Sign up for CNN’s Science of Miracles newsletter. Explore the universe with news of fascinating discoveries, scientific breakthroughs and more.
CNN
–
Deep inside the Earth is a solid metal ball that spins independently of our spinning planet, like a spinning top inside a larger, mysterious top.
This inner core has intrigued researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and how it moves—the speed and direction of its rotation—has been at the center 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 over what exactly is happening — and what it means.
Part of the problem is that the Earth’s deep interior is impossible to observe or examine directly. Seismologists have gathered information about the movement of the inner core by examining how the waves from the large earthquakes that hit the area 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 80s, but it was not until the 90s that seismological evidence was published,” said Dr. Lauren Waszek, a senior lecturer in physical sciences at James Cook. University in Australia.
But researchers debated how to interpret these findings, “primarily because of the challenge of making detailed observations of the inner core, due to its remoteness and the limited data available,” Waszek said. As a result, “studies that followed over the next few years and decades disagreed on the rate of rotation, and also its direction relative to the mantle,” she added. Some analyzes even suggested that the core did not rotate at all.
A promising model proposed in 2023 described an inner core that in the past had rotated faster than the Earth itself, but was now rotating more slowly. For a time, scientists reported, the rotation of the core matched the rotation of the Earth. Then it slowed even more, until the core was moving backwards relative to the liquid layers around it.
At the time, some experts warned that more data were needed to strengthen this conclusion, and now another team of scientists has provided compelling new evidence for this hypothesis about the rotation rate of the inner core. Research published June 12 in the journal Nature not only confirms the underlying slowdown, but supports the 2023 proposal that this underlying slowdown is part of a decades-long pattern of slowing and accelerating.
The new findings also confirm that changes in rotation speed follow a 70-year cycle, said study co-author Dr. John Vidale, Dean’s Professor of Earth Sciences in 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 settles it,” Vidale said. “I think we’ve ended the debate about whether the inner core moves and what its pattern has been for the last two decades.”
But not everyone is convinced the issue is settled, and how a slowing of the inner core might affect our planet is still an open question – although some experts say the Earth’s magnetic field could come into play.
Buried about 3,220 miles (5,180 kilometers) deep inside 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 sun’s surface—about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
The Earth’s magnetic field impinges 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 drag into the core. Over many decades, the push and pull of these forces cause changes in the core’s spin rate, Vidale said.
The breakdown of metal-rich liquid in the outer core generates electrical currents that power Earth’s magnetic field, which shields 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 partially shorten the length of a day.
When scientists try to “see” across the planet, they’re generally following two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. P waves travel through all kinds of matter; S waves travel only through solids or extremely viscous fluids, according to the US Geological Survey.
Seismologists noted in the 1880s that the S waves generated by earthquakes did not travel all the way through the Earth, and so they concluded that the Earth’s core was molten. But some P waves, after passing through Earth’s core, appeared in unexpected places—a “shadow zone,” as Lehmann called it—creating anomalies that were impossible to explain. Lehmann was the first to suggest that irregular P waves could interact with a solid inner core within the liquid outer core, based on data from a massive earthquake in New Zealand in 1929.
By tracing 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 the opposite direction, relative to the mantle.
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 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 turns, Vidale said, it affects the arrival time of the wave. Comparing the timing of the seismic signals as they hit the core revealed changes in the core’s spin over time, confirming the 70-year spin cycle. According to the researchers’ calculations, the nucleus is almost ready to start accelerating again.
Compared to other core seismographic studies that measure individual earthquakes as they move 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 the spin of the core with greater precision, according to Vidale. If his team’s model is correct, the rotation of the core will begin to accelerate again in about five to 10 years.
The seismographs also found 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 inner metal core is not as strong as expected. If it deforms as it rotates, it could affect the symmetry of its rotational speed, he said.
The team’s calculations also suggest that the nucleus has different degrees of rotation 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 that uncertainties remain, she added. As for whether or not the core rotation debate is over, “we need more data and improved interdisciplinary tools to investigate this further,” Waszek said.
Changes in the spin of the core — although they can be tracked and measured — are invisible to people on Earth’s surface, Vidale said. When the core rotates more slowly, the mantle accelerates. This shift causes the Earth to spin faster and the length of a day to shorten. But such rotational shifts translate into just thousandths of a second in the length of the day, 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 the Earth’s deep interior was formed and how activity is connected throughout the planet’s underground layers. The mysterious region where the liquid outer core envelops the solid inner core is particularly interesting, Vidale added. As a place where liquid and solid meet, this boundary is “rife with the potential for activity,” as are the core-mantle boundary and the mantle-crust boundary.
“We can have volcanoes at the inner boundary of the core, for example, where solids and liquids 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 discover its exact role. And there is still much to learn about the general structure of the inner core, Waszek said.
“New and future methodologies will be essential to answering ongoing questions about Earth’s inner core, including that of rotation.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science magazine, Scientific American, and How It Works.
