Uneven Core Gravitational Torque Alters Earth's Rotation Speed

New research points to a secret engine running beneath our feet, quietly stretching and shrinking our days without us even noticing it. A team at the University of Alberta dug into data stretching from 1964 through 2019. They found that Earth is not spinning at a constant pace because something deep inside is pushing and pulling against it.

The culprit lies in the planet's core, a molten ball of iron and nickel sitting atop a rocky mantle. This inner sphere is hot, dense, and far from round. As it spins, its uneven mass creates a gravitational tug on the layer above it. Scientists call this twisting force 'gravitational torque.' It acts like an invisible hand that can speed up or slow down the rotation just enough to add or subtract a few milliseconds from every 24-hour cycle.

Those fractions of a second are too tiny for humans to feel directly. But they matter immensely for technology. GPS systems and global timekeeping rely on razor-sharp measurements of how fast the planet turns. If Earth's spin wobbles, those digital clocks get out of sync unless we account for these hidden shifts.

The study reveals that this gravitational dance happens over a massive 70-year rhythm. It suggests the solid inner core itself might be slowly reshaping over years rather than staying perfectly static. This discovery changes how we think about what drives Earth's rotation.

A new study published in Nature on September 23 offers a fresh look at what happens deep inside our planet. University of Alberta physicists Huifeng Zhang and Mathieu Dumberry are the researchers behind the work. They found that Earth's material remains solid yet slowly yields to surrounding forces. That flexibility turned out to be vital when they tested their calculations. A rigid inner core created changes with the wrong timing, but allowing it to deform brought predictions into closer agreement with observed shifts in day length. Their best estimates suggest this adjustment happens over roughly eight to 10 years, though the wider range of possible timescales stretched from about two to 31 years.

To isolate the effects of the planet's interior, the team removed contributions from atmospheric winds, ocean movements and longer-term processes, including the moon's gradual braking effect on Earth's rotation. They then compared predictions from three competing mechanisms against the remaining changes in day length. A new study suggests that a gravitational tug between the planet's solid inner core and its rocky mantle can alter Earth's rotational speed, making days longer or shorter by a few milliseconds. Magnetic forces and pressure against uneven surfaces at the boundary between the core and mantle produced patterns broadly opposite to those recorded. The gravitational mechanism provided a much closer match.

The best results came when gravity acted as the main driver and the other forces pushed back, leaving a small imbalance that changed the planet's rotation. Their findings nevertheless show how tiny variations measured at Earth's surface can reveal information about the movement, composition and physical behavior of regions deep beneath our feet. The calculations also offered clues about material hidden near the bottom of the mantle. They are consistent with an electrically conducting, iron-rich layer about 1.2 miles thick, although the researchers did not directly discover or sample such a layer. Their findings also support the presence of large accumulations of chemically distinct, warmer material. The material's composition would make it denser, but its higher temperature counteracts that effect, leaving it close to the density of its surroundings.

The shifts amount to a few thousandths of a second, too small for people to feel but important for GPS navigation and global timekeeping. The results additionally favor a form of mantle mineral that deforms relatively easily, helping explain how conditions deep inside Earth influence the gravitational interaction. However, the researchers cautioned that the roughly 70-year pattern should not yet be treated as a reliably repeating cycle. 'Whether this flow structure is periodic and repeats over time, or whether it only reflects the dynamics over the past seven decades, is unknown,' the authors wrote. Their conclusions also depend on the accuracy of existing models of the inner core's rotation and liquid core flows. Some numerical estimates changed by up to 30 percent when different flow models were used. The study does not fully explain shorter fluctuations in day length unfolding over 10 to 30 years. Those changes may be driven more strongly by forces acting at the boundary between the core and mantle. The authors said better models are needed to resolve these remaining uncertainties.