Earth’s inner layers have just got a bit more complicated, with scientists discovering a whole new inner core within the center of the planet.

Research released on February 21 in the journal Nature Communications shows that what was assumed to be the Earth’s innermost layer actually has another inner core within, made of iron and measuring around 400 miles across.

“The innermost inner core’s existence was proposed in 2002, and the radius was estimated at about 300 km [186 miles]. Several lines of research have confirmed it so far, including those from our research group recently. The most recent estimate of its size is about 650 km [404 miles],” Thanh-Son Pham, lead author of the paper and postdoctoral fellow in geophysics at the Australian National University, told Newsweek.

earth layers
Stock image showing the inner layers of the Earth. A new paper states that the inner core may contain an extra, innermost core.
iStock / Getty Images Plus

Before this discovery, the structure of the Earth was assumed to be comprised of four layers: the outer crust, the mantle, the outer core and the inner core. The inner core is 758 miles in diameter, is made of extremely dense iron and nickel, and has a temperature of around 9,800 degrees Fahrenheit.

The outer core is also made of iron and nickel but is instead liquid, churning in massive turbulent flows that generate electrical currents and therefore sustains the Earth’s magnetic field. The mantle is made of mostly iron, magnesium and silicon, and is around 1,865 miles thick, while the crust is a mere 3.1 miles to 43.5 miles thick, depending on location.

An innermost core has been long theorized to exist, but until now, was unconfirmed.

The authors made the discovery of the extra innermost core using records of seismic waves from around the world.

“We analyzed digital records of ground motion, known as seismograms, from hundreds of large earthquakes in the last decade,” Pham said. “Our study becomes possible thanks to the unprecedented expansion of the global seismic networks, particularly the dense networks in the contiguous U.S., the Alaskan peninsula, and over the European Alps. Many co-working seismographs empower the enhancement of weak signals that have undergone several bounces throughout the entire Earth’s bulk.”

earth layers experiment
Ray paths of fivefold reverberating waves along the Earth’s diameter provide a new probe to the distinct internal shell of the Earth’s inner core: the innermost inner core.
Drew Whitehouse, National Computational Infrastructure’s Vizlab, Australian National University

A critical advantage of this method is that it improves the volumetric sampling of the innermost inner core compared to previous studies because the authors were able to take advantage of dense continental-scale networks consisting of several hundreds of seismometers.

“This new study differs from previous studies because it uses seismic waves that bounce multiple times within the Earth, along its diameter and through its center. That way, we obtain a sampling of the inner core in some directions that were unavailable with the existing methods. The more angles we can sample the inner core, the better we can tell about the distinction of anisotropic properties at two regions in the Earth’s inner core.”

They found that this innermost core has a different speed of travel for seismic waves depending on the direction of travel, a property known as anisotropy. This property is not present in the rest of the inner core.

“It could be caused by different arrangements of iron atoms at high temperatures and pressures or the preferred alignment of growing crystals. There is strong evidence that the outer shell of the inner core is anisotropic and that the slowest direction of propagation is in the equatorial plane (and the fastest is parallel to the Earth’s spin axis). Meanwhile, in the innermost part of the inner core, the slowest direction of propagation forms an oblique angle with the equatorial plane. This is critical–this is why, in the title, we say ”distinct” anisotropy in the innermost inner core” Pham said.

The authors found that rather than like the sharp boundaries between the other layers within the Earth, the boundary between this newly discovered innermost core and the rest of the inner core is more gradual.

“The transition from the innermost metallic (solid) ball to the outer shell of the inner core (also metallic, solid shell) seems rather gradational than sharp. This is why we cannot observe it via direct reflections of seismic waves from it. Instead, we use the waves that move through it. This differs from previous studies documenting sharp boundaries between the other Earth’s internal layers,” Pham said.

The authors suggest that this could have been formed by a change in the Earth’s magnetic field, which in turn may re-shape the way we think the Earth was formed 4.5 billion years ago.

“How different layers of the Earth interact and how the convection in the liquid outer core might affect the growth/solidification of the inner core.  The inner core grows (and rotates) in response to the coupling with other layers, which are both gravitational and electromagnetic,” Pham said.

This discovery may also help scientists to understand the formation of other planets in our solar system and beyond.

“Understanding the history of our planet’s magnetic field gives us a glimpse of what might have happened with other planets. Take Mars as an example. We don’t understand yet why it [Mars’ magnetic field] ceased to exist in the past. It could have been because of the cessation of convection in its liquid core, which, in turn, could have been caused by the growth rate of its solid portion (or the miscibility [mixability] of chemical elements present in the liquid outer core).”

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