A gravitational tug between Earth’s solid inner core and its rocky mantle may be altering the length of our days by a few milliseconds, researchers at the University of Alberta have found.
The imperceptible changes in Earth’s rotation are too small for people to feel, but are significant for GPS navigation and global timekeeping, which rely on precise measurements of the planet’s movement.
Physicists Huifeng Zhang and Mathieu Dumberry examined records from 1964 to 2019 to investigate what drives the fluctuations. Their study was published in Nature on September 23.
How the inner core affects Earth’s rotation
Earth’s inner core is a hot, dense sphere made mainly of iron and nickel. It is not perfectly round, and its gravitational pull interacts with uneven concentrations of mass in the surrounding mantle.
That interaction creates a twisting force known as gravitational torque. The force can slightly accelerate or slow the mantle, changing the time taken for the planet to complete one rotation.
The researchers linked the effect to changes unfolding over a period of roughly 70 years. Their findings also indicate that the inner core can gradually alter its shape over several years while remaining solid.
When the team tested its calculations, a rigid inner core produced changes at the wrong time. Predictions came closer to the observed shifts in day length when the core was allowed to deform.
The strongest estimates placed the adjustment period at between eight and 10 years, although the possible timescale ranged from about two to 31 years.
To investigate the planet’s interior, the researchers combined earlier earthquake studies tracking the inner core’s rotation with models of movement in the liquid outer core, reconstructed from changes in Earth’s magnetic field.
They removed the effects of atmospheric winds, ocean movements and longer-term influences, including the gradual braking effect of the Moon, before comparing three possible mechanisms with the remaining changes in day length.
Magnetic forces and pressure acting on uneven surfaces at the boundary between the core and mantle produced patterns broadly opposite to those recorded. The gravitational mechanism offered a much closer match, particularly when other forces acted against it and left a small imbalance affecting the planet’s rotation.
The calculations also provided clues about material near the bottom of the mantle. They were consistent with an electrically conducting, iron-rich layer about 1.2 miles thick, although the researchers did not directly discover or sample it.
The results also supported the presence of large masses of chemically distinct, warmer material. Its composition would make it denser, but its higher temperature may counteract that effect, leaving it close to the density of its surroundings.
The researchers said the findings favoured a type of mantle mineral that deforms relatively easily, helping to explain how conditions deep inside Earth influence the gravitational interaction.
However, they cautioned that the apparent 70-year pattern should not yet be regarded 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.
The conclusions depend on existing models of inner-core rotation and liquid-core flows, with some numerical estimates changing by up to 30 per cent when different flow models were used.
The study does not fully account for shorter fluctuations in day length occurring over periods of 10 to 30 years. The authors said these may be more strongly influenced by forces at the boundary between the core and mantle, and that improved models are needed to resolve the remaining uncertainties.
