- Researchers discovered a donut-shaped region at the ceiling of the outer core
- This lighter region helps stir the liquid metal which generates the magnetic field
Scientists have uncovered a vast donut-shaped structure buried thousands of miles beneath our feet.
Researchers from the Australian National University used seismic waves generated by earthquakes to peer into the Earth’s mysterious molten core.
By tracing the path of these waves through the planet, the researchers found a region a few hundred kilometres thick where they travelled two per cent slower than normal.
This donut-like structure runs parallel to the equator in a ring around the edge of the liquid outer core, and could be responsible for driving our planet’s protective magnetic field.
Professor Hrvoje Tkalčić, lead author of the study, says: ‘The magnetic field is a fundamental ingredient that we need for life to be sustained on the surface of our planet.’
The Earth is made up of four major layers: the surface crust, the semi-molten mantle, a liquid metal outer core, and a solid metal inner core.
When the movement of tectonic plates in the crust creates earthquakes, these produce vibrations that spread out through all the other layers of the Earth.
Using the worldwide network of seismographic stations, researchers can see how the waves spread and make predictions about the conditions below the surface.
Scientists usually only look at the big, powerful wavefronts which travel around the world in the first hour or so after an earthquake.
However, Professor Tkalčić and his co-author Dr Xiaolong Ma were able to detect this structure by studying the faint traces left behind by waves many hours after the initial tremor.
This method revealed that seismic waves travelling near the poles were moving faster than those near the equator.
By comparing their results to different models of the Earth’s interior, Professor Tkalčić and Dr Ma found that this was best explained by the presence of a vast underground ‘torus’, or donut-shaped, region.
They predict that this region is only found at low latitudes and runs parallel to the equator near the ceiling of the outer core where the liquid section meets the mantle.
‘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,’ Professor Tkalčić says.
Thanks to this region’s critical role, their discovery may also have profound implications for the study of life on Earth and other planets.
Earth’s outer core has a radius of around 2,160 miles (3,480km) – making it slightly larger than Mars.
Mainly made of hot nickel and iron, convection currents coupled with the Earth’s rotation force the liquid metal in this layer into long vertical vortices running in a north-south direction, like giant waterspouts.
It is the swirling currents of these liquid metals which act like the dynamo, powering the Earth’s magnetic field.
Since this donut region has ‘floated’ to the top of the liquid outer core, it suggests that it could be rich in lighter elements like silicon, sulphur, oxygen, hydrogen or carbon.
Professor Tkalčić says: ‘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.’
Without that stirring motion to drive the planet’s interior dynamo, the Earth’s magnetic field might not have formed.
Without the magnetic field, the planet’s surface would be exposed to a constant bombardment of charged particles from the sun which can destroy the DNA of living creatures.
This donut-shaped region, therefore, might be a critical piece of the puzzle which explains why life has developed on Earth and what we might look for in habitable planets elsewhere.
