Earth’s core may have begun to slow down relative to the surface

Posted on Wed, Jan 25, 2023 at 1:00 pm

Under the Earth’s surface, the Earth’s core has stopped spinning faster than the planet’s surface and may have begun to slow down relative to it, according to a study whose results should not put an end to the controversy that sparks experts on this subject.

That core, a hot ball the size of Pluto, has stopped spinning faster than the surface and may have slowed down, according to the study published Monday in the journal Nature Geoscience.

This “planet within a planet” is made primarily of iron, 5,000 kilometers below the surface, and is free to move because it floats in the liquid envelope of the outer core.

The exact mechanism for this alternation remains debated. Because what little we know is based on careful analysis of seismic waves, which are caused by earthquakes, when they pass through the center of the planet.

Analyzing seismic wave data over the past 60 years, Xiaodong Song and Yi Yang of Peking University concluded that the core’s rotation slowed to align with the surface around 2009. It has slowed further since then.

Thus, the central core will rotate alternately faster then slower than the rest of the planet, in a motion equivalent to that of a pendulum.

A full cycle (with faster then slower rotation) of this swing is about seventy years, they say. The core would have accelerated its rotation relative to the rest of the Earth in the early 1970s, and the next cycle would have occurred in the mid-40s, according to Chinese researchers.

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– ‘A very careful study’ –

According to them, this rotation will be to some extent related to changes in the length of the day, which are slight differences in the exact time it takes for the Earth to rotate on its axis.

So far, there are few indications of the effect of this rotation on what happens on Earth’s surface. But the authors are convinced that there are physical connections between all the layers that make up the Earth. “We hope that our research will motivate researchers to design and test models that treat the Earth as an integrated dynamical system,” they explain.

Independent experts welcomed this research with interest, but with some reservations.

“This is a very careful study by excellent scientists who used a lot of data,” John Vidal, a seismologist at the University of Southern California, told AFP. But according to him, “none of the existing models explain all the available data really well.”

John Vidal published a study last year indicating that the inner core is swinging faster, changing its rotational speed every six years, according to seismic data from two nuclear explosions dating back to the late 1960s and early 1970s.

A watershed moment close to that indicated by the Chinese researchers’ study, “a coincidence,” according to the American seismologist.

– ‘Russian dolls’ –

Another theory, with a solid foundation according to Mr. Vidal: The inner core only moved significantly between 2001 and 2013, before it has stabilized since then.

For Hrvoje Tkalcic, a geophysicist at the Australian National University, the indoor core cycle is 20 to 30 years, rather than the 70 years suggested by the study in Nature Geoscience.

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“Perhaps all of these mathematical models are incorrect,” he says, because even if they explain the observed data, the latter may answer other models that have yet to be imagined.

Therefore, the geophysical community, according to him, has every reason “to be divided over this discovery, and the topic remains controversial.”

He likens seismologists to doctors “who study the internal organs of a patient with imperfect or limited equipment”. As if we are trying to understand the work of the liver only with the help of ultrasound.

Without a scanner equivalent, “our representation of the Earth’s interior remains ambiguous,” he says, anticipating more surprises in the field. Like the theory that the inner core hides a smaller ball of iron inside, similar to Russian dolls.

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