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By Aristos Georgiou,Newsweek
Researchers from the Australian National University 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 describe how they detected so-called "shear waves" or “J waves” in the inner core. These are a type of wave that can only travel through solid objects.
“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—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.”
Inner core shear waves are so weak that they can’t be observed directly. In fact, detecting them has long 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 researchers used an approach that looked at the similarities between the signals at pairs of receivers after a major earthquake. This technique has been used by the same team to measure the thickness of the ice in Antarctica.
“We’re throwing away the first three hours of the seismogram and what we’re looking at is between three and 10 hours after a large earthquake happens,” Tkalčic said. “We want to get rid of the big signals.”
“Using a global network of stations, we take every single receiver pair and every single large earthquake—that’s many combinations—and we measure the similarity between the seismograms,” he said. “That’s called cross-correlation, or the measure of similarity. From those similarities we construct a global correlogram—a sort of fingerprint of the earth.”
These results were used to demonstrate the existence of J waves and their speed in the inner core. 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, and without that geomagnetic field there would be no life on the Earth’s surface,” he said.
