Earth’s days are not their own: they grow and shrink over decades by a few millisecondsa change too small for most of us to experience in our daily lives, but still significant enough to affect the measures of time we use and to baffle scientists working to decipher the planet’s deep interior. Now a new piece of research in Nature offers a possible cause, one to do with a titanic tug-of-war between Earth’s metallic inner core and the dense, rocky shell that envelopes it.
To investigate the forces acting on the core and mantle, the team, led by University of Alberta researchers Huifeng Zhang and Mathieu Dumberry, reconstructed the torques from the mid-1960s to 2019. They used the observed rotation of the inner core, provided by seismic studies, with models of the flow in the outer core based on the variation of the Earth’s magnetic field over time. Among the various possible mechanisms they examined, they found that gravitational coupling between the deformed inner core and the mass anomalies in the overlying mantle was the closest to explaining observed changes in the length of day both in magnitude and time.
The “core” is basically a solid ball of iron and nickel, about as big as the moon. It isn’t a perfect sphere and it turns a fraction faster or slower than the mantle that’s above it. Since the mantle has these areas of heavy metal concentration, there’s a tug of war between the core’s equally dense protrusions and the mantle’s extreme topography that produces a torque. That torque subtly adjusts the rotation of the mantle – which changes the length of the day you all know so well – and electrical. And topographic forces at the core/mantle boundary “brakes” the core’s movement at roughly the same magnitude that the gravitational tug alters it.
Between the beginning and the end of the period we are examining the processes that ‘operate’ on the core could change the length of the day by a few milliseconds. The cycle is roughly demic-nominal, probably in the century range, and the torque exerted by gravitation appears to be most capable of causing the long-term swings, but the treatment of the other direction at the COS boundary means that they are not allowed to get up to that point. What we end up with is a gradual, subtle change in the speed of rotation of the solid Earth.
More than a technicality in how the planet spins though the results provide fresh limits on properties of the deep-Earth interior, which has a measured base temperature but not a specific form, the authors say. The models are also consistent with computer models of Earth’s dynamo, which creates Earth’s magnetic field.
Researchers have understood for some time that several factors, like deep ocean tides, atmospheric winds and the melting of ice sheets, all shift daylength and contribute to shorter-term variations. But the multidecadal signal has proved more elusive. Thanks to the work of Zhang and Dumberry, the gravitational coupling with the inner core is clearly contributing soon, because of this bridging a major missing link in earth simulation models.
