Earth’s Core May Explain Tiny Changes in the Length of a Day

Small variations in the length of Earth’s 24-hour day can be explained by a battle between forces generated by movement in the planet’s metallic core. This discovery, reported today in Nature¹, suggests that geophysicists may be on the right track to understanding the mysterious structures deep within Earth.

Earth’s rotation rate changes subtly over time, causing the length of a day to vary by just a few milliseconds over several decades. Geophysicists measure these changes by observing Earth’s position relative to distant objects in space.

Researchers have long suspected that these fluctuations are related to interactions between Earth’s core and rocky mantle, but it was unclear which forces were involved, says geophysicist Mathieu Danbury of the University of Alberta in Edmonton, Canada. Scientists know that the core “is responsible for the changes over the last few decades. But how does it happen?” he says. “This is where it gets unclear.”

How Earth’s core can change the length of a day

Earth’s core is a Mars-sized ball at the centre of the planet, made mostly of iron. The inner core is solid and can rotate independently within the molten, flowing outer core. Researchers have proposed several ways that these parts of the core could interact with the rocky mantle surrounding them, slowing or accelerating Earth’s overall rotation.

Has Earth’s inner core stopped its strange rotation?

One possibility is a mechanical torque, or torsional force. Flowing molten material in the outer core can push against irregularities and bulges at the boundary with the mantle, like a river flowing around a rock. Another possibility is an electromagnetic torque, created when the magnetic field generated by flow in the outer core attracts iron-rich parts of the mantle. A third force could arise when gravity pulls dense regions of the inner core towards unusually dense regions of the mantle.

Danbury and fellow University of Alberta colleague Zhang Huifeng developed a model to explore how different combinations of these forces change the length of Earth’s day. Key parts of the model are based on research published in 2023.²

Since the 1960s, researchers have used seismic waves from earthquakes to measure the rotation rate of the inner core relative to the rest of Earth. The study found that the inner core was spinning slightly faster than the rest of the planet until around 2010, when it began rotating more slowly.

Gravity is the dominant force in the model

The researchers found that their model best matched the record of changes in Earth’s day length when gravitational torque was the dominant force. But, like a kind of geophysical tug-of-war, gravity had to be balanced by electromagnetic and mechanical torques pushing in opposite directions. “I didn’t know they were competing against each other until I saw the results,” Chan said.

This push-and-pull battle also appears to affect the inner core’s rotational speed. According to the model, the inner core “wants to align” with the mantle because it is attracted to denser regions within it, Danbury said. However, the outer core’s roughly westward flow gradually shifts the inner core out of position, after which gravity slowly pulls it back towards its eastward position.