Deep within Earth lies a solid metal ball that rotates independently of our spinning planet, like a top whirling inside a larger top, enveloped in mystery. Since its discovery by Danish seismologist Inge Lehmann in 1936, this inner core has fascinated researchers. Its movement - including rotation speed and direction - has been the subject of a decades-long debate. Increasing evidence suggests that the core's spin has changed significantly in recent years, but scientists remain divided on what exactly is happening and what it signifies.
One major challenge is that Earth's deep interior is impossible to observe or sample directly. Seismologists have gathered information about the inner core's motion by examining the behaviour of waves from large earthquakes that reach this area. Differences between waves of similar strengths that passed through the core at different times have allowed scientists to measure changes in the inner core's position and calculate its spin, CNN reported.
"Differential rotation of the inner core was proposed as a phenomenon in the 1970s and '80s, but it wasn't until the '90s that seismological evidence was published," said Dr Lauren Waszek, a senior lecturer of physical sciences at James Cook University in Australia.
Researchers have debated how to interpret these findings. "The challenge of making detailed observations of the inner core, due to its remoteness and limited available data, is the primary reason," said Waszek. Consequently, "studies in subsequent years and decades have disagreed on the rate and direction of the inner core's rotation relative to the mantle," she added. Some analyses even suggested that the core didn't rotate at all.
A promising model proposed in 2023 described an inner core that had previously spun faster than Earth itself but was now spinning slower. For a period, the core's rotation matched Earth's spin. Then, it slowed even further, eventually moving backwards relative to the fluid layers around it.
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 this hypothesis. Research published on June 12 in the journal Nature not only confirms the core's slowdown but also supports the 2023 proposal that this deceleration is part of a decades-long pattern of speed changes.
The new findings also confirm that the changes in rotational speed follow a 70-year cycle, said study co-author Dr John Vidale, Dean's Professor of Earth Sciences at the University of Southern California's Dornsife College of Letters, Arts and Sciences.
"We've been arguing about this for 20 years, and I think this nails it," Dr Vidale said. "I think we've ended the debate on whether the inner core moves and what's been its pattern for the last couple of decades."
However, not everyone is convinced the matter is settled. The implications of the inner core's slowdown for our planet remain an open question, with some experts suggesting that Earth's magnetic field could be affected.
Buried about 3,220 miles (5,180 kilometres) deep inside Earth, the solid metal inner core is surrounded by a liquid metal outer core. Composed mostly of iron and nickel, the inner core 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. Meanwhile, the gravity and flow of the fluid outer core and mantle exert a drag on the core. Over many decades, the push and pull of these forces have resulted in variations in the core's rotational speed, according to Vidale.
The movement of metal-rich fluid 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 a slower-spinning core could potentially impact it and also slightly shorten the length of a day.
Dr Vidale and his coauthors observed seismic waves produced by earthquakes in the same locations at different times for this study. They found 121 examples of such earthquakes occurring between 1991 and 2023 in the South Sandwich Islands, an archipelago of volcanic islands in the Atlantic Ocean to the east of South America's southernmost tip. 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 slower and accelerates at different rates, "which is going to need an explanation," Dr Vidale said. The scientists feel that one possibility could be metal inner core is not as solid as expected.
However, the depth and inaccessibility of the inner core mean that uncertainties remain. Although, changes in core spin can be tracked and measured. The researchers highlighted that when the core spins slowly, the mantle speeds up. This shift makes Earth move faster, and the length of a day shortens.
"In terms of that effect in a person's lifetime?" he said. "I can't imagine it means much."
