Researchers have previously pinpointed a handful of compounding causes that alter how fast our planet spins. Fluctuations across the atmosphere and oceans can slightly accelerate or decelerate the Earth, major earthquakes can redistribute planetary mass, and melting continental ice deposits enormous volumes of water into the oceans.
Deep Earth Forces and the Outer Core
While those influences account for recent and sudden shifts, variations spanning several decades originate far deeper beneath our feet. Beneath the rocky mantle sits the outer core, a 1,400 mile thick layer of ultra-hot liquid metal locked in constant motion. As these massive flows shift, they alter the rotational speed of the rest of the planet.
Unraveling the Transmission Mystery
How these immense forces transmit from one subterranean region to another remained an enduring puzzle for geologists. Friction operating between the core and the mantle is simply too weak to drive the observed changes, prompting scientists to search for alternative mechanisms hidden deep inside the planet.
A New Study in Nature
A study published in Nature proposes that researchers have identified one of these hidden gears driving planetary rotation. They suggest that the gravity of the inner core pushes rotation in one specific direction, while opposing interactions between the core and the mantle fight back.
The Tug-of-War of Planetary Layers
This dynamic stems from the inner core rotating at a different rate than the rest of the planet, causing it to slip out of alignment with mass irregularities in the mantle. The core wants to be aligned, as Mathieu Dumberry, a geophysicist at the University of Alberta involved in the study, explained. Gravity pulls the inner layers back toward a harmonic position, minutely influencing surface rotation.
Reconstructing Decades of Spin Data
The authors successfully reconstructed how this interaction influenced Earth spin between 1964 and 2019 by combining seismic estimates of inner core rotation with fluid models of the outer core. Their gravitational model closely matched both the timing and the magnitude of real gradual changes observed across the decades.
Gravity Claims Victory
Although other unknown factors likely play a role, this model unravels a major component of our variable days. While the system operates as a complex tug-of-war, Dumberry noted that gravity still wins.
