists previously reported in 2023 that the slower rotating core could potentially affect it and also partially shorten the length of the day.
When scientists try to “see” the entire planet, they generally look for two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. P waves travel through all kinds of matter; According to the US Geological Survey, S waves only travel through solids or extremely viscous liquids.
In the 1880s, seismologists noted that the S waves generated by the earthquake did not travel all the way through the Earth, so they concluded that the Earth’s core was molten. But some P waves, after passing through the Earth’s core, appeared in unexpected places—the “shadow zone,” as Lehmann called it—and created anomalies that could not be explained. Lehmann was the first to suggest, based on data from the massive 1929 New Zealand earthquake, that arbitrary P waves could interact with a solid inner core inside a liquid outer core.
By tracking seismic waves from earthquakes that have traveled through Earth’s inner core on similar paths since 1964, the authors of the 2023 study found that the rotation followed a 70-year cycle. In the 1970s, the inner core was spinning slightly faster than the planet. It slowed down around 2008, and from 2008 to 2023 it began to move slightly in the opposite direction, relative to the mantle.
For the new study, Vidale and his co-authors observed seismic waves produced by earthquakes at the same locations at different times. They found 121 examples of such earthquakes that occurred between 1991 and 2023 in the South Sandwich Islands, an archipelago of volcanic islands in the Atlantic Ocean east of the southernmost tip of South America. The researchers also looked at shock waves penetrating the core from Soviet nuclear tests conducted between 1971 and 1974.
When the core rotates, Vidale said, it affects the arrival time of the wave. Comparing the timing of the seismic signals as they touched the core revealed changes in the core’s rotation over time, confirming a 70-year rotation cycle. According to the researchers’ calculations, the core is just about ready to start accelerating again.
Compared to other seismographic studies of the core, which measure individual earthquakes as they pass through the core — regardless of when they occur — using only paired earthquakes reduces the amount of usable data, which “makes the method more challenging,” Waszek said. But it also allowed scientists to measure changes in the core’s spin with greater precision, Vidale said. If his team’s model is correct, the core’s rotation will begin to accelerate again in about five to 10 years.
The seismographs also revealed that during its 70-year cycle, the core’s rotation slows and speeds up at different rates, “which will need an explanation,” Vidale said. One possibility is that the metal inner core is not as strong as expected. If it deforms as it spins, that could affect the symmetry of its spin rate, he said.
The team’s calculations also suggest that the nucleus has different spin rates for forward and backward motion, which adds “an interesting contribution to the discourse,” Waszek said.
But the depth and inaccessibility of the inner core mean that uncertainties remain, she added. As for whether the core rotation debate is truly over, “we need more data and improved interdisciplinary tools to explore this further,” Waszek said.
Changes in the core’s rotation — while they can be observed and measured — are almost imperceptible to humans on Earth’s surface, Vidale said. As the core rotates more slowly, the mantle speeds up. This shift causes the Earth to spin faster and the length of the day to shorten. But such rotational shifts translate into mere thousandths of a second in the length of the day, he said.
“In terms of that influence in a person’s life?” he said. “I can’t imagine it would mean much.
Scientists study the inner core to find out how the deep interior of the Earth was formed and how the activity is connected across all the subsurface layers of the planet. The mysterious region where the liquid outer core envelops the solid inner core is of particular interest, Vidale added. As the place where liquid and solid meet, this boundary is “full of potential for activity,” as are core-mantle and mantle-crust boundaries.
“We could have volcanoes at the boundary of the inner core, for example, where solid and liquid meet and move,” he said.
Because the rotation of the inner core affects motion in the outer core, it is thought that the rotation of the inner core helps power the Earth’s magnetic field, although more research is needed to reveal its exact role. And we still have a lot to learn about the overall structure of the inner core, Waszek said.
“New and upcoming methodologies will be key to answering ongoing questions about Earth’s inner core, including rotation.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works.