Apart from the thin crust on which we live, the structure of the Earth is intangible deep beneath our feet, and as a result difficult to imagine.
Food is often used to demonstrate how the earth is made of four main layers, metaphors reminiscent of a tasty snack (opens in a new tab): graham crackers for the crust, ice cream for the mantle, melted marshmallows for the outer core and chocolate chips for the inner core.
Scientists have long known about a fifth layer: a 400-mile-wide (650-kilometer) metallic ball inside the inner core, aptly named the innermost inner core. Since it was first theorized (opens in a new tab) in 2002, researchers have confirmed (opens in a new tab) its presence repeatedly (opens in a new tab)last in March 2022 (opens in a new tab). But because it is hidden beneath Earth’s various layers and lies deep within the planet’s inner core – which itself is less than 1% of Earth’s volume – the innermost inner core is not well understood.
Related: Earth’s inner core may slow down compared to the rest of the planet
Now, researchers studying seismic waves created by large earthquakes have recorded waves bouncing back and forth like a ping-pong ball along the Earth’s diameter five times – the highest reflection rate ever recorded, breaking the previous record of twice. Observing how these waves, generated when Earth’s tectonic plates suddenly move during earthquakes, are distorted as they travel through Earth’s center is helping scientists bring the elusive inner core into clearer focus.
The team behind the latest research probed the Earth’s center in an innovative way by using three earthquake datasets, each of which saw the core differently, study co-author Hrvoje Tkalčić told Space.com in an email. One of the events they studied was the 7.9 magnitude earthquake that occurred in 2017 in Solomon Islands (opens in a new tab).
“The Earth oscillates like clockwork after a big earthquake, and not just for hours, but days,” said Hrvoje Tkalčić, a geophysicist at the Australian National University and co-author of the latest study. statement (opens in a new tab).
To study the innermost core well, scientists need seismometers located at the opposite ends of earthquakes – points they call antipodes – which are often in the ocean. So they have very little data to work with, thanks to the high costs associated with installing seismic stations in such remote areas.
“The innermost inner core is notoriously difficult to probe by seismic waves,” Tkalčić told Space.com in an email.
Related: How has the Earth’s core stayed as hot as the Sun’s surface for billions of years?
So the team combined seismic data recorded by different data centers around the world about the Great Solomon Islands earthquake and studied a type of seismic wave called the primary, pressure, or P wave. The P wave is the fastest of all seismic waves and the only one that passes through the Earth’s center, so studying it as it crossed the Earth’s center five times illuminated the planet’s deep interior.
Tkalčić’s team found that the wave took 20 minutes to travel the width of the planet. Each time it did, they saw the innermost core’s “anisotropic” property on clear display: Seismic waves passing through the innermost core were slowed in one direction, while those traveling through the outer layer slowed in another direction.
“It simply means that the iron crystals – iron, which is dominant in the inner core – are probably organized in a different way than in the outer shell of the inner core,” Tkalčić said in the same statement (opens in a new tab).
Scientists knew as early as 2003 (opens in a new tab) that the innermost inner core is anisotropic (opens in a new tab), so the latest research bolsters that knowledge with clearer evidence. In the new study, the researchers found that the direction of P waves inside the innermost core is slowest at an “oblique” angle with the equatorial plane, or 50 degrees from the Earth’s axis of rotation.
“This is critical, and is why we can say we have discovered ‘distinct’ anisotropy in the innermost core,” the authors wrote in a piece published by The Conversation (opens in a new tab).
There is strong evidence that slow-moving iron in Earth’s core drives the planet’s geodynamo, leading to the generation of Earth’s global magnetic field. So understanding what happens in the center of the planet will shed light on how the magnetic field behaves and at times reverses.
Although the latest study adds to the growing body of evidence confirming that the innermost core is Earth’s fifth layer, it may take a while for the textbooks to be updated, Tkalčić told Space.com.
“After all, when the inner core was first hypothesized in 1936,” Tkalčić said, “it took some time for (the model) of the Earth to settle and the textbooks to change.”
Shortly after Earth’s innermost core is on its way to textbooks, food analogies will follow. Maybe a dark chocolate in the middle of the chocolate bar?
The research is described in a paper (opens in a new tab) published online February 21 in the journal Nature Communications.
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