The inner core of the Earth may have a whole other core inside

The heart of the earth may have a secret chamber. The planet’s inner core isn’t just a solid ball of nickel and iron, the researchers say, but contains two layers of itself: a distinct middle region tucked inside the outer crust.

Scientists say they have confirmed that this is the inner core using a previously undescribed type of seismic that not only travels through the Earth’s core, but also bounces back and forth through the interior, gathering invaluable information about the structure of the core along the way.

Focusing on earthquakes of magnitude 6 or greater that have struck in recent decades, the researchers compiled data about these earthquakes that had been collected at seismic stations around the world. Combining these signals made it possible to detect even the tiniest reflections of seismic waves. Of the 200 or so analyzed, 16 events produced seismic waves that bounced through the inner core in multiple detections.

The origin, structure and fate of the Earth’s core is very important because the core creates the planet’s magnetic field, which shields the Earth from particles ejected from the Sun and helps keep the planet’s inhabitants safe from excessive radiation.

“Understanding how the magnetic field evolves is important for life on Earth’s surface,” Hrvoje Tkalčić, a seismologist at the Australian National University, said in a statement.

The entire core is about 6,600 kilometers across and consists of two main parts: the liquid outer core and the solid inner core (.SN: 1/23/23). As the iron-rich fluid circulates in the outer core, some of the material cools and crystallizes, sinking to form a solid core. In between, it generates the Earth’s magnetic field.

When this chorus first began to hum is not certain, but some studies suggest it was as recent as 565 million years ago, a fraction of the 4.6 million years of Earth’s lifespan.SN: 1/28/19). That from time to time the chorus hesitated, In small magnetic grains stammering traces kept in the rocks. These data suggest that the planet’s magnetic poles flip-flopped repeatedly over many years, temporarily weakening the magnetic field (SN: 2/18/21). As the crystals cool more and more, the dance will eventually slow down and stop, shutting down the planet’s magnetic field millions or billions of years from now.

Different types and structures of minerals, as well as different amounts of liquid in the subsurface, can change the speed of seismic waves traveling through the Earth, revealing the makeup of the interior. In 2002, researchers noticed that seismic waves traveling through the inner part of the Earth move slightly more slowly in one direction towards the poles of the planet than in others. That suggests something oddity there – a difference in crystal structure, perhaps. That hidden core, the team suggested, was a kind of fossil: a long-preserved remnant of the core’s first formation.

With that observation, Tkalčić and others pored over the seismic data, finding independent lines of evidence supporting the idea of ​​an inner core. Seismic wave reflected, described on February 21 in . Nature Communicationsand yet the existence of this hidden heart is a sure sign of slowness.

Using that seismic data, Tkalčić and seismologist Thanh-Fon Phạm, both of the Australian National University, estimate that this inner core is about 600 kilometers across, or about half the diameter of the full inner core. And the pair could measure the direction of the slowest waves by about 50 degrees relative to the Earth’s rotation axis, providing greater insight into the region.

The exact source of the slow wave is not clear, Tkalčić says. The phenomenon can be related to the structure of the iron crystals, which can otherwise be concentrated further away from the center. Or it could be from a different crystal alignment from some long ago global event that changed how the inner cores solidified from the outer core.

The inner core holds many other mysteries as well. The lighter elements that are present in the smallest parts of the core – hydrogen, carbon, oxygen – flow around the solid iron in a liquid “superionic” state, still complicated by the seismic picture (SN: 2/9/22).

By tracking and reporting the seismic waves that have moved through the interior of the planets multiple times, researchers have done invaluable work that will help nuclear researchers in new ways, says seismologist Paul Richards of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, NY.

Still, the team’s interpretation of the core’s inner structure from those waves is “perhaps more iffy,” said Richards, who was not involved in the work.

One reason for this uncertainty is that, as they fluctuate and fluctuate, they can become weaker and more difficult to record, he says. “Many additional observations will help to determine” what this new information can reveal about the heart of the planet.