Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific breakthroughs and more.
CNN
—
Deep within the Earth lies a solid metal ball that spins independently of our rotating planet, like a top whirling inside a larger top, shrouded in mystery.
This inner core has intrigued researchers since it was discovered by Danish seismologist Inge Lehmann in 1936, and how it moves—its speed and direction of rotation—has been the subject of a decades-long debate. A growing body of evidence suggests that the core’s rotation has changed dramatically in recent years, but scientists remain divided over exactly what’s happening and what it means.
Part of the problem is that the depths of the Earth are impossible to observe or sample directly. Seismologists have gathered information about the motion of the inner core by examining the behavior of waves from large earthquakes that hit the area. Variations between waves of the same intensity 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, a lecturer in physical sciences at James Cook University in Australia.
But researchers have debated how to interpret these results, “mainly because of the difficulty of making detailed observations of the inner core, due to its remoteness and the limited data available,” Waszek said. As a result, “subsequent studies over the next years and decades disagree on how fast it’s rotating, or where it’s going relative to the mantle,” she added. Some analyses have even suggested that the core isn’t rotating at all.
One promising model proposed in 2023 described an inner core that once spun faster than Earth itself but was now spinning more slowly. For a while, the scientists reported, the core’s rotation kept pace with Earth’s. Then it slowed even more, until the core was moving backward relative to the fluid layers surrounding it.
At the time, some experts warned that more data was needed to support this conclusion, and now another team of scientists has provided compelling new evidence to support this hypothesis about the inner core’s rotation speed. The research, published June 12 in the journal Nature, not only confirms the core’s slowdown, it supports the 2023 proposal that this core deceleration is part of a decades-long pattern of slowing and speeding up.
pourplayday/iStockphoto/Getty Images
Scientists study the inner core to understand how the depths of the Earth were formed and how activity connects throughout all of the planet’s subsurface layers.
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 debating this for 20 years, and I think this question is at the heart of the debate,” Vidale said. “I think we’ve settled the debate about whether the inner core is moving or not, and how it’s been changing over the last couple of decades.”
But not everyone is convinced the issue is settled, and how a slowing inner core might affect our planet remains an open question — although some experts say Earth’s magnetic field could come into play.
Buried about 5,180 kilometers deep in the Earth, the solid metal inner core is surrounded by a liquid metal outer core. The inner core is composed primarily of iron and nickel, and is estimated to be as hot as the surface of the sun, about 5,400 degrees Celsius.
Earth’s magnetic field pulls on this solid ball of hot metal, making it spin. At the same time, gravity and the flow of the fluid outer core and mantle pull the core along. Over decades, the push and pull of these forces cause variations in the core’s rotation rate, Vidale explained.
The sloshing of metal-rich fluid in the outer core generates electrical currents that power Earth’s magnetic field, which protects our planet from deadly solar radiation. While the direct influence of the inner core on the magnetic field is unknown, scientists had already reported in 2023 that a slower-rotating core could potentially affect it and also slightly shorten the length of a day.
When scientists try to “see” the planet in its entirety, they typically track two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. According to the U.S. Geological Survey, P waves travel through all types of matter; S waves travel only through solids or extremely viscous liquids.
Seismologists noticed in the 1880s that S waves generated by earthquakes did not pass 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, 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 wayward P waves might interact with a solid inner core within the liquid outer core, based on data from a massive earthquake in New Zealand in 1929.
By tracking seismic waves from earthquakes that have passed through Earth’s inner core on similar paths since 1964, the authors of the 2023 study found that the rotation follows 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 moving slightly in the opposite direction, relative to the mantle.
For the new study, Vidale and his coauthors looked at 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 southern tip of South America. The researchers also looked at core-penetrating shock waves from Soviet nuclear tests conducted between 1971 and 1974.
According to Vidale, the rotation of the core affects the arrival time of the wave. Comparing the propagation times of seismic signals at the contact with the core revealed changes in the rotation of the core over time, confirming the 70-year rotation cycle. According to the researchers’ calculations, the core is about to resume its cruising speed.
Compared with other core seismography 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 difficult,” Waszek said. However, the method also allowed scientists to measure changes in the core’s rotation with greater precision, Vidale said. If his team’s model is correct, the core’s rotation will start to accelerate again in about five to 10 years.
The seismographs also revealed that, over the course of its 70-year cycle, the core’s rotation slows down and speeds up at different rates, “which is going to require some explanation,” Vidale said. One possibility is that the inner metal core isn’t as strong as expected. If it warps as it spins, that could affect the symmetry of its rotation rate, he said.
The team’s calculations also suggest that the nucleus has different rotation rates for forward and backward movements, 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 to whether the debate over the core’s rotation is truly over, “we need more data and improved interdisciplinary tools to study this topic more deeply,” Waszek said.
Changes in the core’s rotation, while they can be tracked and measured, are nearly imperceptible to people on Earth’s surface, Vidale explained. As the core spins more slowly, the mantle speeds up. That change speeds up Earth’s rotation and shortens the length of the day. But such rotational changes result in days that last a few thousandths of a second, he added.
“In terms of the effect on a person’s life?” he said. “I don’t think it means much.”
Scientists study the inner core to understand how the Earth’s depths formed and how activity connects to all the planet’s subsurface layers. The mysterious region where the liquid outer core envelops the solid inner core is particularly interesting, Vidale added. As the meeting place of liquid and solid, this boundary is “filled with potential for activity,” much like the core-mantle boundary and the mantle-crust boundary.
“We could have volcanoes at the boundary of the inner core, for example, where the solid and the fluid meet and move,” he said.
Because the rotation of the inner core affects the motion of the outer core, the rotation of the inner core is thought to help power Earth’s magnetic field, although more research is needed to elucidate its precise role. And there is still much to learn about the overall structure of the inner core, Waszek said.
“New and upcoming methodologies will be essential to answer current questions about the 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 magazine.