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The inner core of the Earth can rotate in different directions

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The inner core of the Earth may have temporarily stopped rotating, according to a study by Chinese scientists. Contrary to appearances, this is not a harbinger of imminent destruction. Rather, the results suggest that a giant ball of iron and nickel that rotates deep beneath the Earth’s surface undergoes regular, tiny changes in speed and direction.

The Earth’s core consists of two layers. Its outer areas are built by a liquid cocktail of elements, most of which are iron and nickel, which determines the existence of the earth’s magnetic field. It surrounds the inner core – a sphere with a radius of about 1220 kilometers, composed of an alloy of the same elements with small admixtures of other atoms. This core rotates relative to the Earth’s outer layers, crust and mantle. According to research published in the journal Nature Geoscience, its rotation cycle may be more complicated than previously thought.

Pause in rotation

Researchers from Peking University based their research on data analysis, the oldest of which came from the 1990s. Their goal was to study the seismic waves generated by double earthquakes – tremors of similar magnitude occurring in the same place. In data from before 2009, researchers noted that the twin waves were moving at different rates through the inner core. This phenomenon can be explained by the fact that the core rotates faster than the mantle and crust of the Earth – successive seismic waves then pass through a slightly different part of the metal sphere.

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Data from 2009-2011 surprised scientists. It turned out that the differences in the rate at which the seismic waves passed through the core disappeared, as if the core had stopped rotating. The results from the following years showed, however, that although the differences returned, they were different than those from a few years ago. The explanation for this phenomenon could be that the nucleus began to rotate in the opposite direction. To confirm their hypothesis, the scientists turned to historical data. A similar pattern appeared in seismic records from the early 1970s.

Visualization of the Earth’s interior. The inner core is marked with the lightest colorRost9/Shutterstock

Regular cycle

Based on the collected data, the researchers suggest that the inner core of the Earth can cyclically change its direction of rotation. The cycle lasts about 70 years – for 35 years the nucleus rotates one way, then stops and begins to rotate the other way. This phenomenon may explain the cyclical, slight differences in the length of days on Earth and magnetic field anomalies.

According to this theory, in the early 1970s, the inner core was not rotating relative to the earth’s crust. Over time, it began to pick up speed and spin eastward, outstripping the speed of rotation of the Earth’s surface. At some point, however, it began to slow down, and in 2009-2011 its turnover stopped. It is currently rotating slowly westward relative to the crust. Over time, it is likely to accelerate and then slow down, reaching another lull in the 2040s.

The scientists noted that these observations prove that the Earth’s layers are dynamically interacting with each other. They occur from the innermost parts of the core to the very surface, driven by gravitational interactions and the transfer of angular momentum from the core and mantle to the crust.

Secrets of the interior of the Earth

According to other scientists, the results of the study may describe the actual state of the inner core, but they do not have to. John Vidale of the University of Southern California in Los Angeles admits that the authors of the study probably succeeded in “identifying the last 10-year period of weaker activity of the nucleus”, but it is difficult to infer its behavior on this basis.

In 2022, Vidale showed that it can be inferred from seismic wave observations that the inner core can reverse its rotation approximately every three years. Other researchers have proposed the hypothesis that the inner core does not move at all, and seismic waves do not travel at different rates through it at all – the core deforms, which causes differences observed by measuring instruments.

Vidale adds that the key to how the Earth’s inner core really “works” lies in further observations.

“All the data points to it being irrelevant to surface life, but we don’t really know what’s going on,” he says. “However, it is our responsibility to unravel this mystery,” he adds.

Nature Geoscience, ScienceAlert

Main photo source: Rost9/Shutterstock

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