Earth's days arenot exactly the same length. Scientists have been recording small variations in for decades. These changes are minuscule—sometimes just a few milliseconds—and mostly imperceptible, but scientists are keen to understand them as they point to a much more monumental change: Our planet’srotation speedis shifting.
Scientists have already pinpointed a handful of compounding causes. Fluctuations in theatmosphereandoceanscan slightly speed up or slow down the Earth’s rotation;major earthquakescan redistribute part of the planet’s mass; and by depositing enormous amounts of water into the oceans,melting continental iceis also a factor.
These influences account for some of the more sudden and recent changes, but on a several decades-long timescale, some of the variation appears to originate from a source much deeper within the Earth. Beneath our planet’srocky mantlelies the outer core—a 1,400 mile thick layer of ultra-hot liquid metal that is in constant motion. As these flows change, they can slightly alter the speed at which the rest of the planet rotates.
However, how these forces are transmitted from one region to another has remained a mystery. Friction between the core and the mantle is too weak, so geologists have been searching for other mechanisms operating in the Earth’s interior.
In anew studypublished this week inNature, researchers believe they have identified one of these hidden gears. They propose that the gravity of the inner core pushes rotation in one direction, while other interactions between the core and the mantle oppose it. The result of this competition, accumulated over decades, would be small accelerations and decelerations in the planet’s spin.
The idea is based on the fact that the inner core does not rotate at the same rate as the rest of the planet, causing it to become slightly misaligned with certain mass irregularities in the mantle. But the core “wants to be aligned,” Mathieu Dumberry, a geophysicist at the University of Alberta involved in the study, said in apress release. What this means is that gravity tries to pull the layers of the inner Earth back into their more harmonic position, which influences—albeit minutely—the rotation of the planet’s surface.
The authors reconstructed how this interaction influenced Earth’s spin between 1964 and 2019 using seismic estimates of the inner core’s rotation and models ofliquid metalflows in the outer core. Theirgravitational modelclosely reproduced both the timing and the magnitude of real gradual changes that had been observed over the decades.
Other yet-unknown factors are likely involved, but this proposal unravels a major component of our variable days. The model also didn’t account for all the forces competing involved, but it didn’t need to to produce a compelling result. Because it might be a tug-of-war, Dumberry said, but “gravity still wins.”
This story originally appeared inWIRED en Españoland has been translated from Spanish.
