Scientists Have Solved the Mystery of a Strange 'Signal' from Earth's Core, Study Reports

Scientists in China say they've solved a major mystery deep inside the Earth by confirming that the inner core wobbles with a predictable rhythm that repeats every 8.5 years. 

The research team says that the finding confirms previous measurements made by the team in 2018 and challenges earlier assumptions about the relationship between Earth’s layers and how their movement affects things like our changing day lengths. The study was published in December in Nature Communications. 

Understanding the solid hunk of iron and nickel, the inner core, floating within a liquidy metal layer, the outer core, is much more difficult than fictional trips to the center of the Earth have made it seem. A lot of what scientists know about the inner core and its movements has come from waves rippling across the planet, generated by massive earthquakes or nuclear bomb tests. Yet, understanding the inner core and its movements is crucial for unraveling mysteries around Earth’s structure, its magnetic field and earthquakes.

In 2018, geophysicist Hao Ding, a co-author of the current study, and colleagues analyzed how the motion of the Earth’s poles changes subtly over time. They noticed a pattern, or harmonic “signal,” repeating every 8.5 years, which they confirmed in this latest study using measurements of how day length varies over time. That pattern, they report in the new study, is the wobbling of the inner core.

Their analysis also led them to conclude that the core must be tilted at around 17 degrees relative to the Earth’s mantle, or crust. This goes against the current thinking that the tilt is much greater and that the spin of the core matches up with that of the mantle. It also implies that the core isn’t a perfect sphere but rather is egg-shaped and denser in the northwestern portion. 

"These deviations offer valuable constraints for the 3D density model of the mantle and question assumptions in the liquidity-core oblate, highlighting potential deviations from a perfectly spherical form calculated using traditional theories," explained Ding in a press statement.

And all this tilt and wobble has a ripple effect. The authors state that the pattern of wobbling of the Earth’s core drives the oddities in the Earth’s overall rotation and day length and may explain other phenomena like the Earth’s changing magnetic field. "The static tilt may also lead to a certain change in the shape of the liquid core, resulting in a change in the fluid motion and a corresponding change in the geomagnetic field," said Ding.

This latest research isn’t the only way to explain such phenomena however. Other theories say that a geophysical tug of war between the Earth’s magnetic field and the gravitational field of the mantle causes the inner core to spin backwards then forwards roughly every 70 years. Or, it’s because the surface of the core isn’t static and smooth but has changing peaks and valleys. 

“I’m not sure there are very many things in Earth sciences that are as unresolved as the motion of the inner core,” John Vidale, seismologist at the University of Southern California, who wasn’t involved in the study, said previously. “It’s a long-standing battle to try to figure out exactly what’s happening.”

While some uncertainties remain, the current study is a step forward in resolving some of these debates. "We aim to delve deeper into the periodic oscillation and differential rotation of the Earth's core, seeking clarity on these conceptual theories that are different and may be difficult to coexist," said Ding.