Earth’s rotation is shifting over decades, and researchers publishing in Nature have discovered that the inner core’s gravity is driving the shift. By testing models from 1964 to 2019, scientists found that gravitational pull competes with electromagnetic and topographic forces between the core and mantle to alter day lengths by milliseconds.
Our planet does not spin at a constant rate, and scientists have tracked these minute shifts for decades. According to wired.com, beneath our rocky mantle lies the outer core—a 1,400 mile thick layer of ultra-hot liquid metal that is in constant motion. While fluctuations in the atmosphere and oceans, melting continental ice, and major earthquakes can cause sudden alterations in how fast the planet turns, changes spanning several decades have long baffled geologists. Friction between the liquid core and the rocky mantle is too weak to transmit these forces, leaving researchers searching for the hidden gears driving gradual accelerations and decelerations in Earth’s spin.
How Gravity and Core Interactions Pull Against Each Other
A new study uncovers a gravitational tug-of-war taking place thousands of kilometers beneath the surface. The solid inner core sits inside a molten outer core and can rotate independently from the mantle above it. Because the inner core does not spin at the exact same rate as the rest of the planet, it falls slightly out of alignment with mass irregularities in the mantle.
Photo: Radar.am
What this means is that gravity tries to pull the layers of the inner Earth back into their more harmonic position, which influences—albeit minutely—the rotation of the planet’s surface.
Photo: Interesting Engineering
Mathieu Dumberry, geophysicist at the University of Alberta, via Wired
That desire for alignment creates a restoring force. Researchers examined three potential ways rotational momentum could transfer between these deep layers: gravitational coupling, electromagnetic coupling near the bottom of the mantle, and topographic coupling along physical irregularities at the core-mantle boundary. When Huifeng Zhang and Mathieu Dumberry tested statistical models against six decades of observations, they found that gravitational torque exerted the strongest influence on the planet’s rotation.
The findings revealed an unexpected competition between these deep forces. While gravity pulls the layers back into a harmonic position, electromagnetic and mechanical effects act in the opposite direction, creating a delicate balance that limits the overall change in Earth’s rotation.
Measuring Milliseconds and Reconstructing Decades of Spin
To verify these dynamics, the research team reconstructed interactions between 1964 and 2019 by combining seismic estimates of inner core rotation with models of liquid metal flows in the outer core. Their gravitational model successfully reproduced both the timing and magnitude of real, gradual changes observed over that span. Observations indicate that core-related processes altered the length of the day by several milliseconds between the early 1970s and 2021, with days generally getting shorter over time even though the rate fluctuates.
Photo: nature.com
This is the part that’s unclear.
Mathieu Dumberry, geophysicist at the University of Alberta, via Interesting Engineering
While other unknown factors likely contribute to the shifting rotation, the gravitational model accounts for a major component of the variation. Although the forces create a complex contest, Dumberry noted that gravity still wins in shaping the multidecadal trend, as reported by wired.com.
Probing the Planet’s Deepest Interior
Because Earth’s deep interior is extremely difficult to observe directly, surface measurements of day length offer a rare window into subterranean mechanics. According to radar.am, the mantle is a rocky layer about 3,000 kilometers thick. The physical properties governing core-mantle coupling remain uncertain, making rotation data an essential constraint for geophysicists trying to build a complete picture of the planet’s core layers.
Earth’s Internal Gravitational Tug-Of-War Is Changing The Length Of Our Days
With these findings established, researchers are already looking toward other cyclical patterns. Zhang is investigating whether similar core mechanisms might account for an observed six-year cycle in the length of Earth’s day, continuing the effort to link surface timekeeping with deep planetary physics.
By combining these improved observations with measurements of Earth’s rotation and inner core motion, we hope to develop a more detailed picture of how Earth’s deep layers interact and how motions thousands of kilometers beneath the surface subtly change the length of our day.
Huifeng Zhang, PhD student at the University of Alberta, via Interesting Engineering
