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. In their new study they claim that this pattern is caused by the internal core wobbling.
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. The current belief is that the tilt of the Earth’s core and its spin are in sync. This also suggests that the core’s shape is not a perfect ball but is more egg-shaped in its northwestern part.
“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 of this wobble and tilt has ripple effects. They claim that Earth’s wobbling pattern is responsible for the strange variations in day and rotational lengths. It may also explain the changing Earth magnetic field. Ding said that the static tilt could also cause a shift in the core’s shape, which would change the fluid movement and the geomagnetic fields.
This research doesn’t explain all of these phenomena. 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.
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