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The inner core of the Earth appears to hold an innermost secret.
Geology textbooks almost inevitably include a cutaway diagram of the Earth showing four neatly delineated layers: a thin outer shell of rock that we live on known as the crust; the mantle, where rocks flow like an extremely viscous liquid, driving the movement of continents and the lifting of mountains; a liquid outer core of iron and nickel that generates the planet’s magnetic field; and a solid inner core.
Analyzing the crisscrossing of seismic waves from large earthquakes, two Australian scientists say there is a distinctly different layer at the very center of the Earth. “We have now confirmed the existence of the innermost inner core,” said one of the scientists, Hrvoje Tkalcić, a professor of geophysics at the Australian National University in Canberra.
Dr. Tkalcic and Thanh-Son Pham, a postdoctoral researcher, estimate that the innermost inner core is about 800 miles wide; the entire inner core is about 1,500 miles wide. Their findings were published on Tuesday in the journal Nature Communications.
While the cutaway diagram appears to depict clear-cut divisions, knowledge about the deep interior of Earth is unavoidably fuzzy. It is nearly 4,000 miles to the center of Earth, and it is impossible to drill more than a few miles into the crust. Most of what is known about what lies beneath comes from seismic waves — the vibrations of earthquakes traveling through and around the planet. Think of them as a giant sonogram of Earth.
Two Harvard seismologists, Miaki Ishii and Adam Dziewonski, first proposed the idea of the innermost inner core in 2002 based on peculiarities in the speed of seismic waves passing through the inner core. Scientists already knew that the speed of seismic waves traveling through this part of the Earth varied depending on the direction. The waves traveled fastest when going from pole to pole along the Earth’s axis and slowest when traveling perpendicular to the axis. The difference in speeds — a few percent faster along polar paths — arises from the alignment of iron crystals in the inner core, geophysicists believe.
But in a small region at the center, the slowest waves were those traveling at a 45-degree angle to the axis instead of 90 degrees, the Harvard seismologists said.
The data available then were too sparse to convince everyone.
The best measurements would be seismic waves traveling from an earthquake’s origin straight down into the Earth and through the innermost inner core. However, detecting those generally requires a seismometer located almost exactly on the other side of the Earth, and that point is in the middle of the ocean.
The new paper takes advantage of the fact that seismic waves also bounce back. Thus a seismometer close to the epicenter could detect the reflection of the wave that traveled through the Earth and bounced back, passing through the innermost inner core twice. They could also be reflected back-and-forth a second time, traveling through the innermost core four times.
In recent years, a multitude of seismometers have been deployed, especially in the United States. Combining signals from multiple instruments enabled the detection of the faint reflections resulting from earthquakes with a magnitude of 6 or larger. “We processed 200 events and found that 16 of them had these bouncing waves,” Dr. Tkalcic said.
For one quake that ruptured in the Solomon Islands in 2017, waves that traveled five times through the innermost core were detected by seismometers that were fortuitously positioned on the other side of the planet.
“Kudos to them for uncovering the observations that further studies might use to unravel the perplexities of the inner core’s structure,” said George Helffrich of the Tokyo Institute of Technology’s Earth-Life Science Institute in Japan who was not involved with the research.
There does not seem to be any significant difference in composition between the outer and innermost parts of the inner core, and the transition appears gradual and not sharp.
Vernon Cormier, a professor of physics at the University of Connecticut who was not involved with the research, said that could point to some change in the Earth’s ancient past. The inner core is fairly young, in geological terms — estimates range from 600 million to a billion years, Dr. Cormier said. That is a fraction of the planet’s 4.5-billion-year history, and the structure of the solid core appears complex. In January, other scientists reported that the speed of spin of the inner core changes.
“The reason people study the inner core structure is they try to link it to the Earth’s magnetic field,” Dr. Cormier said. “People will try to look for some change in the Earth’s magnetic field that may have occurred at the same time as the change in the crystallization of the inner core.”