Earth Surprising Geologists: Inner Core inside the Inner Core

Thanks to a novel application of earthquake-reading technology, a study team at the University of Illinois and colleagues at Nanjing University in China have discovered that the Earth’s inner core has an inner core of its own, which has astonishing properties that could disclose info about our planet. 

Led by Xiaodong Song, a professor of geology at the U. of I., and visiting postdoctoral scientist Tao Wang, the team issued its work in the journal Nature Geoscience on Feb. 9. 

“Even though the inner core is small – smaller than the moon – it has some really interesting features,” said Song. “It may tell us about how our planet formed its history, and other dynamic processes of the Earth. It shapes our understanding of what’s going on deep inside the Earth.”

Scientists use seismic waves from earthquakes to study below the planet’s surface, much like doctors use ultrasound to see inside patients. The team used a technology that collects data not from the initial shock of an earthquake, but from the waves that vibrate in the earthquake’s aftermath. The earthquake is like a hammer hitting a bell; much like a listener hears the strong tone that resonates after the bell strike, seismic sensors gathers a coherent signal in the earthquake’s coda. 

“It turns out the coherent signal improved by the technology is stronger than the ring itself,” said Song. “The basic idea of the technique has been present for a while, and people have used it for other kinds of researches near the surface. But we are looking all the way through the center of the earth.” 

Looking through the core disclosed a wonder at the center of the planet – though not of the type intended by novelist Jules Verne. 

 The inner core, once believed to be a solid ball of iron, has some complex structural properties. The team discovered a separate inner-inner core, about half the diameter of the whole inner core. The iron crystals in the outer layer of the inner core are aligned directionally, north-south. Though, in the inner-inner core, the iron crystals point approximately east-west. 

 Not only are the iron crystals in the inner-inner core aligned in a different way, they behave distinctly from their counterparts in the outer-inner core. This means that the inner-inner core could be made of a dissimilar crystal, or a different phase.

“The fact that we have two regions that are clearly different may tell us something about how the inner core has been developing,” Song said. “For example, over the history of the earth, the inner core might have had a very vivid change in its distortion regime. It might hold the key to how the planet has evolved. We are right in the center – trully, the center of the Earth.”

The U.S. National Science Foundation and the National Science Foundation of China supported this work.



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