New research suggests that Earth’s inner core has been slowing down for 14 years, which could mean the length of a day slightly increases.

Published this month in Nature, the findings go against previous research indicating that the inner core rotates faster than the planet’s surface.

Inner Core

Around 3,000 miles below the surface, Earth’s inner core is a sphere of solid iron-nickel. It’s about the same size as the moon and surrounded by an outer core of liquid iron-nickel outer core, which generates Earth’s magnetic field. Above that outer core is the rocky mantle and, finally, the crust.

Since it can’t be seen or sampled, Earth’s inner core is famously difficult to study. The easiest way to study the core is to get data from seismic waves produced by earthquakes. In this case, the researchers used seismic data from earthquakes and nuclear tests to analyze the inner core’s movement.

Convincing Resolution

Scientists from the University of Southern California and the Chinese Academy of Sciences found that the Earth’s inner core is slowing down in relation to the planet’s surface. This change in the inner core’s movement occurred around 2010. The paper suggests that the slowing results from the churning of the liquid iron outer core and gravitational tugs from parts of the mantle.

The research shows that the inner core is moving slightly slower instead of faster than the Earth’s mantle for the first time in about 40 years. “The inner core had slowed down for the first time in many decades,” said John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences. “Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution.”

Lost In The Noise

The slowing of the inner core is expected to have implications for the length of a day simply because the slower it moves, the bigger the drag factor on the speed at which Earth rotates. Since one rotation takes 24 hours and signifies one day, a slowing inner core will mean it takes longer for Earth to complete a rotation. However, it’s expected to alter it only 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 churning oceans and atmosphere,” said Vidale.

Seismic Data

The study used seismic data from 121 earthquakes between 1991 and 2023 close to the South Sandwich Islands, a chain of uninhabited volcanic islands in the South Atlantic Ocean. Data from nuclear tests conducted by the U.S., France, and the U.S.S.R. between 1971 and 1974 were also used.

“When I first saw the seismograms that hinted at this change, I was stumped,” said Vidale. “But when we found two dozen more observations signaling the same pattern, the result was inescapable.

Wishing you clear skies and wide eyes.

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