Physicists at the University of Alberta have revealed that gravitational tugs between Earth's shifting solid inner core and the rocky mantle drive decadal, millisecond fluctuations in the length of a day.
Published in the journal Nature, the study by doctoral researcher Huifeng Zhang and physics professor Mathieu Dumberry resolves a thirty-year geophysical mystery surrounding how angular momentum is transferred across the planet's deep interior. While an Earth day is conventionally treated as exactly twenty-four hours, atomic clocks show that the rotational period subtly varies by several milliseconds over cycles lasting between ten and seventy years. Scientists have long known that as the fluid outer core accelerates over decades, the mantle slows down to conserve Earth's overall angular momentum, and vice versa.
Until now, however, researchers struggled to identify the physical forces capable of transmitting that rotational motion across thousands of kilometers of molten and solid rock. By modeling historical geophysical records from 1964 to 2019, the researchers determined that Earth's solid inner core is slightly non-spherical, meaning its shifting orientation exerts a gravitational torque on density variations in the overlying mantle. This gravitational pull drags on the mantle, causing it to speed up or slow down and altering planetary day length accordingly.
At the same time, opposing electromagnetic and topographic torques at the core-mantle boundary act in the opposite direction, resisting the gravitational shift and limiting the extent of the swings. Beyond explaining our planet's shifting clock, the findings suggest that Earth's solid inner core is surprisingly pliable, viscously deforming on a timescale of roughly ten years under intense interior temperatures and pressures.