Earth's Inner Core's Gravitational Tug Alters Day Length, Study Finds

A hidden force deep within Earth, originating from the planet's solid inner core, has been subtly altering the length of our days for decades, according to new research.

A study published in the journal Nature reveals that a gravitational interaction between the Earth's solid inner core and its rocky mantle can change the planet's rotational speed, resulting in days that are milliseconds longer or shorter.

These minute changes are imperceptible to humans but have implications for precision technologies like GPS navigation and global timekeeping, which rely on accurate measurements of Earth's rotation.

Researchers from the University of Alberta analyzed data spanning from 1964 to 2019 to understand the mechanisms behind these variations. Their findings suggest that the Earth's inner core, a dense ball primarily composed of iron and nickel, is not perfectly spherical. As it rotates, its gravitational pull interacts with uneven mass distributions in the mantle.

This interaction creates a 'gravitational torque,' a twisting force that can slightly speed up or slow down the mantle, thereby affecting the time it takes for Earth to complete one rotation. The scientists linked this gravitational tug to a roughly 70-year pattern in Earth's rotation, though it remains uncertain if this pattern is cyclical.

The study also indicates that Earth's solid inner core can gradually change shape over several years. This flexibility is crucial, as the researchers found that a rigid inner core model did not align with the observed shifts in day length. Allowing the inner core to deform, however, brought their predictions closer to reality.

These adjustments are estimated to occur over approximately eight to 10 years, with a wider possible range from two to 31 years. The research team, consisting of physicists Huifeng Zhang and Mathieu Dumberry, combined existing studies on inner core rotation, tracked via earthquake waves, with models of the liquid outer core's movement, reconstructed from changes in Earth's magnetic field.

To isolate the internal processes, the researchers accounted for and removed external influences such as atmospheric winds, ocean currents, and the long-term lunar braking effect on Earth's rotation. They then compared their findings against three potential mechanisms driving the day-length changes.

Mechanisms involving magnetic forces and pressure against uneven surfaces at the core-mantle boundary produced patterns that were largely opposite to the recorded observations. The gravitational mechanism, however, provided a significantly closer match.

The calculations also offered insights into the composition of the deep mantle, suggesting the presence of an electrically conducting, iron-rich layer about 1.2 miles thick, though this layer was not directly observed. The findings also support the existence of large accumulations of chemically distinct, hotter material. While this material would naturally be denser, its elevated temperature counteracts this, resulting in a density similar to its surroundings.

Furthermore, the study favors a type of mantle mineral that deforms relatively easily, which could help explain how deep-Earth conditions influence the gravitational interaction. The authors cautioned, however, that the observed 70-year pattern should not yet be definitively treated as a 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 stated. Their conclusions also rely on the accuracy of current models for inner core rotation and liquid core flows, as numerical estimates varied by up to 30 percent when different flow models were used.

The study does not fully account for shorter fluctuations in day length that occur over 10 to 30 years, which may be more strongly influenced by forces at the core-mantle boundary. The authors noted that improved models are needed to resolve these remaining uncertainties. Nevertheless, the findings demonstrate how subtle variations measured at the Earth's surface can reveal information about the movement, composition, and physical behavior of regions deep within the planet.