The innermost part of our planet, the core, might be lopsided. This is the suggestion based on a model by seismologists at UC Berkeley. This approach could help explain why seismic waves travel differently through the inner core. The hypothesis is detailed in Nature Geoscience.
There is a lot we don’t understand about what goes on a few thousand kilometers under our feet. We know that the Earth has a solid inner core, that has been crystalizing for at least 500 million years, but maybe much longer than that. This crystallization releases heat, keeping the outer core nice and molten. That’s good for us since the motion of the outer core generates the magnetic field that protects us from cosmic radiation.
The crystallization of the inner core, though, doesn’t appear to be uniform. By studying the motion of some of the seismic waves that can travel through the inner core, scientists noticed that some directions are better, making the waves go through them more quickly.
To explain this, the new model suggests that the core grows faster on one side (underneath Indonesia) than it does on the other (underneath Brazil) by about 60 percent. The final result is iron crystals that are preferentially orientated along the planet’s axis of rotation.
“The simplest model seemed a bit unusual — that the inner core is asymmetric,” lead author Dr Daniel Frost said in a statement. “The west side looks different from the east side all the way to the center, not just at the top of the inner core, as some have suggested. The only way we can explain that is by one side growing faster than the other.”
The model also narrows the possible age range of the inner core, leading to more mysteries. What was heating up the outer core and producing the magnetic field before the inner core crystallization? The team suggests that the separation of lighter elements from iron might have done the trick.
“We provide rather loose bounds on the age of the inner core — between half a billion and 1.5 billion years — that can be of help in the debate about how the magnetic field was generated prior to the existence of the solid inner core,” explained Professor Barbara Romanowicz, emeritus director of the Berkeley Seismological Laboratory. “We know the magnetic field already existed 3 billion years ago, so other processes must have driven convection in the outer core at that time.”
There are limitations in this model based on certain assumptions and on the data on the interior of our planet. Studying what goes on within the Earth is easier said than done, so more seismic data will be needed to validate this model.