We are used to thinking of a day as a constant value — exactly 24 hours. But Earth does not rotate at a perfectly stable speed. The length of a day varies by milliseconds, and now scientists have proposed an explanation for these fluctuations, linking them to the movement and deformation of the planet’s inner core.

A study by physicists from the University of Alberta, published on September 23 in Nature, shows that small changes in the rotation speed of the solid inner core can create a gravitational torque that affects the rotation of the mantle and, consequently, the length of a day, as reported by Sciencedaily.

The Core and Mantle Exchange Angular Momentum

Earth is not a single solid body rotating like a monolith. Its liquid outer core and the surrounding mantle can rotate at slightly different speeds.

For about three decades, scientists have known that the rotation speed of the liquid core changes over spans of decades. Observations of Earth’s magnetic field show that the core can gradually speed up over several decades and then slow down.

The mantle responds in the opposite way. If the core speeds up, the mantle slows down slightly. When the core slows down, the mantle speeds up.

The reason is the law of conservation of Earth’s total angular momentum. If one part of the planet gains more rotational momentum, another part must compensate for that change.

Only a very small change in the mantle’s rotation speed is enough to affect the length of a day by several milliseconds.

A Gravitational “Brake” Operates Inside Earth

Until now, one question remained: exactly how do the core and mantle transfer angular momentum to each other?

Huifeng Zhang, a physics graduate student at the University of Alberta, and Professor Mathieu Dumberry propose one possible answer. According to their model, an important role may be played by the solid part of the core — the inner core, located at the very center of Earth.

It is not perfectly spherical. Because of this, its mass is distributed unevenly. If the inner core changes its rotation speed or position slightly, its irregularities can interact gravitationally with irregularities in the mass distribution of the mantle.

The resulting gravitational torque can slightly alter the mantle’s rotation speed. And along with it, the length of a day changes as well.

This means that changes in Earth’s day length may be linked not only to processes on the surface or in the liquid core, but also to what is happening at the very center of the planet.

Opposing Forces Act at the Core-Mantle Boundary

Gravitational interaction is not the only mechanism affecting Earth’s rotation. At the boundary between the core and mantle, another torque is at work, associated with friction and electromagnetic interaction.

It resists changes in rotation and effectively opposes the transfer of angular momentum between the planet’s different layers.

According to the researchers, the observed fluctuations in day length may arise from the constantly changing balance between these mechanisms: the gravitational torque associated with the inner core and the torque at the core-mantle boundary.

These are extremely small changes, but modern methods of measuring Earth’s rotation make it possible to detect them.

The Inner Core Turned Out to Be More Dynamic

The study also provides insight into how quickly Earth’s deepest internal structure can change.

Calculations by Zhang and Dumberry show that the inner core is capable of viscous deformation on a timescale of about ten years. In other words, although it is made of solid material, over sufficiently long periods of time its shape can gradually change under the influence of internal forces.

This makes Earth’s interior far more dynamic than one might assume by simply viewing the core as a motionless solid sphere.

Changes in day length by milliseconds are only an outward trace of these processes. Precise measurement of the planet’s rotation makes it possible to use this as a kind of sensor through which scientists try to understand what is happening nearly 6,400 kilometers beneath our feet.