Looking into the world's "deepest mystery" with the help of seismic waves, researchers have found that the Earth's inner core has an inner core of its own, which has surprising properties that could reveal information about our planet.
"Even though the inner core is small - smaller than the moon - it has some really interesting features," said Xiaodong Song, professor of geology at University of Illinois.
"It may tell us about how our planet formed, its history, and other dynamic processes of the Earth. It shapes our understanding of what is going on deep inside the Earth," Song added.
Researchers use seismic waves from earthquakes to scan below the planet's surface, much like doctors use ultrasound to see inside patients.
The team 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.
The inner core, once thought to be a solid ball of iron, has some complex structural properties.
The team 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, the study noted.
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 scientists pointed out.
"The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving," Song said.
The study appeared in the journal Nature Geoscience.