Earth’s day length has subtly shifted over the past several decades, according to a new study.
Scientists say a gravitational tug between the planet’s solid inner core and its rocky mantle can change Earth’s rotation speed, making individual days longer or shorter by a few milliseconds.
Although these minute variations are impossible for people to notice, they are important for GPS navigation and global timekeeping systems, both of which rely on highly precise measurements of Earth’s rotation.
Researchers from the University of Alberta examined records covering the period from 1964 to 2019 to determine what is driving these barely detectable changes.
Earth’s inner core is a hot, dense sphere made primarily of iron and nickel, but it is not perfectly round.
As the core rotates, its gravity interacts with unevenly distributed masses within the mantle.
This interaction generates a twisting force called gravitational torque. It can slightly accelerate or slow the mantle, altering the time Earth needs to complete one full rotation.
The researchers connected this gravitational interaction with an approximately 70-year pattern in Earth’s rotation, though it remains unclear whether the cycle repeats.
Scientists have identified a hidden force deep inside Earth that has been changing the length of our days for decades
The findings also indicate that Earth’s solid inner core can gradually change shape over several years. While its material remains solid, it slowly yields to the forces acting on it.
That ability to deform played a crucial role when the researchers tested their calculations.
When the inner core was treated as rigid, the predicted changes occurred at the wrong times. Allowing the core to deform produced results that more closely matched the observed variations in day length.
The researchers’ best estimates place the adjustment period at roughly eight to 10 years, although the possible timescale ranged from about two to 31 years.
The study was published in Nature on September 23. It was carried out by University of Alberta physicists Huifeng Zhang and Mathieu Dumberry.
The pair combined previous research using earthquake waves to monitor the inner core’s rotation with models of liquid movement in the outer core, reconstructed from changes in Earth’s magnetic field.
To focus on processes taking place inside the planet, the team removed the effects of atmospheric winds, ocean circulation and longer-term influences such as the Moon’s gradual braking of Earth’s rotation.
They then compared the remaining changes in day length with predictions based on three competing mechanisms.
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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 acting against uneven surfaces at the boundary between the core and mantle generated patterns that were broadly opposite to those recorded.
The gravitational explanation, by contrast, produced a far closer match to the observed changes.
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 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.
