Earth Core Gravitational Forces Drive Shifting Length of Days

Small variations in Earth’s rotation speed are shortening days by mere milliseconds, driven by a hidden tug-of-war beneath our feet. According to a study published on September 23 in the journal Nature, geophysicists have discovered that gravitational forces within the planet’s metallic core dictate this slowdown, outcompeting opposing electromagnetic and mechanical torques.

Decoding the Shifting Length of a Day

Earth’s standard day is defined as 24 hours based on the time it takes the planet to complete one rotation, but the actual speed of this rotation varies constantly across different time scales. While atmospheric and ocean processes trigger short-term changes, and interactions with the Moon impact the planet over long periods, fluctuations observed over recent decades have proven difficult to explain. Observational data reveals that between the early 1970s and 2021, mechanisms originating in Earth’s core altered the length of a day by just a few milliseconds, continuing a general trend where days are gradually growing shorter over time.

Mathieu Dumberry, a geophysicist at the University of Alberta in Edmonton, Canada, suspected that these shifts involved interactions between the Earth’s core and its surrounding mantle. The planet’s core is roughly the size of Mars and consists primarily of iron. Its solid inner core is surrounded by a liquid outer core that can rotate independently from the rocky mantle, altering the planet’s overall rotation speed through three potential mechanisms.

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Friction, Magnetic Pulls, and Gravitational Tugs

The first possibility involves mechanical torsion generated when fluid flow in the liquid outer core pushes against rough patches at the boundary shared with the mantle, much like a river flowing around rocks. A second mechanism occurs when magnetic fields created by fluid movement in the outer core pull on iron-rich regions of the mantle, producing electromagnetic torque. The third force arises from gravity pulling dense sections of the inner core toward dense regions of the mantle.

To explore how these forces combine to alter Earth’s rotation, Dumberry and colleague Huifeng Zhang developed a specialized model. Part of their framework builds on a 2023 study that utilized seismic waves from earthquakes to measure the inner core’s rotation speed relative to the rest of the planet starting from the 1960s. That previous work revealed the inner core rotated slightly faster than the rest of the planet before beginning a deceleration phase around 2010.

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Gravity Wins the Core-Mantle Tug-of-War

When running their model, the researchers found that the simulation matched observational day-length data most closely when gravitational torque acted as the dominant force. Electromagnetic and mechanical torques pushed in opposite directions to establish equilibrium, leaving interaction forces among the solid inner core, liquid outer core, and rocky mantle to control the planet’s rotational velocity. This underground competition also dictates the rotational behavior of the inner core itself.

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According to the model, the inner core attempts to align with the mantle because its dense zones are pulled toward corresponding dense areas in the mantle. Over time, however, this alignment is disrupted by westward-flowing currents in the liquid outer core, only to be pulled slowly back toward the east by gravitational attraction. Ultimately, Dumberry concluded that gravity emerges victorious in this subterranean competition between torques.

The subtle variations in planetary rotation offer a rare window into physical phenomena happening thousands of kilometers beneath the surface. By combining improved observations with measurements of Earth’s rotation and inner core motion, we hope to provide a more detailed picture of how the deep layers of the Earth interact and change the length of a day at a very difficult-to-detect level, Zhang shared with Gizmodo.