Earth’s days aren’t exactly the same length. Scientists have been recording small variations in our planet’s rotation for decades, sometimes as little as a few milliseconds, but these changes are significant enough to point to a much bigger shift: our planet’s rotation is shifting.
Several factors contribute to these fluctuations, including atmospheric and oceanic changes, major earthquakes, and the melting of continental ice. However, a new study published in Nature suggests that the gravitational battle within the Earth’s core may be responsible for some of the longer-term variations.
The Earth’s outer core, a 1,400-mile thick layer of ultra-hot liquid metal, is in constant motion. As these flows change, they slightly alter the speed at which the rest of the planet rotates. However, the mechanisms by which these forces are transmitted from one region to another remain 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 a new study, researchers propose that the gravity of the inner core pushes rotation in one direction, while other interactions between the core and the mantle oppose it. This competition, over decades, results in 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. Gravity tries to pull the layers back into their more harmonic position, albeit minutely, influencing 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 of liquid metal flows in the outer core. Their gravitational model closely reproduced both the timing and the magnitude of real gradual changes observed over the decades. This proposal unravels a major component of our variable days.
