Earth's Days Stretch and Shrink by Milliseconds, and the Inner Core's Gravity Is Why
University of Alberta physicists report in Nature that the solid inner core tugs on lumps in the mantle, a 'gravitational torque' that explains decades-long swings in the length of a day.
A day on Earth is not exactly 24 hours, and it never has been. Measured with atomic clocks, the length of a day wanders by a few thousandths of a second over periods of decades. Scientists have known for about 30 years that this drift is tied to what happens deep inside the planet. Now two physicists at the University of Alberta say they have found the mechanism that links the two.
In a study published in the journal Nature, doctoral student Huifeng Zhang and professor Mathieu Dumberry report that small changes in how fast Earth's solid inner core spins exert a "gravitational torque" on dense and light patches within the mantle, the 3,000-kilometer-thick rocky shell above the core that includes the crust. That tug speeds up or slows down the mantle's rotation, and with it the length of the day we experience at the surface.
The broad picture has been clear for a while. Changes in Earth's magnetic field show that the liquid outer core speeds up its rotation over a few decades, then slows down again. Because the planet's total spin, or angular momentum, has to stay constant, the mantle does the opposite, slowing when the core speeds up and speeding up when the core slows. Those swings can lengthen or shorten a day by a few milliseconds. What nobody had pinned down was exactly how the core and mantle trade that spin back and forth.
Zhang and Dumberry's answer involves two competing forces. The inner core is not a perfect sphere, and the mantle contains uneven "mass anomalies," regions that are slightly denser or lighter than their surroundings. When the inner core's rotation shifts, gravity between its lopsided shape and those anomalies pulls on the mantle. At the same time, a second kind of torque acts at the boundary where the liquid core meets the mantle, through friction and electromagnetic drag. That core-mantle boundary torque resists the gravitational tug and limits how much the day can change. The researchers conclude that small shifts in the balance between the two torques are ultimately behind the observed multidecadal variations in day length.
The work also says something about the inner core itself. To fit the observations, the solid inner core must "deform viscously," Zhang and Dumberry write, reshaping itself slowly over roughly 10 years, more like very stiff putty than rigid rock. That gives seismologists and geophysicists a new constraint on the physical properties of a region roughly 5,000 kilometers beneath our feet that no one can sample directly.
The finding matters beyond curiosity. Timekeepers track tiny variations in Earth's rotation to keep civil time in step with the planet, and navigation and satellite systems depend on precise models of how Earth spins. Knowing that the inner core's gravity is one of the hands on the dial gives researchers a physical explanation, rather than just a statistical pattern, for part of that wobble in the clock. The paper is titled "Gravitational torque drives multidecadal variations in length of day."
