Science & Technology

Earth’s Inner Core Rotation Has Slowed Down

Researchers have found that the rotation of Earth’s inner core is slowing down in relation to the planet surface.

In a new study, scientists from the University of Southern California (USC) provide evidence that the inner core began to decrease its speed more than a decade ago, and that it is slowly and smoothly rotating on a reversing path.

Earth’s Inner Core

The inner core is a solid iron-nickel sphere around the size of the moon. It is surrounded by the liquid iron-nickel outer core, encased by a solid rocky mantle. The inner core is located more than 3,000 miles beneath the Earth’s surface. It is estimated to be as hot as the surface of the sun, with about 5,400 degrees Celsius.

Earth’s Inner Core Rotation Has Slowed Down
Earth’s structure

However, the inner core presents a challenge to researchers because scientists cannot view or visit it. Instead, they must use the seismic waves of earthquakes to create renderings of the inner core’s movements.

New Findings

According to the study, published in Nature, USC researchers analyzed seismic data recorded from various earthquakes and nuclear tests to understand the inner core’s movement. The analysis of the speed and interaction of the seismic waves within the Earth’s layers enables researchers to estimate the position and movement of the inner core.

The study found that since around 2010, the Earth’s inner core has been backtracking relative to its speed in the previous decades. Moreover, it is moving slower than the Earth’s surface.

In this regard, John Vidale, Dean’s Professor of Earth Sciences at the USC Dornsife College of Letters, Arts and Sciences, said in a statement: “When I first saw the seismograms that hinted at this change, I was stumped.”

“The inner core had slowed down for the first time in many decades. Other scientists have recently argued for similar and different models, but our latest study provides the most convincing resolution,” he added.

Magnetic Field

Earth’s magnetic field pulls the inner core, causing it to spin. Meanwhile, the gravity and the movement of the liquid outer core and mantle affect the core. Vidale explained that over many decades, the push and pull of these forces cause variations in the core’s rotational speed.

In this study, researchers analyzed seismic data recorded from 121 repeating earthquakes occurred near the South Sandwich Islands in the Atlantic Ocean between 1991 and 2023. Additionally, they have used data from several nuclear tests across the world.

Earth’s Inner Core Rotation Has Slowed Down
Earth’s inner core rotation is slowing down

Vidale attributed the slowing speed of the inner core to the churning of the liquid iron outer core surrounding it, which generates Earth’s magnetic field, as well as gravitational tugs from the dense regions of the overlying rocky mantle.

Backtracking Impact

The change in the inner core’s movement have implications for the Earth’s surface. Vidale said it may alter the length of a day by fractions of a second, although it may be hard to notice.

Vidale explained that when the core spins more slowly, the mantle’s speed increases. This makes Earth rotates faster, shortening the length of a day, according to CNN. “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,” he said.

But USC scientists look forward to charting the trajectory of the inner core in greater detail to discover the reasons behind its backtracking. “The dance of the inner core might be even more lively than we know so far,” Vidale added.

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