Scientists Finally Have Direct Proof That the Earth's Core Is Solid

Researchers from the Australian National University (ANU) are confident they have found direct proof, for the first time, that the Earth's inner core is solid—albeit a little bit squishy.

In a paper published in the journal Science, Hrvoje Tkalčić and Than-Son Phạm both at ANU describe how they detected so-called "shear waves" in the inner core. These are a type of wave propagated by earthquakes that travel through the Earth's interior and can only move in solid material.

"The inner core is a planet within our own planet, buried thousands of kilometers beneath our feet," Tkalčić told Newsweek. "The progress on the studies of Earth's interior has been tremendous, but we are still in a discovery stage."

"We found the inner core is indeed solid, but we also found that it's softer than previously thought," Tkalčić said in a statement. "It turns out—if our results are correct—that the inner core shares some similar elastic properties with gold and platinum," he continued. "The inner core is like a time capsule; if we understand it we'll understand how the planet was formed, and how it evolves."

Shear waves travelling through the inner core are thought to be so weak that they can't be observed directly. In fact, detecting them has been considered the "Holy Grail" of global seismology since scientists first predicted the center of the Earth was solid in the 1930s and 1940s.

"The waves generated by earthquakes that propagate through the bulk of the Earth could be divided into two categories: compressional (P) waves and shear (S) waves," Tkalčić said. "In compressional waves, the ground particles oscillate in the same way in which the waves propagate, like in sound waves, but in the shear waves, the particles oscillate perpendicular to the direction of wave motion."

"If we can measure the speed by which they propagate, that can tell us quite a lot about the physical (elastic) properties of the rocks through which they propagate," he said. "We can measure these properties in rock physics labs at crustal or mantle conditions, but it gets extremely tricky to simulate the formidable conditions of the inner core. And without that knowledge, it is challenging to calculate the exact temperature, composition or even the age of the inner core."

To find these waves, the researchers used a global network of pairs of receivers which measure activity in the Earth's interior. After major earthquakes, the team measured the similarity of the observed signal collected by thousands of receiver pairs in order to construct a global correlogram—a kind of "fingerprint" of the Earth. The results contained evidence of of shear waves in the inner core and indicated the speed that they were travelling at.

"Both P and S waves can reach all the way down to the Earth's liquid outer core, but shear waves won't propagate through [it]," Tkalčić said. "Instead, P waves continue their journey through the liquid outer core and, if the inner core is indeed solid, [they] will partition their energy at the inner-outer core boundary in a way that a fraction will keep propagating as P waves and [another] fraction will be converted to S waves."

"[The S waves] make their way to the other side of the inner core, where they will again be converted to P waves," he said. "The simplest manifestation of shear waves in the inner core are so-called PKJKP waves. These have been mysterious and difficult to observe."

The first indirect proof that the inner core is solid came in the 1970s, after a paper published by Adam Dziewonski and Freeman Gilbert. However, PKJKP wave observations in the inner core would be more direct proof—the "Holy Grail" that seismologists have been searching for.

"There have been several attempts before us and a number of papers have claimed that they observed PKJKP waves in the seismic wavefield," Tkalčić said. "However, a recent study showed that these results were likely the observations of scattered energy, and thus the scientific community stayed divided and generally unconvinced. This work hopefully concludes a quest for the Holy Grail and provides desired constraints on inner core solidity. However, I expect that refinements of our measurements will follow in the near future."

The new work has important implications for how we investigate the Earth's interior, and indeed those of other planets, according to the authors.

"We have unleashed the potential of a relatively new method that has been on the rise for the last decade or so, and we have shown how the method can revolutionize global seismology and future studies of other planets," Tkalčić said.

"We are limited in global seismology and studies of the Earth's interior by an uneven geographical distribution of large earthquakes and receivers around the world," he said. "[The field] will, therefore, only develop further by the building of more observational infrastructure and through innovative approaches, such as the one described in this research."

Despite the new insights, there is much that remains a mystery about the center of the Earth.

"For instance, we don't know yet what the exact temperature of the inner core is, what the age of the inner core is, or how quickly it solidifies, but with these new advances in global seismology, we are slowly getting there," Tkalčić said.

"The understanding of the Earth's inner core has direct consequences for the generation and maintenance of the geomagnetic field. Without that geomagnetic field there would be no life on the Earth's surface," he said.

This article has been updated to include additional comments from Hrvoje Tkalčić.