Scientists have confirmed that the Earth's inner core has slowed down so much that it is moving backwards. Know what this could mean

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There is a solid metal ball inside the Earth that spins independently of our spinning planet, like a top spinning inside a larger top, shrouded in mystery.

This inner core has fascinated researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and how it spins – its rotation speed and direction – has been at the center of a decades-long debate. Mounting evidence suggests that the core's rotation has changed dramatically in recent years, but scientists are divided over what exactly is happening – and what it means.

Part of the trouble is that it is impossible to see or sample the Earth's deep interior directly. Seismologists have obtained information about the motion of the inner core by examining the behavior of waves from large earthquakes that occur in the region. Variations between waves of equal strength passing through the core at different times enabled scientists to measure changes in the state of the inner core and calculate its rotation.

“Differential rotation of the inner core was proposed as a phenomenon in the 1970s and '80s, but seismological evidence wasn't published until the '90s,” said Dr. Lauren Waszek, senior lecturer in physics at James Cook University in Australia.

But researchers debated how to interpret these findings, “mainly because of the challenge of making detailed observations of the inner core, due to its remoteness and the limited available data,” Waszek said. As a result, “studies conducted over the following years and decades disagreed on the rate of rotation and its direction with respect to the mantle,” he said. Some analyses even proposed that the core does not rotate at all.

A promising model proposed in 2023 describes an inner core that spun faster than Earth in the past, but is now spinning slower. For a while, the core's rotation matched Earth's, the scientists reported. Then it slowed even more, until the core began moving backward relative to the fluid layers surrounding it.

At the time, some experts cautioned that more data was needed to confirm this conclusion, and now another team of scientists has offered new evidence for this hypothesis about the inner core's rotation rate. The research, published June 12 in the journal Nature, not only confirms the core's slowdown, but it supports the 2023 proposal that this core slowdown is part of a decades-long pattern of slowing and increasing speed.

Scientists study the inner core to learn how Earth's deep interior formed and how activities in all subsurface layers of the planet are connected.

The new findings also confirm that changes in rotation speed follow a 70-year cycle, said Dr. John Widell, co-author of the study and dean's professor of Earth sciences at the University of Southern California's Dornsife College of Letters, Arts and Sciences.

“We've been debating this for 20 years and I think this puts a seal on it,” Widell said. “I think we've put an end to the debate on whether the inner core moves and what the pattern has been over the last few decades.”

But not everyone is convinced the matter is settled, and exactly what effects the slowing of the inner core will have on our planet is still an open question — though some experts say Earth's magnetic field could play a role.

Located about 3,220 miles (5,180 kilometers) deep inside Earth, the solid metallic inner core is surrounded by a liquid metallic outer core. The inner core is made up mostly of iron and nickel, and it is estimated to be as hot as the surface of the sun — about 9,800 degrees Fahrenheit (5,400 degrees Celsius).

Earth's magnetic field pulls on this solid ball of hot metal, causing it to spin. Also, gravity and flow of the liquid outer core and mantle pull on the core. Over several decades, the push and pull of these forces causes the core's rotation speed to change, Vidale said.

The splashing of metal-rich fluids in the outer core generates electric currents that power Earth's magnetic field, which protects our planet from deadly solar radiation. Although the inner core's direct effect on the magnetic field is unknown, scientists previously reported in 2023 that the slow-spinning core could potentially affect it and even slightly shorten the length of the day.

When scientists try to “see” through the planet, 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 pass through all types of matter; S waves only pass through solids or highly viscous fluids.

Seismologists discovered in the 1880s that S waves generated by earthquakes did not pass 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 — what Lehmann called the “shadow zone” — creating anomalies that were impossible to explain. Lehmann first suggested that stray P waves might interact with a solid inner core within the liquid outer core, based on data from a major earthquake in New Zealand in 1929.

By tracking seismic waves from earthquakes that have passed along similar paths through Earth's inner core since 1964, the authors of the 2023 study found that the spin follows a 70-year cycle. Until the 1970s, the inner core was spinning slightly faster than the planet. It slowed down around 2008, and began moving slightly counterclockwise relative to the mantle from 2008 to 2023.

For the new study, Wiedel and his co-authors observed seismic waves generated by earthquakes in the same location at different times. They found 121 instances of such earthquakes 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 core-penetrating shock waves generated by Soviet nuclear tests conducted between 1971 and 1974.

When the core rotates, it affects the timing of the wave arrivals, Widell said. Comparing the timing of seismic signals hitting 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 ready to pick up speed again.

Compared with other seismic studies of the core that measure individual earthquakes passing 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 more precisely, according to Vidale. If his team's model is correct, core rotation will begin to speed up again in about five to 10 years.

The seismographs also revealed that the core's rotation slowed and accelerated at different rates during the 70-year cycle, “which would require an explanation,” Wiedel said. One possibility is that the metallic inner core is not as solid as expected. If it deforms as it spins, that could affect the symmetry of its rotation motion, he said.

The team's calculations also show that the core's rotation rate is different for forward and backward motion, which makes “an interesting contribution to the discussion,” Waszek said.

But the depth and inaccessibility of the inner core mean that uncertainties remain, he said. As for whether the debate about core rotation is truly over, Waszek said, “We need more data and better interdisciplinary tools to investigate this further.”

Changes in core spin — though they can be tracked and measured — are nearly imperceptible to people on Earth's surface, Widell said. When the core spins more slowly, the mantle's speed increases. This shift causes Earth to spin faster, and the length of the day to shorten. But such rotational changes amount to a mere thousandth of a second in the length of the day, he said.

“What effect would that have on a person's lifetime?” he asked. “I can't imagine it means much.”

Scientists study the inner core to learn how Earth's deep interior formed and how activity in all of the planet's subsurface layers is connected. The mysterious region where the liquid outer core envelops the solid inner core is particularly interesting, Vidale said. As a place where liquid and solid meet, this boundary is “filled with the potential for activity,” as are the core-mantle boundary and the boundary between the mantle and crust.

“For example, volcanoes may occur at the inner core boundary, where solids and fluids meet and move,” he said.

Because the rotation of the inner core affects motion in the outer core, the rotation of the inner core helps power Earth's magnetic field, though more research is needed to know its exact role. There's still much to learn about the overall structure of the inner core, Waszek said.

“New and upcoming methods will be central to answering ongoing questions about Earth's inner core, including its rotation.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American, and How It Works magazine.

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