Deep inside the Earth lies a solid metal ball that spins independently of the Earth, spinning on its axis, like a spinning top inside a larger, mysterious top. Since its discovery in 1936 by Danish seismologist Inge Lehmann, this inner core has fascinated researchers. Its motion – its speed and direction of rotation – has been the subject of debate for decades. In recent years, increasing evidence suggests that the rotation of the inner core has undergone a major change, but scientists remain divided on what exactly is happening and what it might mean.
One major challenge is the impossibility of directly observing or sampling the Earth’s depths. Seismologists have gleaned information about the movement of the inner core by studying the behavior of waves from large earthquakes that reach this region. Differences in waves of similar strength that pass through the core at different times have allowed scientists to measure the inner core’s changing position and calculate its rotation. CNN report.
“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.
Researchers have debated how to interpret these findings. “The inner core is difficult to observe in detail, mainly because it is so far away and the available data is limited,” Wazek said. As a result, “studies in the ensuing years and decades have not agreed on the speed and direction of the inner core’s rotation relative to the mantle,” she added. Some analyses have even suggested that the inner core doesn’t rotate at all.
A promising model proposed in 2023 describes an inner core that previously rotated faster than Earth, but is now rotating slower. For a period of time, the inner core’s rotation matched Earth’s rotation. Then, the inner core slowed down even more, eventually moving backwards relative to the surrounding fluid layer.
At the time, some experts cautioned that more data was needed to support this conclusion. Now, another team of scientists has presented compelling new evidence to support the hypothesis. Their study, published June 12 in the journal Nature, not only confirms the core’s slowing, but also supports a 2023 suggestion that this slowing is part of a multi-decade pattern of velocity change.
Dr John Vidale, chair of the Department of Earth Sciences at USC’s Doon Seif College of Arts and Letters and a co-author of the study, said the new findings also confirm that the changes in rotation speed continue on a 70-year cycle.
“We’ve been talking about this for 20 years, and I think this announcement is the right one,” Dr. Vidale said. “I think this ends the debate about whether the inner core moves and what the pattern has been over the past few decades.”
But not everyone is convinced the issue is settled: How the slowing of the inner core would affect Earth remains an open question, with some experts suggesting that Earth’s magnetic field could be affected.
Buried about 3,220 miles (5,180 kilometers) beneath Earth, the solid metallic inner core is surrounded by a liquid metallic outer core. Made up primarily of iron and nickel, the inner core is as hot as the surface of the Sun, estimated at about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
Earth’s magnetic field tugs on this solid ball of hot metal, causing it to spin, while gravity and the currents of the fluid outer core and mantle provide resistance. Over the decades, these pushes and pulls have changed the speed at which the core spins, Vidar said.
The movement of metal-rich fluid in the outer core generates electric currents that power Earth’s magnetic field and protect the planet from harmful solar radiation. The direct effect of the inner core on the magnetic field is not fully understood, but scientists reported in 2023 that a slower-rotating core could affect the magnetic field and slightly shorten the length of the day.
For their study, Dr. Vidale and his co-authors monitored 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.
The study revealed that the core is now rotating much slower and accelerating at a different rate, “but this needs an explanation,” Dr. Vidale said. Scientists think one possibility could be that the metallic inner core is not as solid as expected.
However, uncertainties remain due to the depth and inaccessibility of the inner core. Nevertheless, changes in the rotation of the core can be tracked and measured. The researchers emphasized that a slower rotation of the core increases the speed of the mantle. This change makes the Earth move faster and shortens the length of the day.
“In terms of the impact it has on people’s lives?” he says. “I don’t think it’s going to mean much.”
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