(CNN) — The rotation of Earth’s inner core may have stopped and may even reverse, a new study suggests.
The Earth is made up of the crust, the mantle, and the inner and outer cores. The solid inner core is located about 5,000 kilometers below the Earth’s crust and is separated from the semi-solid mantle by the liquid outer core, which allows the inner core to rotate at a speed other than the rotation of the Earth itself.
With a radius of just over 3,500 km, Earth’s core is about the size of Mars. It is made up mostly of iron and nickel, and contains about a third of the Earth’s mass.
In research published Monday in the academic journal Nature Geoscience, Yi Yang, an associate research scientist at Peking University, and Xiaodong Song, a professor at the same university, studied the seismic waves from earthquakes that have passed through the inner core of the Earth. Earth following similar trajectories since the 1960s to deduce how fast the inner core is spinning.
The result was unexpected. Since 2009, seismic records, which previously changed over time, showed little difference. This, they said, suggested that the inner core’s rotation had stopped.
“We show striking observations indicating that the inner core has nearly ceased its rotation in the last decade and may be undergoing a reversal,” they wrote in the study.
“When you look at the decade between 1980 and 1990 you see a clear change, but when you look at 2010 to 2020 you don’t see much change,” Song added.
The spin of the inner core is driven by the magnetic field generated in the outer core and balanced by the gravitational effects of the mantle. Knowing how the inner core spins could shed light on how these layers and other processes interact deep within the Earth.
However, the speed of this rotation, and its variation, is debated, says Hrvoje Tkalcic, a geophysicist at the Australian National University, who was not involved in the study.
“The inner core doesn’t stop completely,” he said. The study’s finding, he said, “means that the inner core is now more in sync with the rest of the planet than it was a decade ago, when it was spinning a little faster.”
“Nothing cataclysmic is happening,” he added.
Song and Yang argue that, according to their calculations, a small imbalance in electromagnetic and gravitational forces could slow down and even reverse the rotation of the inner core. They believe this is part of a seven-decade cycle, and that the tipping point before the one they detected in their data around 2009/2010 occurred in the early 1970s.
Tkalcic, author of the book “The Earth’s Inner Core: Revealed by Observational Seismology,” said “the study’s data analysis is robust.” However, the study’s conclusions “should be taken with caution” as “more data and innovative methods are needed to shed light on this exciting problem.”
Song and Yang agreed on the need for further investigation.
Study of the center of the Earth
Tkalcic, who devotes an entire chapter of his book to the rotation of the inner core, suggested that the inner core cycle lasts for 20 to 30 years, instead of the 70 proposed in the latest study. He explained why these variations occur and why it is so difficult to understand what happens in the innermost parts of the planet.
“The objects of our studies are buried thousands of kilometers below our feet,” he said.
“We use geophysical inference methods to infer the internal properties of the Earth, and caution is needed until multidisciplinary findings confirm our hypotheses and conceptual frameworks,” he explained.
“Seismologists can be thought of as doctors who study the internal organs of patients’ bodies using limited or imperfect equipment. So, despite advances, our picture of Earth’s interior remains blurry, and we are still in the discovery phase.