By Angus Dalton
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Taking an X-ray of the Earth is a little complicated, mainly because you have to wait for an earthquake.
But tuning in to seismic rumbles to measure what’s happening thousands of kilometres beneath our feet is the bread and butter of global seismologist Professor Hrvoje Tkalčić.
Seismic waves refracted inside the Earth are picked up by sensors (green dots) at the surface listening for the rumble of earthquakes (red dots). The technique uncovered the Earth’s “innermost inner core” in 2023.Credit: Hrvoje Tkalcic
Now the Canberra scientist has discovered a previously unknown doughnut-shaped structure within the Earth’s core, and it’s key to the invisible shield that protects Earth from cancer-causing radiation in space.
This shield – Earth’s magnetic field – also deflects lashes of solar plasma that could inflict mass power outages, and prevents solar wind stripping away our atmosphere until we’re as barren as Mars.
It is invisible, save for when solar-charged particles get trapped in the magnetic field, slamming into and exciting atmospheric oxygen and nitrogen atoms. As the atoms unleash extra energy, the skies light up with the spectacular dancing flares around the poles we call the aurora.
A view of the aurora australis from Sandringham beach, Melbourne, this year.Credit: Kris Waring
But this awe-inducing shield is quavering, and anomalies in its make-up have some scientists worried it could be due to weaken and flip.
So what could this inner-Earth doughnut tell us about the magnetic field that life on Earth depends on?
Rewriting the textbooks
Tkalčić called for a rewrite of the textbooks last year when, after studying the reverberations of 200 earthquakes over a decade, he unveiled a fifth layer of the Earth’s core: a searing ball of iron and nickel at the very heart of our planet.
Tkalčić and his colleague Dr Thanh-Son Phạm, both from the Australian National University, described this new layer, the “innermost inner core”, as a “distinct internal metallic ball embedded in the inner core like the most petite Russian nesting doll”.
Global seismologist from the Australian National University, Professor Hrvoje Tkalčić, uses earthquakes to peer into the inner Earth.Credit: Tracey Nearmy/ANU
For his latest discovery, Tkalčić and his colleague Dr Xiaolong Ma looked deeper into signals from earthquakes. Most studies use data from the first hour of a seismic rumble, when an earthquake’s echo is loudest. But Tkalčić and Ma turned to the hours afterwards – the “coda” of the music piece, he says – when the faint signals resemble random wiggles that don’t mean much.
But when they compared the wiggles from different seismographs across the world, Tkalčić and Ma were able to tease out a more sensitive ultrasound picture, and uncovered a hidden, mysterious ring where seismic waves travelled more slowly.
That seemed odd: waves should travel at the same speed throughout the entire stewing core.
Tkalčić and his colleague Dr Xiaolong Ma have unveiled a new doughnut-shaped region of the Earth’s liquid outer core.Credit: Drew Whitehouse, National Computational Infrastructure, ANU
Tkalčić believes the doughnut-shaped region, which sits parallel to the equator underneath the Earth’s 2900-kilometre-thick solid mantle, is made up of a higher concentration of lighter elements such as oxygen, sulphur, silicon, carbon and hydrogen compared with the rest of the outer core.
Why that’s important
A magnetic field is generated whenever a particle with a charge is in motion. A negatively charged electron whizzing around an atom, for example, generates a tiny magnetic field. So does electricity running through a wire.
The electrically conductive liquid metal within the Earth’s core is constantly moving due to convection, and this creates the magnetic field around the planet, a mechanism known as the geodynamo.
Without this geodynamo, NASA says, Earth’s biosphere would not exist as it does today because life would be exposed to damaging levels of radiation from the sun and space. That’s why understanding this doughnut filled with lighter elements is so important.
An animation of Earth’s magnetic field protecting the planet from a coronal mass ejection.Credit: NASA GSFC/CIL/Bailee DesRocher
“The buoyancy of these light chemical elements really participates in the convection,” Tkalčić says. “And of course, without the convection in the liquid core, the geodynamo would stop and the Earth would lose the geomagnetic field and the protective shield.”
And the magnetic field isn’t a stable beast. In 2019 scientists made an emergency update to the world magnetic model to redefine what GPS systems use as true north because magnetic north has been wandering towards Siberia by 50 kilometres a year.
NASA is also monitoring a weakened “dent” in the magnetic field called the South Atlantic Anomaly, where the field is so weak satellites risk short-circuiting blasts from solar radiation.
Some scientists have speculated these anomalies point towards an imminent pole reversal, or flip.
Models showing what the Earth’s magnetic poles may resemble during a flip (right). Credit: University of California, Santa Cruz/Gary Glatzmaier
In 2021 Australian scientists discovered a spike in atmospheric radiocarbon in ancient kauri trees preserved in New Zealand sediment for 40,000 years, which they took as evidence of a breakdown in the magnetic field caused by the poles reversing for about 800 years before swapping back.
“Early humans around the world would have seen amazing auroras, shimmering veils and sheets across the sky,” Professor Alan Cooper, of the South Australian Museum, said of the deteriorated field. “It must have seemed like the end of days.”
Cooper and his colleagues theorised the breakdown might have contributed to climate shifts, the extinction of Australia’s megafauna and even an explosion in cave painting as humans of the time kept indoors.
Untangling the mystery
We have no idea when the magnetic field may weaken next. But to tease out the secrets of Earth’s shield, we have to know exactly what’s going on in its molten generator under our feet.
It’s very difficult to peer inside the Earth’s core as Tkalčić does, so many studies are carried out through numerical calculations to model the geodynamo. The discovery of the doughnut shaped layer beneath the Earth’s mantle will help scientists more accurately model what’s happening in the core and with the magnetic field. “Only when you have some initial conditions can you start simulating the dynamo,” says Tkalčić.
To end this geomagnetic story with a human perspective: I ask Tkalčić how it feels to have potentially uncovered an entirely new region within what we can safely say is the most important known planet in the universe?
“It’s always a dream to be the first to realise something and to leave this sort of legacy to your students and others who are in the same field,” he says.
“Of course, this has to be scrutinised and confirmed, hopefully, by using different methods. But it really feels fulfilling. You know, this is why I’m a scientist.”
Examine, a free weekly newsletter covering science with a sceptical, evidence-based eye, is sent every Tuesday. You’re reading an excerpt – sign up to get the whole newsletter in your inbox.