An international team of geologists, headed by Prof Xiaodong Song from the University of Illinois at Urbana-Champaign and Nanjing University in China, has found that the Earth’s inner core has an inner core of its own, which has surprising properties that could reveal new information about our planet.
The inner core of our planet has an inner core of its own, with crystals aligned in a different direction. Image credit: Lachina Publishing Services.
“Even though the inner core is small – smaller than the moon – it has some really interesting features. 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,” said Prof Song, who is a co-author of the paper published in the journal Nature Geoscience.
Scientists use seismic waves from earthquakes to scan below the planet’s surface, much like doctors use ultrasound to see inside patients. Prof Song and his colleagues used a technology that gathers data not from the initial shock of an earthquake, but from the waves that resonate in the earthquake’s aftermath.
“It turns out the coherent signal enhanced by the technology is clearer than the ring itself. 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,” Prof Song said.
Looking through the core revealed a surprise at the center of the planet – the inner core, once thought to be a solid ball of iron, has some complex structural properties.
The researchers found a distinct 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.
However, in the inner-inner core, the iron crystals point roughly east-west.
Not only are the iron crystals in the inner-inner core aligned differently, they behave differently from their counterparts in the outer-inner core. This means that the inner-inner core could be made of a different type of crystal, or a different phase.
“The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving,” Prof Song said.
“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. We are right in the center – literally, the center of the Earth,” he said.
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Tao Wang et al. Equatorial anisotropy in the inner part of Earth’s inner core from autocorrelation of earthquake coda. Nature Geoscience, published online February 09, 2015; doi: 10.1038/ngeo2354
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