Once, more than a century ago, the center of the Earth was the domain of science fiction. In one famed tale, all you had to do to get down there was find passage through an Icelandic volcano that led to a world of ancient creatures and things that go bump in the night. The novel’s protagonist then reemerged in southern Italy via volcano — the back exit from a subterranean world.
The truth about the Earth’s center may be more difficult to penetrate, but it’s no less dizzying to contemplate. And now, research published on Monday in Nature Geoscience adds a new, previously unknown wrinkle to the saga: Thanks to clues left by earthquakes’ seismic waves, a team of researchers from China and the United States has discovered there’s an even smaller inner core within the Earth’s core.
And what’s more, the inner core can be read like a tree’s rings. “Even though the inner core is small — smaller than the moon — it has some really interesting features,” Xiadong Song, a professor of geology at the University of Illinois, said in a statement Monday. “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 with the Earth.”
What scientists speculate is going on there represents a departure from earlier thinking. The inner core, which rests about 3,100 miles beneath the surface, was long believed to be a solid ball of iron. But it’s more complicated than that. There’s a hidden nucleus, about the half the diameter of the outer core, with iron crystals. Those iron crystals align east-west. But the iron crystals in the outer shell align north-south.
Scientists can’t directly probe the Earth’s core — though one did propose sending a grapefruit-sized probe down there. Instead, they study the core using seismic data by scrutinizing how those waves change as they pass through the planet’s varying layers, many of which have varying densities. “The waves are bouncing back and forth from one side of the Earth to the other side of the Earth,” Song told BBC.
But Song and his co-authors used a novel approach in the research.
They didn’t focus on seismic waves triggered by earthquakes themselves, but those that rang through the planet after quakes had passed. The process wasn’t dissimilar to how doctors use ultrasound to see inside a patient, a University of Illinois release noted: “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.”
And it turned out, Song said, the reverberations in the aftermath of a quake gave a much clearer glimpse into the Earth. “The coherent signal enhanced by the technology is clearer than the ring itself,” Song said. “The basic idea of the method has been around for awhile, 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.”
What they saw down there wasn’t what they were expecting. The iron crystals comprising the inner-inner core may be a different type of crystal from those in the outer-inner core. “The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving,” Song added.
Simon Redfern of the University of Cambridge took a look at the findings and thought they could mean big things. Like reading a tree’s rings, “probing deeper into the solid inner core is like tracing back in time, to the beginnings of its formation,” he told the BBC. If Song’s research is accurate, the fact that the very heart of the Earth is so different from the next layer suggests something “very substantial happened to flip the orientation of the core.”
Now the question becomes, what could that possibly be?