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According to a new study published in the journal Nature, research has confirmed that Earth’s inner core is recessing, or slowing down, compared to the planet’s surface. Nature.
Scientists have debated the movement of the inner core for decades. Some studies suggest that the innermost parts of the Earth rotate faster than the surface, while others argue the opposite. According to the authors, the new study provides “unequivocal evidence” that the inner core began to slow down around 2010 and is now rotating slower than the Earth’s surface.
“When we first saw the seismic records suggesting this change, we were perplexed,” John Vidale, a researcher at the University of Southern California and co-author of the study, said in a press release. “But when we found 24 more observations showing the same pattern, the result was inevitable: the inner core’s velocity had slowed for the first time in decades. Other scientists have recently argued for similar and different models, but our latest work presents the most compelling conclusion.”
Last year, a similar study by Chinese scientists warned that Earth’s inner core — a ball of almost pure iron more than 3,100 miles (5,000 km) deep that is hotter than the sun — is slowing down and rotating in the opposite direction to the planet’s surface. While the phenomenon would have no noticeable effect on daily life, it could slightly change the length of the day and deform the Earth’s crust.
Research suggests Nature, The inner core is thought to be retreating retrogradely relative to the planet’s surface because, for the first time in about 40 years, it is moving slightly slower than Earth’s mantle rather than faster. Compared to its speed over the past few decades, the inner core is slowing down.
Previous studies have shown that this lack of synchronization is a cycle-like phenomenon, and that several decades ago the inner core was rotating faster than the mantle, but in the 2010s, a phase shift occurred and the Earth’s core began to rotate slower than the surface.
The inner core is a large sphere made of iron and nickel, surrounded by an outer core. The outer core is made of the same material, but in a liquid state. The inner core is roughly the same size as the moon. Since the inner core cannot be seen or visited, it is very difficult to understand. Scientists use seismic waves from earthquakes to describe the movement of the inner core.
Vidale and his colleague Wang Wei of the Chinese Academy of Sciences analyzed waves produced by 143 pairs of identical earthquakes that occurred in the same place and produced identical seismograms.
For the study, researchers collected and analyzed 121 earthquakes that occurred around the South Sandwich Islands between 1991 and 2023. They also looked at earthquakes caused by the same nuclear bomb detonated by the Soviet Union between 1971 and 1974, as well as repeated nuclear tests from France and the United States from other studies of the inner core. The study agrees that between 2003 and 2008, the Earth’s core was rotating faster than its surface. From that year to the present, the inner core slowed down by a third and began moving in the opposite direction to the Earth’s crust.
Vidal believes the inner core’s slowing down is due to friction with the liquid-metal outer core — a process essential for life on Earth and which creates the dynamo effect that generates the Earth’s magnetic field and protects it from cosmic radiation. He adds that the slowing down is also caused by gravitational pull from the denser region of the rocky mantle above it.
Xiaodong Song, a researcher at the Institute of Theoretical and Applied Geophysics at Peking University, who published a study on the inner core in 2023, explains that the new study confirms his findings and “clearly demonstrates that a reversal is actually occurring.” He adds, “What’s important is that we’re talking about the rotation of the inner core relative to the Earth’s surface. That is, the inner core rotated slightly faster than the Earth’s rotation before 2009, but synchronized around 2009, and has been rotating slightly slower since 2009. This discovery impressively demonstrates that the Earth is very dynamic and that its deep movements are vividly visible to the human eye.”
How changes in the inner core’s motion might affect the Earth’s surface is up for speculation. Vidal points out that a reversal could change the length of a day by just a split second. “Changes of a thousandth of a second would be barely noticeable amid the din of disturbances in the ocean and atmosphere,” he says. “The inner core may be even more dynamic than we knew.”
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