While most of us take the earth under our feet for reality, written in its complex layers, like the pages of a book, is the history of the Earth. Our history.
Now researchers have found more evidence for a whole new chapter deep into the Earth’s past – the inner core of the Earth seems to have another, even deeper core inside it.
“Traditionally, we have been taught that the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” said Australian National University geophysicist Joanne Stephenson.
Our knowledge of what lies beneath the Earth’s crust has been deduced especially from what volcanoes have revealed and seismic waves have whispered. From these indirect observations, scientists have calculated that the hot inner core, with temperatures exceeding 5,000 degrees Celsius (9,000 Fahrenheit), represents only a percentage of the total volume of the Earth.
Now, Stephenson and his colleagues have found more evidence that the inner core of the Earth could have two distinct layers.
“It’s very interesting – and it could mean we have to rewrite textbooks!” she added.
The team used a search algorithm to navigate and match thousands of models of the inner core with data observed over several decades about how long seismic waves travel to Earth, collected by the International Seismological Center.
Differences in seismic wave paths through the Earth’s layers. (Stephenson et al., Journal of Geophysical Research: Solid Earth, 2021)
So what’s down there? The team looked at several models of inner core anisotropy – how differences in its material change the properties of seismic waves – and found that some were more likely than others.
While some models believe that the material of the inner core channels seismic waves faster parallel to the equator, others argue that the mixture of materials allows faster waves more parallel to the Earth’s axis of rotation. Even then, there are arguments about the exact degree of difference in certain angles.
This study failed to show too many variations with depth in the inner core, but found that there was a change in the slow direction at an angle of 54 degrees, with the faster direction of the waves running parallel to the axis.
“We have found evidence that may indicate a change in the structure of iron, which probably suggests two separate cooling events in Earth’s history,” Stephenson said.
“The details of this great event are still a bit mysterious, but we’ve added another piece to the puzzle when it comes to our knowledge of the inner core of the Earth.”
These new findings may explain why some experimental evidence has been incompatible with our current models of the structure of the Earth.
The presence of an innermost layer has been suspected for some time, with indications that the iron crystals that make up the inner core have different structural alignments.
“We are limited by the distribution of earthquakes and global receivers, especially at the polar antipodes,” the team wrote in their paper, explaining that the missing data diminish the certainty of their findings. But their findings align with other recent studies of inner core anisotropy.
A new method under development could soon fill some of these data gaps and allow scientists to corroborate or contradict their findings and hopefully translate more stories written into this early layer of Earth’s history.
This research was published in Journal of Geophysical Research.