A fifth layer discovered in Earth’s core, confirms scientists

In a new study, scientists have confirmed the existence of a fifth layer in the Earth’s core. The Earth’s innermost core comprises four known layers, including the crust, mantle, outer liquid, and inner solid core. The fifth layer, also known as the innermost inner core, is located at the Earth’s center, within the inner core, as described in a study published in Nature Communications on February 21, 2023.

The innermost inner core is made of iron and nickel, the same materials as the rest of the inner core. However, the difference between the two parts of the inner core could stem from how iron atoms are arranged to form a solid. Alternatively, the iron in the innermost inner core could have solidified and grown in a different direction than the rest of the inner core.

The Earth’s inner core as a whole was liquid in the early years of the Earth’s existence, turning into a solid as the Earth cooled. The idea that the Earth could hold a fifth layer was proposed in 2002. Since then, researchers have been trying to get more data to learn about the elusive inner core.

To study the Earth’s interiors, scientists rely on seismic waves, which are shockwaves generated during an earthquake. These waves behave differently as they pass through diverse materials, such as traveling slower through hot materials.

However, scientists face limitations in their ability to sample the innermost inner core because waves from large earthquakes following a few set paths have been studied repeatedly, leaving the rest of the inner core unexplored. To overcome this limitation, scientists shortlisted waves from earthquakes that bounce from the point of origin to the opposite end multiple times.

Only larger earthquakes will radiate enough energy for these waves to survive multiple journeys through the center of the Earth. Researchers then studied the time seismic waves took to traverse the internal Earth. They confirmed previous studies that showed that the innermost inner core and the rest of the inner core slow seismic waves in different directions. In contrast, the outer shell of the inner core slows down the waves in the equatorial plane.

The differences in speed suggest that the iron in the innermost inner core and the rest of the inner core could be arranged differently. Hrvoje Tkalčić from the Australian National University (ANU) and one of the study’s authors explained that “this is an interesting study that supports previous claims that the inner core has some internal structure.”

The duo calculated the innermost inner core’s radius to be around 650 kilometers, while the inner core as a whole is 1,221 kilometers in radius, which is one percent of Earth’s volume. Tkalčić expects that the transition from the innermost inner core to the outer shell of the inner core is likely gradual, which could inform researchers of two things: the rate at which the transition happened and the age of the inner core during the transition.

Peter Driscoll, from Carnegie Institution for Science, who was not involved in the study, described the phenomenon of differences in speed in a particular direction as interesting. He explained that it has been studied extensively in the upper mantle, but it’s much harder to see in the inner core because there are so few waves that pass through it, and they are much noisier than shallow upper mantle waves.

The discovery of the fifth layer in the Earth’s core could lead to a better understanding of the Earth’s inner workings, such as the rate at which the transition from the innermost inner core to the outer shell of the inner core happened and the age of the inner core during the transition.