LOS ANGELOS – Earth’s inner core, a solid sphere of iron located deep within our planet, has slowed its rotation, according to new research. Scientists from the University of Southern California say their discovery challenges previous notions about the behavior of the inner core and raises intriguing questions about its influence on Earth’s dynamics.
The inner core, a mysterious realm located nearly 3,000 miles below our feet, has long been known to rotate independently of Earth’s surface. Scientists have spent decades studying this phenomenon, believing it plays a crucial role in generating our planet’s magnetic field and shaping convection patterns in the liquid outer core. Until now, it was widely accepted that the inner core was gradually spinning faster than the rest of the Earth, a process known as super-rotation. However, this latest study, published in the journal Nature, reveals a surprising twist in this narrative.
“When I first saw the seismograms that hinted at this change, I was shocked,” says John Vidale, Dean of Earth Sciences in USC Dornsife’s College of Letters, Arts and Sciences, in a statement. “But when we found two dozen other observations that signaled the same pattern, the result was inescapable. The inner core had slowed for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most compelling solution.”
Spinning slow, reverse tempo
By analyzing seismic waves generated by repeated earthquakes in the South Sandwich Islands from 1991 to 2023, researchers found that the rotation of the inner core had not only slowed, but had actually opposite direction. The team focused on a particular type of seismic wave called PKIKP, which travels through the inner core and is recorded by seismic arrays in northern North America. By comparing the waveforms of these waves from 143 pairs of repeated earthquakes, they noticed a distinct pattern.
Many of the earthquake pairs displayed seismic waveforms that changed over time, but surprisingly, they later reverted to their earlier counterparts. This observation suggests that the inner core, after a period of super-rotation from 2003 to 2008, had begun to sub-rotate, or rotate more slowly than the Earth’s surface, essentially reverting to its previous path. The researchers found that from 2008 to 2023, the inner core rotated two to three times slower than its previous super-rotation.
The study’s findings present a fascinating picture of the rotational dynamics of the inner core. Matching waveforms observed in multiple pairs of earthquakes indicate moments when the inner core returned to the positions it had occupied in the past, relative to the mantle. This model, combined with insights from previous studies, reveals that the rotation of the inner core is much more complex than a simple, stable super-rotation.
The researchers found that the super-rotation of the inner core from 2003 to 2008 was faster than its subsequent sub-rotation, suggesting an asymmetry in its behavior. This difference in rotation speed implies that the interactions between the inner core, outer core and mantle are more complicated than previously thought.
Limitations: Pieces Of The Core Puzzle
While the study provides compelling evidence for the slowing and reversal of inner core rotation, the study naturally has some limitations. Spatial coverage of seismic data is relatively sparse, especially in the North Atlantic, due to the presence of mushroom layers that prevented continuous coring. Moreover, the Earth system model used in the study, despite its sophistication, is still a simplified representation of the complex dynamics at play.
The authors emphasize the need for additional high-resolution data from a wider range of locations to strengthen their findings. They also call for continued refinement of Earth system models to better capture the intricacies of the behavior of the inner core and its interactions with the outer core and mantle.
What do the findings mean for the future?
The discovery of the deceleration and reversal of the rotation of the inner core has major implications for our understanding of the Earth’s interior and its influence on the dynamics of our planet. The behavior of the inner core is closely related to the Earth’s magnetic field and convection patterns in the outer core.
This study also raises fascinating questions about the possible consequences of changing the spin of the inner core on the Earth’s surface. Could these changes in rotation affect the Earth’s magnetic field, climate, or even the length of our days? Vidale suggests that it is possible that the variation could lead to changes in the length of a day by fractions of a second. “It’s very hard to notice, on the order of a thousandth of a second, almost lost in the noise of the oceans and the oscillating atmosphere,” he says.
Future research will undoubtedly explore these intriguing possibilities in greater depth.
“The dance of the inner core may be even more vibrant than we know so far,” adds Vidale.
This article was reviewed by StudyFinds Editor-in-Chief Steve Fink.