Earth’s core consists of a solid iron-nickel ball rotating within a layer of liquid metal. But that ball may not be as simple as it seems: new research suggests the inner core contains its own inner core.
If so, this so-called innermost inner core may record an early phase in Earth’s evolution.
Earth scientists have long observed that seismic waves from earthquakes move differently through the heart of the inner core than they do through its upper layers, indicating some sort of change in texture. Only a few usable earthquake waves have been detected passing through the innermost part of this inner core, however, so scientists have very few data points with which to study that area’s composition.
In research published on February 21 in the journal Nature Communications, seismologists peered into the inner core using a new method that tracks the echoes from earthquakes, discovering a change in the way the waves travel in the about 1,300-kilometer (808-mile) diameter of the innermost core.
“This is a new way to sample the innermost inner core,” says study co-author Thành Sơn Phạm, a postdoctoral researcher in seismology at the Australian National University (ANU). “We strengthen existing evidence for the existence of the innermost inner core, which should be a ball the size of about half of the inner core.” The latter spans just more than 2,400 kilometers in diameter.
Not all seismologists agree that the observations are proof of a distinct innermost inner core, however. The change in the waves’ behavior is likely accurate and meshes with previous research, says Dan Frost, a seismologist at the University of South Carolina, who was not involved in the study. But, he says, the results could be linked to a gradual change within the core rather than an abrupt and distinct transition. “I think it is an unnecessary splitting into multiple layers,” Frost says.
The only way to peer deep into Earth’s interior is to use earthquake waves like a scanner. By analyzing how waves change as they travel through the planet, researchers can learn the properties of the materials they passed through. To do this with the innermost inner core requires a strong earthquake that occurs on one side of the planet and seismic instruments to pick up the waves on exactly the opposite side. Otherwise, the wave signal won’t pass directly through the heart of the core.
Phạm and his co-author Hrvoje Tkalčić, a seismologist at ANU, used a different strategy. Because of the ever expanding number of seismic sensors deployed around the globe, it’s increasingly possible to detect very weak seismic signals, Phạm says. He and Tkalčić collected signals from large quakes above magnitude 6.0 that generate waves that bounce around the globe repeatedly. These echoing waves are small because they lose energy with each pass through the planet. As they ripple through and around the interior, however, they pass through the inner core multiple times. The researchers added up these repeated faint signals. “We can record signals which used to be very weak in the past, but now it can be enhanced,” Phạm says.
The results revealed a greater difference in how the angle of the waves impacted their speed—a phenomenon called anisotropy—in the very center of the inner core, compared with its outermost region.
The new research isn’t likely to settle the debate on what this change in waves means, which dates back to 2002. Proponents of a distinct innermost heart of the inner core suggest that some dramatic event in Earth’s history may have altered the way the latter solidifies and grows. If that’s the case, the distinct layers could act like a planetary time capsule, Phạm says.
But other geoscientists argue that the iron-nickel alloy that makes up the inner core simply becomes gradually more organized in its crystallization pattern at greater depths. This change in organization, in turn, affects how earthquake waves move through the innermost inner core. By this hypothesis, the inner core has been solidifying basically consistently throughout Earth’s history, and the distinction of an innermost inner core isn’t very meaningful. This crystallization transition would happen gradually, leaving no distinct border between the innermost inner core and the rest of the inner core.
Though Frost contests Phạm and Tkalčić’s interpretation of their findings, he praises the use of multiple faint echoes of earthquakes to look at the very center of the planet. “What they’ve been doing is very valuable because it changes what is required to see inside of Earth,” Frost says. “It’s making more earthquakes accessible to us.”