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Deep down the Earth is a solid ball of metal that spins independently of our spinning planet, like the top of a whirlpool around a larger, mysterious top.
This inner core has intrigued researchers since it was discovered by a Danish seismologist Inge Lehmann in 1936, and how it moves — its rotational speed and direction — has been the subject of debate for many years. A growing body of evidence suggests that the core’s spin has changed dramatically in recent years, but scientists are still divided on exactly what’s happening—and what it means.
Part of the trouble is that the Earth’s deep interior cannot be observed or sampled directly. Seismologists have obtained information about the motion of the inner core by examining how waves from large earthquakes that affect this area behave. Variations between waves of similar strengths that passed through the heart at different times allowed scientists to measure changes in the position of the inner heart and calculate its spin.
“Differential rotation of the inner core was suggested as a phenomenon in the 1970s and 80s, but it wasn’t until the 90s that seismological evidence was published,” said Dr. Lauren Waszek, senior lecturer in physical sciences at James Cook. University in Australia.
But the researchers debated how to interpret these findings, “primarily 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, “subsequent studies over the years and years to come disagree about the rate of rotation, and also about its direction in relation to the mantle,” she said. Some analysts even suggested that the core did not rotate at all.
One promising model proposed in 2023 described an inner core that had spun faster than Earth itself in the past, but was now spinning more slowly. For a while, the scientists reported, the rotation of the heart matched the rotation of the Earth. Then it slowed even more, until the heart was moving backwards relative to the fluid 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 strong new evidence for this hypothesis about the heart’s rotation rate. Research published on June 12 in the journal Nature not only confirms the central deceleration, it supports the 2023 suggestion that this central deceleration is part of a multi-year pattern of slowing and speeding up.
The new results also confirm that the changes in rotational speed follow a 70-year cycle, said study coauthor 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 the last 20 years, and I think this is telling,” Vidale said. “I think we’ve ended the debate about whether the inner core is moving, and what the pattern has been over the last few decades.”
But not everyone is convinced the matter is settled, and how a slowing of the inner core might affect our planet remains an open question – although some experts say the Earth’s magnetic field could come into play.
Magnetic attraction
Buried about 3,220 miles (5,180 kilometers) deep inside the Earth, the solid inner metal core is surrounded by an outer liquid metal core. The inner core is made mostly of iron and nickel, and is estimated to be as hot as the surface of the sun – around 9,800 degrees Fahrenheit (5,400 degrees Celsius).
The Earth’s magnetic field floats by this solid ball of hot metal, causing it to spin. At the same time, the gravity and flow of the outer core pull fluid and the mantle at the core. Over many years, the push and pull of these forces cause variations in the heart’s rotational speed, Vidale said.
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. Although the direct influence of the inner core on the magnetic field is unknown, scientists previously reported in 2023 that a slower spinning core could affect it and also shorten the length of the day slightly.
When scientists try to “see” all the way through the planet, they usually track two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. P waves move through all kinds of material; S waves only move through highly viscous solids or liquids, according to the US Geological Survey.
Seismologists noticed in the 1880s that S waves generated by earthquakes did not travel 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, appeared in unexpected places – a “shadow zone,” as Lehmann called it – creating anomalies that could not be explained. Lehmann was the first to suggest that path P waves could be interacting 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 passed through Earth’s inner core along similar paths since 1964, the authors of the 2023 study found that the spin followed a 70-year cycle. By the 1970s, the inner core was spinning slightly faster than the planet. It slowed down around 2008, and from 2008 to 2023 it started to move slightly backwards, relative to the mantle.
Heartbreak in the future
For the new study, Vidale and his co-authors observed seismic waves that produce 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 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 heart is spinning, Vidale said, that 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 rotation over time, confirming the 70-year rotation cycle. According to the researchers’ calculations, the heart is ready to start accelerating again.
Compared to other seismographic studies of the core 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,” the more challenging method,” Waszek said. However, this allowed scientists to measure changes in core rotation with greater precision, according to Vidale. If his team’s model is correct, the core rotation will begin to accelerate again in five to 10 years.
The seismographs also showed that during its 70-year cycle, the core spins and accelerates at different rates, “which will require an explanation,” Vidale said. One possibility is that the inner metal core is not as solid as expected. If it deforms as it rotates, that could disrupt the symmetry of its rotational speed, he said.
The team’s calculations also suggest that the heart’s forward and backward motion have different rates of rotation, which adds “interesting interest to the discourse,” Waszek said.
But the depth and inaccessibility of the inner heart means there are always uncertainties, she said. As for whether or not the debate about central rotation is over, “we need more data and improved interdisciplinary tools to investigate this further,” Waszek said.
‘Full of potential’
Changes in heart rate — although they can be tracked and measured — are almost imperceptible to humans on Earth’s surface, Vidale said. When the core spins slower, the mantle speeds up. This change makes the Earth rotate faster, and the length of a day shortens. But such rotation shifts translate to milliseconds in the length of a day, he said.
“In terms of that effect in human life?” he said. “I can’t imagine it means much.”
Scientists study the inner core to learn how Earth’s deep interior is formed and how activity across subsurface layers connects the entire planet. The mysterious region where the liquid outer core covers the solid inner core is particularly interesting, Vidale said. As a place where liquid and solid meet, this boundary is “charged with activity potential,” as is the core-mantle boundary and the mantle-crust boundary.
“We may have volcanoes on the inner core boundary, for example, where solid and fluid are coming together and moving,” he said.
Because the spinning of the inner core affects movement in the outer core, the rotation of the inner core is thought to contribute to the power of the Earth’s magnetic field, although more research is needed to resolve its exact role. And there’s still a lot to learn about the overall structure of the heart, Waszek said.
“New and upcoming methodologies will be key to answering the ongoing questions about the Earth’s interior, 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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