Earth’s inner core may have started spinning in the other direction, study finds

Earth’s inner core, a hot iron ball the size of Pluto, has stopped spinning in the same direction as the rest of the planet and may even be spinning the other way, research suggested Monday.

Approximately 3,100 miles below Earth’s surface, this “planet within a planet” can spin independently because it floats on its outer core of liquid metal.

Exactly how the inner core spins has been a matter of debate among scientists, and the latest research is expected to prove controversial.

What little we know about the inner core comes from measuring tiny differences in seismic waves, created by earthquakes or sometimes nuclear explosions, as they pass through the center of the Earth.

Seeking to track the movements of the inner core, new research published in the journal Nature Geoscience analyzed seismic waves from repeated earthquakes over the past six decades.

Study authors Xiaodong Song and Yi Yang, from China’s Peking University, said they found that the inner core’s rotation “almost stopped around 2009 and then turned in the opposite direction.”

“We think the inner core rotates, relative to the Earth’s surface, from side to side, like a swing,” they told AFP.

Diagram of the Earth's interior showing the crust.

“One oscillation cycle is about seven decades long,” meaning it changes direction about every 35 years, they said.

They said it previously changed direction in the early 1970s and predicted the next change in direction would be in the mid-2040s.

The researchers said this rotation roughly aligns with changes in what’s called “day length,” small variations in the exact time it takes Earth to spin on its axis.

So far there is little to indicate that what the inner core does has much of an effect on us surface dwellers.

But the researchers said they believed there are physical links between all of Earth’s layers, from the inner core to the surface.

“We hope that our study can motivate some researchers to build and test models that treat the whole Earth as an integrated dynamical system,” they said.

“The geophysical community will be divided”

Experts not involved in the study expressed caution about their findings, pointing to several other theories and warning that many mysteries remain about the center of the Earth.

“This is a very careful study by excellent scientists who provide a wealth of data,” said John Vidale, a seismologist at the University of Southern California.

“(But) none of the models explains all the data very well in my opinion,” he added.

Vidale published research last year that suggests the inner core oscillates much faster, changing direction every six years or so. His work was based on seismic waves from two nuclear explosions in the late 1960s and early 1970s.

That time frame is at the point where Monday’s research says the inner core last changed direction, which Vidale called “sort of a coincidence.”

Another theory, which Vidale said has good evidence to support it, is that the inner core only moved significantly between 2001 and 2013 and has held steady ever since.

Hrvoje Tkalcic, a geophysicist at the Australian National University, has published research suggesting that the inner core cycles every 20 to 30 years, instead of the 70 proposed in the latest study.

“It’s very likely that these mathematical models are all wrong because they explain the observed data, but the data doesn’t require them,” Tkalcic said.

“Therefore, the geophysical community will be divided on this finding and the issue will remain controversial.”

He likened seismologists to doctors “who study the internal organs of patients’ bodies with imperfect or limited equipment.”

Lacking something like a CT scan, “our picture of Earth’s interior is still fuzzy,” he said, predicting more surprises to come.

That could include more information on the theory that the inner core might have another iron ball inside, like a Russian doll.

“Something’s going on and I think we’re going to figure it out,” Vidale said. “But it may take a decade.”

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