Evrim Yazgin
Cosmos science journalist
A newly discovered doughnut-shaped region thousands of kilometres beneath the surface of the Earth is helping scientists understand the dynamics of our planet’s magnetic field.
The discovery, made by Australian National University (ANU) seismologists, is published in the journal Science Advances.
Earth, like ogres and onions, is layered.
Our planet’s crust – the outer layer – is 30 to 70km thick. Beneath this the mantle extends for about 3,000 km and makes up about 84% of the Earth’s total volume. And beneath the mantle is the core.
But the core also has layers – a solid inner core, and a liquid outer core.
The doughnut-shaped structure sits within the outer core. It is parallel to the equator and is found only at lower latitudes.
“We don’t know the exact thickness of the doughnut, but we inferred that it reaches a few hundred kilometres beneath the core-mantle boundary,” says co-author Hrvoje Tkalčić, a professor of geophysics at ANU.
The team found the doughnut by observing the patterns of seismic waves produced by earthquakes.
But unlike other research, which focuses on the first hour of seismic wave signals, the ANU-led study looked at waveforms many hours after the earthquake.
“By understanding the geometry of the paths of the waves and how they traverse the outer core’s volume, we reconstructed their travel times through the Earth,” Tkalčić explains.
Tkalčić’s team found a doughnut-shaped region where the waves slowed down.
“The peculiar structure remained hidden until now as previous studies collected data with less volumetric coverage of the outer core by observing waves that were typically confined within one hour after the origin times of large earthquakes,” Tkalčić says.
Much more remains to be learned about Earth’s outer core.
The outer core is mostly liquid iron and nickel. The movement of the electrically conductive liquid metals creates the planet’s magnetic field which protects us from harmful cosmic radiation.
Knowing more about the outer core could help explain how this phenomenon, critical to the existence of life on our planet, came to be.
“Our findings are interesting because this low velocity within the liquid core implies that we have a high concentration of light chemical elements in these regions that would cause the seismic waves to slow down. These light elements, alongside temperature differences, help stir liquid in the outer core,” Tkalčić says.
“The dynamics of Earth’s magnetic field is an area of strong interest in the scientific community, so our results could promote more research about the magnetic field on both Earth and other planets.”
