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A new technique that scans the deepest interior of our planet suggests that the heart of the Earth is not all that it seems.
Hrvoje Tkalčić and Thanh-Son Phạm, both of the Australian National University, reanalyzed the echoes of massive earthquakes spreading inside the planet.
They calculated that the solid inner core is softer than expected.
Their new method, published in Science Today, raises his own questions, namely: what's going on there?
"Initially, we thought that it could act of pure iron in the solid inner core, but this is not guaranteed," said Louis Moresi, a geophysicist at the University of Melbourne, who did not participate in the study.
"Maybe all this is now appreciated by this new work."
Peel the layers of the Earth
To get an idea of the structure of what is inside our planet, seismologists analyze seismic waves triggered by earthquakes.
A global network of instruments called seismometers captures the tremors and faint tremors that travel around the Earth.
This is not a new technique. Seismometers have existed since 1880.
Until the 1930s, scientists thought that the Earth contained an enormous reservoir of liquid rock in the center, which was wrapped in a solid mantle and crusted over.
But after the magnitude 7.3 earthquake of 1929 that hit the island of South New Zealand, the instruments in Europe recorded waves that would not have been possible if the core was liquid.
Danish seismologist Inge Lehmann realized that there must be a solid component somewhere.
In 1936, she published her discovery: this Earth had an inner core.
A few years later, the American mineral physicist, Francis Birch, postulated that the inner core was solid iron.
And we now think that the liquid outer core is arranged in rotating columns that generate the magnetic field of the Earth.
"Rotating fluids drive electricity and generate magnetic fields, it is a kind of autonomous dynamo that we see in other places like the Sun and Jupiter," said Professor Moresi.
"A very important element of the Earth's core is the fact that the inner core exists.
"This allows a lot of fluid to flow into this relatively stable cylinder set.
"But if you want to study the inner core, you must first study through the outer core, which makes the task even more difficult."
S waves and P waves
To probe the nucleus, scientists analyze how the S and P waves bounce around the planet.
For decades, seismologists have been looking for signs of S waves in the inner core.
Their speed can tell you about the rigidity, or rigidity, of the material.
And even though S-waves can not cross the liquid outer core, they do exist in the solid inner core.
Indeed, when a P wave propagating in the outer core strikes the inner core, part of its energy is converted into S waves.
These S waves can propagate through the inner core to reach the outer liquid core, where they are again converted to P waves.
How to see the invisible
Professor Tkalčić and his PhD student, Dr. Phm, analyzed historical earthquake data with a magnitude of at least 6.8 between 2010 and 2016.
But instead of examining the properties of the individual signals, they analyzed the similarities between pairs of waveforms.
This produced mathematical constructs called "global correlograms", which the pair used to calculate the velocity of the S waves through the inner core.
It turned out that the S waves were about 2.5% slower than expected. And as S waves move rapidly through a stiffer material, the result is that the inner core is also more flexible than expected.
What is the cause of this squishiness?
The new work does not say with certainty, said Professor Tkalčić, and is currently under debate.
This could be an intrinsic property of iron in the hot and pressurized conditions of the center of the Earth.
Or maybe pockets of melt, enriched with lighter elements, are trapped inside the inner core.
A better map for the interior of the Earth
According to Professor Moresi, having a better idea of the center of the Earth means that geophysicists can refine their reference models.
"If you measure the waves that go through the nucleus or bounce off, we can use them for images elsewhere," he said.
Understanding the properties of the core will also help to determine if it is spinning faster than the overlying mantle.
And we could get a more accurate picture in the next few years.
Professor Tkalčić and his team will soon establish a network of seismometers on and around Macquarie Island, halfway between New Zealand and Antarctica.
It is a region with a lot of earthquakes, but very few instruments.
The one-year experiment will not only help us understand what is going on beneath the surface in this part of the world, but it will also add to our knowledge of the core.
Professor Tkalčić stated that it would be "like a big telescope directed towards the center of the earth".
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