Deep inside Earth is a solid metal ball that rotates independently of our spinning planet, like a top whirling around inside a bigger top, shrouded in mystery.
This inner core has intrigued researchers since its discovery by Danish seismologist Inge Lehmann in 1936, and how it moves 鈥 its rotation speed and direction 鈥 has been at the center of a decades-long debate. A growing body of evidence suggests the core鈥檚 spin has changed dramatically in recent years, but scientists have remained divided over what exactly is happening 鈥 and what it means.
Part of the trouble is that Earth鈥檚 deep interior is impossible to observe or sample directly. Seismologists have gleaned information about the inner core鈥檚 motion by examining how waves from large earthquakes that ping this area behave. Variations between waves of similar strengths that passed through the core at different times enabled scientists to measure changes in the inner core鈥檚 position and calculate its spin.
鈥淒ifferential rotation of the inner core was proposed as a phenomenon in the 1970s and 鈥80s, but it wasn鈥檛 until the 鈥90s that seismological evidence was published,鈥 said Dr. Lauren Waszek, a senior lecturer of physical sciences at James Cook University in Australia.
But researchers argued over how to interpret these findings, 鈥減rimarily due to the challenge of making detailed observations of the inner core, due to its remoteness and limited available data,鈥 Waszek said. As a result, 鈥渟tudies which followed over the next years and decades disagree on the rate of rotation, and also its direction with respect to the mantle,鈥 she added. Some analyses even proposed that the core didn鈥檛 rotate at all.
One promising model proposed in 2023 described an inner core that in the past had spun faster than Earth itself, but was now spinning slower. For a while, the scientists reported, the core鈥檚 rotation matched Earth鈥檚 spin. Then it slowed even more, until the core was moving backward relative to the fluid layers around it.
At the time, some experts cautioned that more data was needed to bolster this conclusion, and now another team of scientists has delivered compelling new evidence for this hypothesis about the inner core鈥檚 rotation rate. Research published June 12 in the journal Nature not only confirms the core slowdown, it supports the 2023 proposal that this core deceleration is part of a decades-long pattern of slowing down and speeding up.
The new findings also confirm that the changes in rotational speed follow a 70-year cycle, said study coauthor Dr. John Vidale, Dean鈥檚 Professor of Earth Sciences at the University of Southern California鈥檚 Dornsife College of Letters, Arts and Sciences.
鈥淲e鈥檝e been arguing about this for 20 years, and I think this nails it,鈥 Vidale said. 鈥淚 think we鈥檝e ended the debate on whether the inner core moves, and what鈥檚 been its pattern for the last couple of decades.鈥
But not all are convinced that the matter is settled, and how a slowdown of the inner core might affect our planet is still an open question 鈥 though some experts say Earth鈥檚 magnetic field could come into play.
Buried about 3,220 miles (5,180 kilometers) deep inside Earth, the solid metal inner core is surrounded by a liquid metal outer core. The inner core is made mostly of iron and nickel, and it is estimated to be as hot as the surface of the sun 鈥 about 9,800 degrees Fahrenheit (5,400 degrees Celsius).
Earth鈥檚 magnetic field yanks at this solid ball of hot metal, making it spin. At the same time, the gravity and flow of the fluid outer core and mantle drag at the core. Over many decades, the push and pull of these forces cause variations in the core鈥檚 rotational speed, Vidale said.
The sloshing of metal-rich fluid in the outer core generates electrical currents that power Earth鈥檚 magnetic field, which protects our planet from deadly solar radiation. Though the inner core鈥檚 direct influence on the magnetic field is unknown, scientists had previously reported in 2023 that a slower-spinning core could potentially affect it and also fractionally shorten the length of a day.
When scientists attempt to 鈥渟ee鈥 all the way through the planet, they are generally tracking two types of seismic waves: pressure waves, or P waves, and shear waves, or S waves. P waves move through all types of matter; S waves only move through solids or extremely viscous liquids, according to the US Geological Survey.
