Deep within the Earth lies a solid metallic ball that spins independently of our spinning planet, like a peak inside a larger peak, shrouded in mystery. Since its discovery by Danish seismologist Inge Lehmann in 1936, this inner core has fascinated researchers. Its motion—including the speed and direction of its rotation—has been the subject of decades of debate. Mounting evidence suggests that the core’s rotation has changed dramatically in recent years, but scientists remain divided over exactly what’s happening and what it signifies.
One major challenge is that it is impossible to directly observe or sample the Earth’s deep interior. Seismologists have gathered information about the motion of the inner core by examining the behavior of waves from large earthquakes that reach this region. The differences between waves of similar magnitude that pass through the core at different times have allowed scientists to measure changes in the position of the inner core and calculate its rotation. CNN It has been reported.
“Differential rotation of the inner core was proposed as a phenomenon in the 1970s and 1980s, but seismic evidence was not published until the 1990s,” said Dr Lauren Waszczyk, a senior lecturer in physical sciences at James Cook University in Australia.
Researchers have debated how to interpret these results. “The challenge of making detailed observations of the inner core, due to its distance and the limited data available, is the main reason,” Wasek said. “As a result, studies in the years and decades since have differed on the rate and direction of the inner core’s rotation relative to the mantle.” Some analyses have even suggested that the core isn’t rotating at all.
In 2023, a promising model was proposed describing an inner core that once rotated faster than Earth itself, but now rotates more slowly. For a while, the core’s rotational speed matched that of Earth. Then it slowed even more, eventually moving backward relative to the surrounding liquid layers.
At the time, some experts cautioned that more data was needed to support this conclusion. Now, another team of scientists has provided compelling new evidence for the hypothesis. The research, published June 12 in the journal Nature, not only confirms the core’s slowdown, but also supports a 2023 suggestion that the slowdown is part of a long-term pattern of speed changes.
The new findings also confirm that changes in rotation speed follow a 70-year cycle, said Dr. John Vidal, study co-author and professor of earth sciences at the University of Southern California Dornsife College of Letters, Arts and Sciences.
“We’ve been arguing about this for twenty years, and I think this is the answer,” Dr. Vidal said. “I think we’ve closed the debate about whether the inner core is moving and what its pattern has been for the past two decades.”
But not everyone is convinced that the matter is settled. The effects of the slowing inner core on our planet remain an open question, with some experts suggesting that the Earth’s magnetic field may be affected.
The solid metal inner core lies 3,220 miles (5,180 kilometers) deep inside the Earth, surrounded by a liquid metal outer core. The inner core is mostly made of iron and nickel, and is estimated to be as hot as the surface of the Sun—about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
Earth’s magnetic field pulls on this solid ball of hot metal, causing it to spin. At the same time, the gravity and flow of the liquid outer core and mantle exert a tug on the core. Over many decades, this push and pull has led to variations in the core’s rotational speed, Vidal said.
The movement of mineral-rich fluids in the outer core generates electrical currents that power Earth’s magnetic field, which protects our planet from harmful solar radiation. Although the inner core’s direct influence on the magnetic field is not fully understood, scientists reported in 2023 that the slowly rotating core may be affecting it and may even be causing the length of the day to shorten slightly.
In the study, Dr. Vidal and his colleagues observed seismic waves generated by earthquakes in the same locations at different times. They found 121 examples of such earthquakes that occurred between 1991 and 2023 in the South Sandwich Islands, an archipelago of volcanic islands in the Atlantic Ocean east of the southernmost tip of South America. The researchers also looked at core-penetrating shock waves from Soviet nuclear tests conducted between 1971 and 1974.
The research revealed that the core is now spinning much more slowly and accelerating at different rates, which needs to be explained, Dr. Vidal said. One possibility, the scientists believe, is that the metallic inner core is not as solid as expected.
However, the depth and inaccessibility of the inner core means that doubts remain. Although changes in the core’s rotation can be tracked and measured, the researchers highlighted that when the core rotates slowly, the mantle speeds up. This shift makes the Earth move faster, and the length of the day becomes shorter.
“In terms of how that impacts a person’s life?” he said. “I can’t imagine that means much.”
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