Artistic representation of a cross-section of the Earth, showing its different layers

Artist’s rendering of a cross-section of the Earth. The innermost layer, the inner core, is a 1,500 mile wide ball of iron.
CHRISTOPH BURGSTEDT/SCIENTIFIC PHOTO LIBRARY via Getty Images

For decades, scientists have studied the behavior of Earth’s inner core, a solid ball of iron at the heart of our planet that spans about 1,500 miles across, or nearly 70% the size of the Moon. . Some say that at the bottom of our feet, this innermost layer is rotating – and at a different speed than the rotation we experience on the surface.

In a new study, researchers hypothesize that over the past few decades, the rate of rotation of the inner core has in turn gradually slowed, synchronized with the rotation of the surface, and then slowed further. . Now the inner core is slightly behind, they say.

The idea is not alarming – the change is part of a normal 70-year cycle, scientists proposed Monday in nature geoscience. The inner core switches between rotating a little faster and a little slower than the surface, matching the speed of the surface roughly every 35 years, they write.

But not all scientists agree on the details of the rotation of the inner core, and some are not at all convinced that it rotates.

“No matter which model you like, there’s data that disagrees,” said John Vidale, a seismologist at the University of Southern California who didn’t contribute to the new research. New York Times Robin George Andrews.

Buried about 3,200 miles below the planet’s surface, the inner core reaches around 9,000 to 13,000 degrees Fahrenheit. It is not accessible for samples or direct measurements of its spin. Instead, researchers used seismic waves from earthquakes to learn more. An earthquake sends these waves deep into the Earth and through the inner core, and scientists pick them up with sensors on the other side of the planet.

In 1936, scientists used this technique to discover the existence of the inner core. Sixty years later, in 1996, two researchers realized that the time it took for seismic waves to pass through the center of the Earth changed over time, which signaled changes in the inner core, according to Scientific news“Nikk Ogasa. Based on these findings, the pair hypothesized that the inner core was spinning slightly faster than the rest of the Earth.

One of these scientists, Xiaodong Song, a geophysicist at Peking University in China, is also a co-author of the new study. In it, the researchers examined digital seismic records from the 1980s to 2021, as well as paper records of seismic activity from the 1960s and 1970s. The data suggests that around 2009, the inner core had slowed its rotation to little nearly the same speed as the Earth’s surface. Since then, it has evolved slightly more slowly, according to the researchers.

“Most of us assumed that the inner core was spinning at a slightly different regular rate than the Earth,” said Paul Richards, a seismologist at Columbia University, who did not contribute to the new paper but did. co-authored the 1996 article with Song. them Washington Postit is Carolyn Y. Johnson. “This article shows that the evidence for [faster] the rotation is strong before about 2009 and essentially dies out in the following years.

The same synchronization occurred in the early 1970s, they found, suggesting that the rotation of the inner core coincides with that of the Earth’s surface about every 35 years.

This periodic change in rotation could be due to a tug-of-war effect between Earth’s liquid outer core and the solid mantle, writes the Times. As molten metals move through the outer core, they generate electromagnetic forces that influence the inner core to spin. But the mantle’s gravity pulls in the opposite direction, slowing the rotation of the inner core. A full cycle of this process takes about 70 years, the researchers write in the new study.

Other scientists have different interpretations. Based on data from nuclear explosions in the 1970s, Vidale co-authored a paper in June that posits that inner core rotation follows a six-year cycle instead of 70 years, per Reverseit is Jon Kelvey.

Lianxing Wen, a seismologist at Stony Brook University who did not contribute to the study, does not think the inner core spins any differently than the surface at all, he tells the Job. Instead, changes to the surface of the inner core over time could drive the seismic patterns discovered by the team.

“This study misinterprets seismic signals that are caused by episodic changes in the surface of the Earth’s inner core,” Wen said in an email. Job.

Scientists study the inner core because its properties can affect the Earth’s rotational speed, and therefore the length of a day, as well as the planet’s magnetic field, for example. Scientific news.

But they still have more to learn. “I keep thinking we’re close to figuring this out,” Vidale said. Nature News“Alexandra Witze. “But I’m not sure.”

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