In fact, the Earth may have two solid inner cores: the inner core and the shell, which does not yet have a name.
Photos from open sources
It seems that the internal structure of the Earth simply has no end. A new study shows that a component known as the “inner core” is made up of an innermost core and a middle core. Although both are likely composed of solid iron and nickel, it seems that they can be separated and should be considered separately if we are to understand the formation and behavior of our planet.
Humankind’s knowledge of the inner workings of the Earth has been shaped by earthquakes. Seismic waves that occur at this time bounce off the planet’s boundaries or bend like reflected light waves. However, the deeper we go, the more difficult it is to study them, due to the fact that only waves passing almost directly through the center of the planet are affected, and also because of the noise created by interactions at the boundaries further.
Although the hypothesis of the innermost core has been put forward for more than 20 years, its existence has remained in question. Dr. Thanh-Song Phem and Professor Hrvoe Tkalcic of the Australian National University write in a new paper that they have finally found evidence for their theory. An article with the results of the work of scientists was published in the journal Nature Communications.
To find the answer, the researchers used the arrival time delays of seismic waves from 200 earthquakes of magnitude 6 or more that bounced back and forth across the Earth, in some cases traveling five times.
Photos from open sources / Diagram of the layers of the Earth and seismic waves reflected from the crust and slowed down when passing through different layers. The innermost core is about half the diameter of the inner core, but small compared to the outer core and mantle / Image: Drew Whitehouse, National Computing Infrastructure Vizlab, Australian National University
Phem and Tkalcic describe the inner core as a solid “metal ball” with a diameter of 650 kilometers. It is smaller than Ceres, but larger than any other main belt asteroid. Seismic P-waves travel through it 4 percent slower when they follow a path inclined 50 degrees to the Earth’s axis of rotation. The authors interpret this as an indication that it has a crystalline structure caused by the arrangement of iron atoms at very high temperatures and pressures. Movement through the rest of the solid core is slowest in the direction of the equatorial plane.
“This inner core is like a time capsule of the Earth’s evolutionary history. It is a fossilized record that serves as a gateway to the events of our planet’s past. Events that took place on Earth hundreds of millions or billions of years ago,” Tkalchich said.
The authors of the study speculate that a major global event caused the inner core to separate from the rest of the inner core, though they don’t yet know what that was.
The inner core as a whole is thought to grow as portions of the liquid outer core solidify. However, it is not known whether the boundary between the innermost core and the surrounding shell is shifting.
The authors report that the work has been made possible by the growing number of seismic sensors distributed around the planet. By building a wide network and analyzing data from many locations, the authors were able to track seismic waves in unprecedented detail, including those traveling from north to south almost straight through the Earth. They also developed methods for amplifying the signal from distant seismic waves, distinguishing it from localized noise.
The authors suggest that future studies will explore the boundary between the innermost core and the shell around it.
Last year, Tkalcic and a colleague questioned the notion of Mars’ internal structure, providing evidence that its mantle still contains mobile magma.