Deep inside the Earth there is a solid metal ball that spins independently of our planet’s rotation, like a top spinning inside a larger top, shrouded in mystery.
This inner core has intrigued researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and how it moves — its speed and direction of rotation — has been the center of a decades-long debate. A growing body of evidence suggests that the rotation of the nucleus has changed dramatically in recent years, but scientists are still divided over what exactly is happening — and what it means.
Part of the problem is that Earth’s deep interior is impossible to directly observe or sample. Seismologists gain information about the movement of the inner core by examining how waves from large earthquakes that hit this area behave. Variations between waves of similar forces 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 only in the 1990s that seismological evidence was published,” said Dr. Lauren Waszek, professor of physical sciences at James Cook University in Australia. .
But researchers have debated how to interpret these findings, “primarily due to the challenge of making detailed observations of the inner core due to its distance and limited data available,” Waszek said. As a result, “studies that followed in the following years and decades disagree about the rate of rotation and also its direction relative to the mantle,” she added. Some analyzes even proposed that the nucleus did not rotate at all.
A promising model proposed in 2023 described an inner core that once rotated faster than Earth itself but now spins more slowly. For a time, scientists reported, the core’s rotation coincided with the Earth’s rotation. Then it slowed down even more, until the core began to move backward relative to the fluid layers around it.
At the time, some experts warned that more data was needed to reinforce this conclusion, and now another team of scientists has provided compelling new evidence for this hypothesis about the inner core’s rotation rate. Research published on June 12 in Nature magazine not only confirm the core slowdown, but also support the 2023 proposal that this core slowdown is part of a decades-long pattern of slowing and accelerating.
The new findings also confirm that changes in rotational speed follow a 70-year cycle, said study co-author Dr. John Vidale, Professor of Earth Sciences in the Dornsife College of Letters, Arts and Sciences at the University of Southern California.
“We’ve been arguing about this for 20 years, and I think this settles the issue,” Vidale said. “I think we have settled the debate about whether the inner core moves and what its pattern has been over the last few decades.”
But not everyone is convinced the issue is resolved, and how a slowdown 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.
Magnetic attraction
Buried about 5,180 kilometers deep inside the Earth, the solid metal inner core is surrounded by a liquid metal outer core. The inner core is composed mainly of iron and nickel, and is estimated to be as hot as the surface of the sun, at around 5,400°C.
The Earth’s magnetic field pulls on this solid ball of hot metal, causing it to spin. At the same time, gravity and flow from the fluid outer core and mantle drag the core along. Over many decades, the push and pull of these forces cause variations in the core’s rotational speed, Vidale said.
The oscillating motion of the metal-rich fluid in the outer core generates electrical currents that power Earth’s magnetic field, which protects our planet from deadly solar radiation. Although the inner core’s direct influence on the magnetic field is unknown, scientists previously reported in 2023 that a slower-rotating core could potentially affect it and also weakly shorten the length of a day.
When scientists try to “see” all the way across the planet, they generally track two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. of matter; S waves move only through extremely viscous solids or liquids, according to the U.S. Geological Survey.
Seismologists noticed in the 1880s that S waves generated by earthquakes did not pass completely through the Earth, and so concluded that the Earth’s core was molten. But some P waves, after passing through the Earth’s core, emerged in unexpected places — a “shadow zone,” as Lehmann called it — creating anomalies that were impossible to explain.
Lehmann was the first to suggest that deflected P waves could be interacting with a solid inner core within the liquid outer core, based on data from a large earthquake in New Zealand in 1929.
By tracking seismic waves from earthquakes that have passed through Earth’s inner core along 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 rotating slightly faster than the planet. It slowed down around 2008, and from 2008 to 2023 it began to move slightly in reverse relative to the mantle.
Future of Core Rotation
For the new study, Vidale and his co-authors observed seismic waves produced by earthquakes in the same locations at different times. They found 121 examples of such earthquakes occurring 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 analyzed penetrating shock waves in the test core Soviet nuclear weapons conducted between 1971 and 1974.
When the core rotates, Vidale said, it affects the wave’s arrival time. Comparing the timing of the seismic signals as they touched the core, he revealed changes in the core’s rotation over time, confirming the 70-year rotation cycle. According to the researchers’ calculations, the nucleus is about to start accelerating again.
Compared to other core seismographic studies that measure individual earthquakes as they pass through the core — regardless of when they occur — using only paired earthquakes reduces the amount of usable data, “making the method more challenging,” Waszek said. However, in doing so, it also allowed scientists to measure changes in the nucleus’ rotation with greater precision, according to Vidale. If his team’s model is correct, core rotation will begin to accelerate again in about five to ten years.
The seismometers also revealed that, during its 70-year cycle, the core’s rotation slows down and speeds up at different rates, “which is going to need an explanation,” Vidale said. One possibility is that the inner metal core is not as solid as expected. If it deforms as it spins, that could affect the symmetry of its rotation speed, he said.
The team’s calculations also suggest that the nucleus has different rotation 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 means uncertainties remain, she added. As for whether the debate over core rotation is truly over, “we need more data and improved interdisciplinary tools to investigate this further,” Waszek said.
“Full of potential”
Changes in the core’s rotation — although they can be tracked and measured — are virtually imperceptible to people on Earth’s surface, Vidale said. When the core spins more slowly, the mantle speeds up. This change causes the Earth to rotate faster, and the length of a day shortens. But such rotation changes translate into mere thousandths of a second in day length, he said.
“In terms of that effect on a person’s life?” he said. “I can’t imagine it means much.”
Scientists study the inner core to learn how Earth’s deep interior formed and how activity connects throughout the planet’s subsurface layers. The mysterious region where the liquid outer core surrounds the solid inner core is especially interesting, Vidale added. As a place where liquid and solid meet, this boundary is “full of potential for activity,” just like the core-mantle boundary and the mantle-crust boundary.
“We can have volcanoes on the boundary of the inner core, for example, where the solid and fluid meet and move,” he said.
Because the rotation of the inner core affects motion in the outer core, the rotation of the inner core is thought to help power Earth’s magnetic field, although more research is needed to unravel its precise role. And there’s still a lot to learn about the structure of the inner core, Waszek said.
“New and emerging methodologies will be central to answering open questions about Earth’s inner core, including rotation.”
Earth’s core has stopped rotating and may reverse, study suggests
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