Though the seismic waves from earthquakes are best known for their destructive capabilities, in the hands of geologists, they can be powerful tools of discovery. A research team at the University of Illinois has just used the rumbles from quakes to more closely examine the inner core of our planet, and what they found there was quite a surprise. It seems there's another core inside the inner core that measures about half its diameter.
What demarcates this "inner-inner core" is that the iron crystals it contains are oriented on an east-west axis, unlike the iron crystals in the "outer-inner core" which organize along a north-south axis.
"The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving," Xiaodong Song, a professor of geology at UI who worked on the project with visiting postdoctoral researcher Tao Wang, said in a University of Illinois report about the findings. "For example, over the history of the Earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved."
While multiple components of the inner core have been suggested before, this is the first time the difference in polarity has been noted. "Indeed, the layering of the inner core has been suggested more than 10 years ago, at shallow depths of the inner core and at deeper parts of the inner core as well," Song told Crave. "Everyone assumed before the crystal alignment was north-south. But here we found alignment in the inner-inner core to be nearly east-west."
If all this inner and inner-inner talk sounds confusing, perhaps a quick geology refresher is in order. The Earth consists of three layers: the crust where we live; the mantle, a layer of scalding-hot liquid rock; and the core. The core consists of a liquid outer core containing mainly nickel and iron and a solid inner core made up mostly of iron. Even though the inner core is even hotter than its surroundings, the intense pressure at the Earth's center means the inner core is unable to melt and remains solid, according to a National Geographic entry about the topic.
And now we can add another layer to our Earth's composition: the inner-inner core, which is still mostly solid iron, but has a different polarity than the substance surrounding it.
In "unearthing" the inner-inner core, the research team relied on seismic sensors that pick up the waves that penetrate the planet after an earthquake hits, known as the quake's coda. "The earthquake is like a hammer striking a bell; much like a listener hears the clear tone that resonates after the bell strike, seismic sensors collect a coherent signal in the earthquake's coda," the report says.
"It turns out the coherent signal enhanced by the technology is clearer than the ring itself," said Song. "The basic idea of the method has been around for a while, and people have used it for other kinds of studies near the surface. But we are looking all the way through the center of the Earth."
The researchers' findings were published in the journal Nature on Monday.
Update, Wednesday at 9:45 a.m. PT: Added more detail and a response from Professor Xiaodong Song.