by The Earth’s core is a massive iron-nickel sphere spinning in a layer of liquid metal. But this ball might 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 could record an early phase in Earth’s evolution.
Geoscientists have long observed that seismic waves from earthquakes travel differently through the heart of the inner core than through its upper layers, suggesting some sort of texture change. However, few useful earthquake waves have been detected penetrating the innermost part of this inner core, leaving scientists with very few data points with which to study the composition of this area.
In research published February 21 in the journal nature communicationUsing a new method that tracks earthquake echoes, seismologists peered into the inner core and discovered a change in the way the waves propagate in the innermost core, about 1,300 kilometers (808 miles) across.
“This is a new way to study the innermost inner core,” says Thành Sơn Phạm, co-author of the study, a postdoctoral fellow in seismology at the Australian National University (ANU). “We are strengthening existing evidence for the existence of the innermost inner core, which should be a sphere about half the size of the inner core.” The latter is just over 2,400 kilometers in diameter.
However, not all seismologists agree that the observations are evidence of a distinct innermost inner core. The change in the waves’ behavior is likely accurate and consistent 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 associated with a gradual change within the core rather than an abrupt and distinct transition. “I think it’s an unnecessary layering,” says Frost.
The only way to see deep inside the earth is to use earthquake waves like a scanner. By analyzing how waves change as they travel through the planet, researchers can learn about the properties of the materials they have passed through. Doing this with the innermost inner core requires a strong earthquake occurring on one side of the planet and seismic instruments to pick up the waves on the exact opposite side. Otherwise, the wave signal will not go straight through the heart of the nucleus.
Phạm and his co-author Hrvoje Tkalčić, a seismologist at ANU, followed a different strategy. With the ever-increasing number of seismic sensors deployed around the globe, it’s increasingly possible to detect very faint seismic signals, says Phạm. He and Tkalčić collected signals from large tremors, greater than magnitude 6.0, that create waves that repeatedly bounce around the globe. These echo waves are small because they lose energy with each passage through the planet. However, as they curl through and around the interior, they pass through the inner core several times. The researchers added these repeated weak signals. “We can record signals that used to be very weak, but now they can be amplified,” Phạm says.
The results showed a greater difference in how the angle of the waves affects their speed – a phenomenon called anisotropy – at the center of the inner core compared to its outermost region.
The new research is unlikely to end the debate about what this change in waves, which dates back to 2002, means. Proponents of a distinct innermost heart of the inner core surmise that a dramatic event in Earth’s history may have altered the way in which the latter finds itself, solidifies, and grows. If that’s the case, the different layers could act like a planetary time capsule, Phạm says.
However, other geoscientists argue that the iron-nickel alloy that makes up the inner core simply gradually becomes more organized in its crystallization pattern at greater depths. This change in organization, in turn, affects how earthquake waves travel through the innermost inner core. According to this hypothesis, the inner core has been essentially constantly solidifying throughout Earth’s history, and the distinction of an innermost inner core is not very meaningful. This crystallization transition would be gradual, leaving no distinct boundary between the innermost inner core and the rest of the inner core.
Although Frost questions Phạm and Tkalčić’s interpretation of their findings, he praises the use of multiple weak echoes from earthquakes to get a good look at the center of the planet. “What they’re doing is very valuable because it changes what it takes to see inside the Earth,” Frost says. “It makes more earthquakes accessible to us.”
