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Deep within the Earth, a massive metal ball spins independently of our orbiting planet. It resembles a spinning top inside a larger top, shrouded in mystery.
This inner core has intrigued researchers since its discovery by the Danish seismologist Inge Lehmann in 1936, and how it moves — its rotational speed and direction — has been the focus of debate for decades. A growing body of evidence suggests that the core’s rotation has changed dramatically in recent years, but scientists have remained divided over what exactly is happening — and what it means.
Part of the problem is that it is impossible to directly observe or sample the Earth’s deep interior. Seismologists have gathered information about the motion of the inner core by studying how waves from large earthquakes that hit this area behave. Variations between waves of similar magnitude that passed through the core at different times have 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 senior lecturer in physical sciences at James Cook University in Australia.
But researchers disagreed about how to interpret the findings, “mainly because of the challenge of making detailed observations of the inner core, given its remoteness and the limited data available,” Waszek said. As a result, “studies over the years and decades that followed disagree about the rotation rate, and also the direction relative to the mantle,” she added. Some analyses even suggested that the core wasn’t rotating at all.
A promising model proposed in 2023 described an inner core that used to spin faster than Earth itself, but now spins more slowly. For a while, the scientists reported, the core’s rotation matched Earth’s. Then it slowed even more, until the core was moving backward relative to the fluid layers around it.
At the time, some experts cautioned that more data was needed to support 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 June 12 in the journal Nature not only confirms the core’s slowing, it also supports the 2023 proposal that this core slowdown is part of a decades-long pattern of slowing and speeding up.
The new findings also confirm that the changes in rotation speed follow a 70-year cycle, said study co-author Dr. John Vidale, a 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 point,” Vidale said. “I think we’ve ended the debate about whether the inner core moves and what the pattern has been over the last few decades.”
But not everyone is convinced the matter has been resolved, and how a slowing of the inner core could affect our planet is still an open question — though some experts say Earth’s magnetic field could play a role.
Magnetic attraction
Buried about 3,220 miles (5,180 kilometers) deep within the Earth, the solid metallic inner core is surrounded by a liquid metallic outer core. The inner core is composed mostly of iron and nickel, and is estimated to be as hot as the surface of the sun — about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
The Earth’s magnetic field pulls on this solid ball of hot metal, causing it to 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 causes variations in the core’s rotation rate, Vidale says.
The sloshing of metal-rich fluid in the outer core generates electric currents that drive Earth’s magnetic field, which shields our planet from deadly solar radiation. While the inner core’s direct influence on the magnetic field is unknown, scientists reported earlier in 2023 that a more slowly spinning core could potentially influence it, even fractionally shortening the length of a day.
When scientists try to “see” all the way through the planet, they generally track two types of seismic waves: compressional waves, or P waves, and shear waves, or S waves. P waves travel through all types of matter; S waves travel only through solids or extremely viscous fluids, according to the U.S. Geological Survey.
Seismologists in the 1880s noticed that S waves generated by earthquakes did not pass all the way through the Earth, and 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 traveled along similar paths through Earth’s inner core since 1964, the authors of the 2023 study found that the spin followed a 70-year cycle. In the 1970s, the inner core spun slightly faster than the planet. Around 2008, it slowed down, and from 2008 to 2023, it began to move slightly backward, relative to the mantle.
Future nuclear spin
For the new study, Vidale and his coauthors observed seismic waves produced by earthquakes at the same locations at different times. They found 121 examples of such quakes that occurred 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.
When the core spins, Vidale says, it affects the arrival time of the wave. Comparing the timing of seismic signals as they hit the core revealed changes in the core’s rotation over time, confirming the 70-year rotation cycle. According to the researchers’ calculations, the core is nearly ready to start speeding up 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, “making the method more challenging,” Waszek said. However, doing so also allowed scientists to measure changes in core rotation with greater precision, Vidale said. If his team’s model is correct, core rotation will speed up again in about five to 10 years.
The seismographs also revealed that the core’s rotation slows and speeds up at different rates during the 70-year cycle, “which needs an explanation,” Vidale said. One possibility is that the metallic inner core isn’t as solid as expected. If it deforms as it rotates, that could affect the symmetry of its rotation rate, he said.
The team’s calculations also suggest that the core has different rotation rates for forward and backward motion, adding “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 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 nuclear spin — while they can be tracked and measured — are virtually imperceptible to people on Earth’s surface, Vidale said. When the core slows down, the mantle speeds up. This shift causes the Earth to spin faster and the length of a day to shorten. But such rotational shifts translate into mere thousandths of a second in day length, he said.
“As for that effect in someone’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 across all of the planet’s subsurface layers. The mysterious region where the liquid outer core envelops the solid inner core is especially interesting, Vidale added. As a place where liquid and solid meet, this boundary is “filled with potential for activity,” much like the core-mantle boundary and the mantle-crust boundary.
“For example, we could have volcanoes at the boundary of the inner core, where solid and liquid matter meet and move,” he said.
Because the inner core’s rotation affects the motion of the outer core, it’s thought that the inner core’s rotation helps power Earth’s magnetic field, though more research is needed to unravel its precise role. And there’s still much to learn about the inner core’s overall structure, Waszek said.
“New and future methodologies will be critical to answering the ongoing questions about the Earth’s inner core, including rotation.”
Mindy Weisberger is a science journalist and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.
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