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Deep within the Earth lies a solid metal sphere that spins independently of the Earth, which spins on its axis, like a spinning top inside a mysterious larger top.
The inner core has intrigued researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and its motion (its speed and direction) has been the center of debate for decades. In recent years, increasing evidence suggests that the rotation of the inner core has changed dramatically, but scientists remain divided on what exactly is happening and what it means.
Part of the problem is that it’s impossible to directly observe or sample the Earth’s depths. Seismologists have gleaned information about the movement of the inner core by studying how waves from large earthquakes in the region behave. Variations between waves of similar strength that passed through the core at different times have allowed scientists to measure the inner core’s changing position and calculate its rotation.
“Differential rotation of the inner core was proposed as a phenomenon in the 1970s and 1980s, but seismological evidence was only published in the 1990s,” said Dr Lauren Wazek, senior lecturer in physical sciences at James Cook University in Australia.
But researchers debated how to interpret these findings. “The main reason is that it’s difficult to look at the inner core in detail because of its great distance and the limited data available,” Wazek said. As a result, “studies conducted over the ensuing years and decades have not agreed on its rotation speed and orientation relative to the mantle,” she added. Some analyses have suggested that the inner core doesn’t rotate at all.
A promising model Proposed in 2023 The inner core, which once rotated faster than Earth, is now rotating slower, the scientists explain. For a time, the inner core’s rotation matched Earth’s rotation on its axis, the scientists say. Then, the inner core began to rotate more slowly, eventually moving backwards relative to the surrounding fluid layer.
At the time, some experts cautioned that more data was needed to support this conclusion, but now another team of scientists has presented compelling new evidence about the inner core’s rotation speed that supports this hypothesis. Nature This not only confirms the slowdown in the core, but also supports the 2023 suggestion that this core slowdown is part of a multi-decade pattern of deceleration and acceleration.
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Scientists study the inner core to learn how the deep Earth formed and how activity is connected across all of Earth’s subsurface layers.
The new findings also confirm that changes in rotation speed occur on a 70-year cycle, said the study’s co-authors. Dr. John VidaleDawn Saif is Chair of the Department of Geosciences in the College of Arts and Sciences at the University of Southern California.
“We’ve been talking about this for 20 years, and I think this really answers the question,” Vidale said. “I think this ends the debate about whether the inner core moves and what the pattern has been for the last 20 years.”
But not everyone is convinced the issue is solved, and how the slowing of the inner core will affect Earth remains an open question, although some experts say Earth’s magnetic field may play a role.
Buried about 3,220 miles (5,180 kilometers) beneath Earth, the solid metallic inner core is surrounded by a liquid metallic outer core. The inner core is made up mainly of iron and nickel and is estimated to be as hot as the surface of the Sun – about 9,800 degrees Fahrenheit (about 5,400 degrees Celsius).
Earth’s magnetic field tugs on this solid ball of hot metal, causing it to spin. At the same time, gravity and the currents of the fluid outer core and mantle drag on the core. Over decades, these pushes and pulls cause changes in the core’s rotation speed, Vidar said.
The pulsations of metal-rich fluid in the outer core generate electric currents that power Earth’s magnetic field and protect the planet from deadly solar radiation. The inner core’s direct effect on the magnetic field is unknown, but scientists have reported it before. 2023 A slow-rotating nucleus could affect that, shortening the length of the day slightly.
When scientists try to “see through” the entire Earth, they typically track two types of seismic waves: pressure waves (P waves) and shear waves (S waves). P waves can pass through any type of material, while S waves can only pass through solids or very viscous liquids. United States Geological Survey.
Seismologists realized in the 1880s that S waves produced by earthquakes didn’t penetrate the entire planet, leading them to conclude that the Earth’s core was molten. But some P waves penetrated the Earth’s core and appeared in unexpected places, in what Lehmann calls the “shadow region.” Called it — producing unexplained anomalous phenomena. Lehmann, using data from a major earthquake in New Zealand in 1929, first suggested that P waves may be interacting with a solid inner core within a liquid outer core.
The authors of the 2023 study tracked seismic waves from earthquakes that have traveled similar paths through Earth’s inner core since 1964, and found that its rotation varies with a 70-year cycle. Until the 1970s, the inner core rotated a little faster than Earth. Around 2008, its rotation slowed, and between 2008 and 2023, it began to move slightly in the opposite direction relative to the mantle.
In the new study, Vidale and his co-authors looked at seismic waves produced by earthquakes that occurred in the same place at different times. They found 121 examples of similar earthquakes that occurred between 1991 and 2023 in the South Sandwich Islands, a group of volcanic islands in the Atlantic Ocean east of the southernmost 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 rotates, it affects the time it takes for seismic waves to reach us,” Videlle said. By comparing the timing of when seismic signals hit the core, they were able to see that the core’s rotation changes over time, confirming that it rotates every 70 years. The researchers calculated that the core will soon start to accelerate again.
Other seismological studies of the core measure individual earthquakes passing through the core regardless of when they occur, but using only paired earthquakes reduces the amount of usable data and “makes this method more difficult,” Wazek said. But doing so has allowed scientists to more precisely measure changes in the core’s rotation, Vidale said. If his team’s model is correct, the core’s rotation will start to speed up again in about five to 10 years.
The seismometers also revealed that the core’s rotation speed slows down and speeds up on a 70-year cycle. “This requires an explanation,” Vidal 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 the rotation speed, he said.
The team’s calculations also show that the core rotates at different speeds forward and backward, which Wazek said is an “interesting contribution to the debate.”
But uncertainties remain, Wazek added, due to the inner core’s depth and difficulty in accessing it. As for whether the debate over core rotation is truly over, “we need more data and improved interdisciplinary tools to investigate further,” Wazek said.
Vidar said that while changes in the core’s rotation can be tracked and measured, they are virtually undetectable to humans on Earth. When the core slows down, the mantle speeds up. This change causes the Earth to rotate faster on its axis, shortening the length of its day. But these changes in rotation amount to just a thousandth of a second in the length of a day, he said.
“In terms of the impact it has on a person’s life?” he said. “I don’t think it matters much.”
Scientists study the inner core to learn how the Earth’s depths formed and how activity connects throughout the planet’s subsurface layers. The mysterious region where the liquid outer core encases the solid inner core is particularly intriguing, Vidal added. This boundary, where liquid and solid meet, is “full of potential activity,” just like the core-mantle boundary and the mantle-crust boundary.
“For example, there may be volcanoes at the inner core boundary where solids and fluids meet and move,” he said.
Because the rotation of the inner core influences the movement of the outer core, it’s thought to help power Earth’s magnetic field, but more research is needed to understand its exact role, and there’s still a lot to learn about the inner core’s overall structure, Wazek said.
“New and emerging methods will be central to answering continuing questions about the 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.