Scientists uncover hidden force deep inside Earth that is changing the length of our days

Scientists have identified a hidden force deep inside Earth that has been changing the length of our days for decades. 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. The changes are too small for people to feel, but they matter for GPS navigation and global timekeeping, which depend on precise measurements of Earth’s rotation. Researchers at the University of Alberta analyzed records spanning 1964 to 2019 to investigate what drives these tiny changes. Earth's inner core, a hot, dense ball made mostly of iron and nickel, is not perfectly spherical. As it rotates, its gravitational pull interacts with uneven concentrations of mass in the mantle. That interaction creates a twisting force, known as ‘gravitational torque,’ that can slightly speed up or slow down the mantle, changing how long Earth takes to complete a rotation. The researchers linked this gravitational tug to changes in Earth’s rotation that unfold over a roughly 70-year cycle. Scientists have identified a hidden force deep inside Earth that has been changing the length of our days for decadesThe findings also suggest that Earth's solid inner core can slowly change shape over a period of years. Its material remains solid as it slowly yields to the forces around it.That flexibility proved important when the researchers tested their calculations. A rigid inner core produced changes with the wrong timing, while allowing it to deform brought the predictions into closer agreement with the observed shifts in day length.Their best estimates suggest this adjustment happens over roughly eight to 10 years, although the wider range of possible timescales stretched from about two to 31 years.The study, published in Nature on September 23, was conducted by University of Alberta physicists Huifeng Zhang and Mathieu Dumberry.They combined earlier research that used earthquake waves to track the inner core's rotation with models of movement in the liquid outer core, reconstructed from changes in Earth's magnetic field.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 millisecondsMagnetic 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.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. 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.