Earth’s inner core may have started spinning in the opposite direction, the study says

Earth’s inner core may have started spinning in the opposite direction, the study says

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 in the opposite direction, research suggested Monday.

About 5,000 kilometers below Earth’s surface, this “planet within the planet” can spin independently because it floats in its liquid metal outer core.

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

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

To track the inner core’s movements, new research published in the journal Nature Geoscience analyzed seismic waves from repetitive earthquakes over the past six decades.

The study’s authors, Xiaodong Song and Yi Yang of China’s Peking University, said they found that the rotation of the inner core “came to a near standstill around 2009 and then spun in the opposite direction.”

“We believe that the inner core, relative to the Earth’s surface, rotates back and forth, like a swing,” they told AFP.

Diagram of the Earth's interior showing the crust
Diagram of the Earth’s interior showing the crust (aluminium, silicate), the mantle (magnesium, silicate), and the core (iron, nickel). Color illustration.

Photo by DeAgostini/Getty Images


“One cycle of the swing is about seven decades,” meaning it changes direction about every 35 years, they said.

Rather, they said it changed direction in the early 1970s and predicted the next reversal would be in the mid-1940s.

The researchers said this rotation roughly corresponds to changes in what’s called “daylength” — slight variations in the exact time it takes Earth to rotate 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 believe there are physical connections between all layers of the Earth, from the inner core to the surface.

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

“The geophysical community will be divided”

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

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

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

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

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

Another theory – which Vidale said there is good evidence for – is that the inner core only moved significantly between 2001 and 2013 and has remained ever since.

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

“These mathematical models are most likely all incorrect because they explain the observed data but are not required by the data,” Tkalcic said.

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

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

In the absence of something like a CT scan, “our picture of the inner Earth is still fuzzy,” he said, predicting even more surprises.

That could be more about a theory that the inner core might contain another iron ball – like a Russian doll.

“Something is going on and I think we’ll work it out,” said Vidale. “But it could take ten years.”

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