While most of us take the ground beneath our feet for granted, Earth’s history is written in its intricate layers, like the pages of a book. Our story.
Research shows that there are little-known chapters in this story, deeply rooted in Earth’s past. In fact, Earth’s inner core appears to contain another, even more inner core.
“Traditionally, we have been taught that the Earth has four main layers: the crust, the mantle, the outer core and the inner core,” said Joanne Stephenson, a geophysicist at the Australian National University. explained in 2021.
Our knowledge of what lies beneath the Earth’s crust has been inferred primarily from what volcanoes have divulged and what seismic waves have whispered.
From these indirect observations, scientists calculated that the extremely hot inner core, with temperatures exceeding 5,000 degrees Celsius (9,000 Fahrenheit), represents only 1 percent of Earth’s total volume.
But a few years ago, Stephenson and his colleagues discovered that Earth’s inner core may actually have two distinct layers.
“This is very exciting – and it could mean we have to rewrite the textbooks!” Stephenson explained at the time.
The team used a search algorithm to search and match thousands of models of the inner core with decades-long observed data on how long it takes seismic waves to pass through the Earth, collected by the International Seismological Center.
Differences in the paths of seismic waves through the layers of the Earth. (Stephenson et al.Journal of Geophysical Research: Solid Earth, 2021)
So what’s down there? The team examined some models of the inner core anisotropy – how differences in its material composition change the properties of seismic waves – and found that some were more likely than others.
While some models suggest that the inner core material channels seismic waves faster parallel to the equator, others indicate that the mixture of materials allows for waves faster and more parallel to the Earth’s rotation axis. Even then, there are debates about exactly how much difference there is from certain angles.
The study here didn’t show much variation with depth in the inner core, but it did find that there was a change in the slow direction up to an angle of 54 degrees, the fastest direction of the waves being parallel to the axis.
“We found evidence that could indicate a change in the structure of iron, perhaps suggesting two distinct cooling events in Earth’s history,” Stephenson said. said.
“The details of this great event remain a bit mysterious, but we have added another piece of the puzzle when it comes to our knowledge of Earth’s inner core.”
These results may explain why some experimental evidence is incompatible with our current models of the Earth’s structure.
The presence of an innermost layer was suspected Beforewith indications that the iron crystals that make up the inner core have different structural alignments.
“We are limited by the distribution of global earthquakes and receptors, particularly at the polar antipodes,” the team wrote in their paper, explaining the missing data diminishes the certainty of their conclusions.
But their findings align with other studies of innermost core anisotropy.
Future research could fill some of these data gaps and allow scientists to corroborate or contradict their findings and hopefully translate more of the stories written during this early layer of Earth’s history.
This research was published in the Geophysical Research Journal.
An earlier version of this article was published in March 2021.