Key Points:
- Earth’s inner core has slowed its rotation relative to the mantle and surface since 2010.
- This supports a 2023 study suggesting the inner core reverses its rotation approximately every 35 years.
- The mantle’s gravitational pull might slow and distort the inner core’s oscillation.
- Researchers are divided on whether the core’s rotation is independent or influenced by its surface shape.
A recent study in Nature reveals that Earth’s inner core has been rotating slower than the mantle and surface since around 2010. This corroborates a controversial finding from last year, suggesting the inner core may reverse its rotation relative to the Earth’s outer layers every 35 years.
Geophysicist John Vidale from the University of Southern California explains that while the inner core still rotates in the same direction as the mantle and surface, it now appears to move slower, creating the illusion of reverse rotation. This phenomenon is similar to a truck decelerating next to a bus. To a bus driver, the truck seems to move backward, although both vehicles are still moving forward to a pedestrian.
The 2023 study, which suggested a reversal in the inner core’s rotation, faced skepticism. Some researchers, like seismologist Lianxing Wen from Stony Brook University, argued that the inner core’s rotation wasn’t independent but influenced by the shifting shape of its surface. Other scientists believed the rotation fluctuated over shorter periods, proposing oscillation cycles ranging from 6 to 30 years.
In the new study, Vidale and his team analyzed seismic waves from repeating earthquakes in the South Sandwich Islands from 1991 to 2023. They discovered that out of 200 waveform comparisons, 25 matched, indicating the inner core reversed its rotation around 2008 and now rotates less than half as fast in the new direction. This slower backtracking might be caused by the gravitational influence of the mantle, which makes up 70% of Earth’s mass and may be deforming the inner core.
Vidale suggests this gravitational pull might be distorting the inner core’s oscillation. He agrees with the 2023 study’s conclusion that the inner core’s rotation likely oscillates on a 70-year cycle. However, Wen maintains that the inner core’s surface changes could explain the seismic data without invoking independent rotation.
Geophysicist Hrvoje Tkalčić from the Australian National University believes the truth might lie between these perspectives, noting that more data is needed to resolve the debate. As assumptions about Earth’s interior are necessary, different interpretations arise. Tkalčić emphasizes the need for continued observation to gain a clearer understanding.
Vidale is optimistic that the inner core’s rotation will soon reenter a vigorous phase, providing further insights. Observing the core over the next five to ten years could help clarify past and future rotational behavior.