Seismologists noted in the 1880s that S waves generated by earthquakes didn鈥檛 pass all the way through Earth, and so they concluded that Earth鈥檚 core was molten. But some P waves, after passing through Earth鈥檚 core, emerged in unexpected places 鈥 a 鈥渟hadow zone,鈥 as Lehmann called it 鈥 creating anomalies that were impossible to explain. Lehmann was the first to suggest that wayward P waves might be interacting with a solid inner core within the liquid outer core, based on data from a massive earthquake in New Zealand in 1929.
By tracking seismic waves from earthquakes that have passed through the Earth鈥檚 inner core along similar paths since 1964, the authors of the 2023 study found that the spin followed a 70-year cycle. By the 1970s, the inner core was spinning a little faster than the planet. It slowed around 2008, and from 2008 to 2023 began moving slightly in reverse, relative to the mantle.
For the new study, Vidale and his coauthors observed seismic waves produced by earthquakes in the same locations at different times. 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鈥檚 southernmost tip. The researchers also looked at core-penetrating shock waves from Soviet nuclear tests conducted between 1971 and 1974.
When the core turns, Vidale said, that affects the arrival time of the wave. Comparing the timing of seismic signals as they touched the core revealed changes in core rotation over time, confirming the 70-year rotation cycle. According to the researchers鈥 calculations, the core is just about ready to start speeding up again.
Compared with other seismographic studies of the core that measure individual earthquakes as they pass through the core 鈥 regardless of when they occur 鈥 using only paired earthquakes reduces the amount of usable data, 鈥渕aking the method more challenging,鈥 Waszek said. However, doing so also enabled scientists to measure changes in the core rotation with greater precision, according to Vidale. If his team鈥檚 model is correct, core rotation will start speeding up again in about five to 10 years.
The seismographs also revealed that, during its 70-year cycle, the core鈥檚 spin slows and accelerates at different rates, 鈥渨hich is going to need an explanation,鈥 Vidale said. One possibility is that the metal inner core isn鈥檛 as solid as expected. If it deforms as it rotates, that could affect the symmetry of its rotational speed, he said.
The team鈥檚 calculations also suggest that the core has different rotation rates for forward and backward motion, which adds 鈥渁n interesting contribution to the discourse,鈥 Waszek said.
But the depth and inaccessibility of the inner core mean that uncertainties remain, she added. As for whether or not the debate about core rotation has truly ended, 鈥渨e need more data and improved interdisciplinary tools to investigate this further,鈥 Waszek said.
Changes in core spin 鈥 though they can be tracked and measured 鈥 are all but imperceptible to people on Earth鈥檚 surface, Vidale said. When the core spins more slowly, the mantle speeds up. This shift makes Earth rotate faster, and the length of a day shortens. But such rotational shifts translate to mere thousandths of a second in day length, he said.
鈥淚n terms of that effect in a person鈥檚 lifetime?鈥 he said. 鈥淚 can鈥檛 imagine it means much.鈥
Scientists study the inner core to learn how Earth鈥檚 deep interior formed and how activity connects across all the planet鈥檚 subsurface layers. The mysterious region where the liquid outer core envelops the solid inner core is especially interesting, Vidale added. As a place where liquid and solid meet, this boundary is 鈥渇illed with potential for activity,鈥 as are the core-mantle boundary and the boundary between mantle and crust.
鈥淲e might have volcanoes on the inner core boundary, for example, where solid and fluid are meeting and moving,鈥 he said.
Because the spinning of the inner core affects movement in the outer core, inner core rotation is thought to help power Earth鈥檚 magnetic field, though more research is required to unravel its precise role. And there is still much to be learned about the inner core鈥檚 overall structure, Waszek said.
鈥淣ovel and upcoming methodologies will be central to answering the ongoing questions about Earth鈥檚 inner core, including that of rotation.鈥
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.
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