More than 5,000 kilometers below us, the Earth’s inner solid metal core hadn’t been discovered until 1936.
Almost a century later, we are still struggling to answer basic questions about when and how he first graduated.
These are not easy puzzles to solve.
We cannot collect samples directly from the inner core, so the secret to unraveling its mysteries lies in the collaboration between seismologists, who indirectly obtain samples through seismic waves, geodynamicists, who create models of their dynamics, and mineral physicists, who study the behavior of iron alloys at high pressures and temperatures.
By combining these disciplines, scientists have come up with an important clue about what is happening miles below our feet.
In a new study, they reveal how the Earth’s inner core is growing faster on one side than the other, which may help explain the age of the inner core and the intriguing history of the Earth’s magnetic field.
The Earth’s core was formed at the very beginning of our planet’s 4.5 billion-year history, in the first 200 million years.
Gravity pulled the heavier iron towards the center of the young planet, leaving the rocky silicate minerals to form the mantle and crust.
The formation of Earth retained a lot of heat within the planet.
The loss of this heat and warming from continuous radioactive decay has driven our planet’s evolution.
Heat loss inside the Earth drives the intense flow of liquid iron in the outer core, which creates the Earth’s magnetic field.
Meanwhile, cooling deep inside the Earth helps power the tectonic plates, which shape our planet’s surface.
As the Earth cooled over time, the temperature at the planet’s center ended up dropping below the melting point of iron at extreme pressures, and the inner core began to crystallize.
Today, the radius of the inner core continues to grow by around 1 mm each year, which equates to the solidification of 8,000 tons of cast iron every second.
In billions of years, this cooling will eventually cause the entire core to become solid, leaving the Earth without its protective magnetic field.
This solidification can be supposed to create a homogeneous solid sphere, but this is not the case.
In the 1990s, scientists realized that the speed of seismic waves traveling through the inner core varied unexpectedly.
This suggested that something asymmetric was going on in the inner core.
Specifically, the east and west halves of the inner core showed different seismic wave velocity variations.
The eastern part of the inner core is below Asia, the Indian Ocean and the western Pacific Ocean, while the western part lies under the Americas, the Atlantic Ocean and the eastern Pacific.
The new study analyzed this mystery, using new seismic observations combined with geodynamic modeling and estimates of how iron alloys behave at high pressure.
They found that the eastern inner core located below the Indonesia Banda Sea is growing faster than the western side below Brazil.
You can imagine this uneven growth as trying to make ice cream in a one-sided freezer: ice crystals form only on the side of the ice cream where cooling is effective.
On Earth, uneven growth is caused by the rest of the planet sucking heat more quickly from some parts of the inner core than from others.
But unlike ice cream, the solid inner core is subject to gravitational forces qwhich distribute new growth evenly through a gradual flow process, which maintains the spherical shape of the inner core.
This means that the Earth is not at risk of tipping over, although this uneven growth is recorded in seismic wave velocities in our planet’s inner core.
This approach could help us understand, then, how old might the inner core be?
When the researchers compared their seismic observations with their flux models, they found that the inner core — at the center of any core that formed long before — is likely to be between 500 million and 1,500 million years old.
According to the study, the younger end of this age group matches the best, although the oldest matches an estimate made by measuring changes in the strength of the Earth’s magnetic field.
Whichever number is correct, it is clear that the inner core is relatively young, somewhere between one-ninth and one-third the age of Earth.
This new work introduces a powerful new model of the inner core.
However, a number of physical assumptions the authors made would have to be true for this to be correct.
For example, the model only works if the inner core consists of a specific crystalline phase of iron, about which there is some uncertainty.
And our irregular inner core makes the Earth unusual?
In fact, many planetary bodies have two halves that are somewhat different from each other.
On Mars, the surface of the northern half is lower, while the southern half is more mountainous.
The crust on the visible face of the Moon is chemically different from the far side.
On Mercury and Jupiter, it is not the surface that is irregular but the magnetic field, which does not form a mirror image between north and south.
So while the causes of all these asymmetries vary, Earth appears to be well matched as a slightly asymmetrical planet in a solar system of uneven celestial bodies.
* Jessica Irving is Professor of Geophysics at the University of Bristol, UK.
Sanne Cottaar is Professor of Global Seismology at Cambridge University, also in the UK.
This article was originally published on the academic news site The Conversation and republished by the BBC under a Creative Commons license.