The inner core of the Earth seems to hold a more intimate secret.
Geology textbooks almost inevitably include a cutaway diagram of the Earth showing four clearly delineated layers: a thin outer layer of rock on which we live known as the crust; the mantle, where rocks flow like an extremely viscous liquid, driving the movement of the continents and the uplifting of mountains; a liquid iron-nickel outer core that generates the planet’s magnetic field; and a solid inner core.
By analyzing the crisscrossing of seismic waves from large earthquakes, two Australian scientists say there is a distinctly different layer at the 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 across; the entire inner core is about 1,500 miles across. His findings were published Tuesday in the journal Nature Communications.
While the cutaway diagram appears to represent well-defined divisions, knowledge of the Earth’s deep interior is inevitably confusing. It is almost 4,000 miles from the center of the Earth, and it is impossible to drill more than a few miles into the crust. Most of what is known about what lies below comes from seismic waves: vibrations from earthquakes that travel through and around the planet. Think of them as a giant sonogram of the 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 faster when they went from pole to pole along the Earth’s axis and slower when they traveled perpendicular to the axis. Geophysicists believe that the difference in speeds, a few percent faster along the polar paths, arises from the alignment of iron crystals in the inner core.
But in a small region in 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 at the time was too scant to convince everyone.
The best measurements would be the seismic waves that travel from the source of an earthquake directly toward the Earth and through the innermost inner core. However, detecting them usually 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 article takes advantage of the fact that seismic waves also recover. Therefore, a seismometer near the epicenter could detect the reflection of the wave that traveled through the Earth and bounced back, passing 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. The combination of signals from multiple instruments allowed the detection of weak reflections resulting from earthquakes with a magnitude of 6 or greater. “We processed 200 events and found that 16 of them had these bouncing waves,” Dr. Tkalcic said.
For an earthquake that struck the Solomon Islands in 2017, seismometers haphazardly placed on the other side of the planet detected waves that traveled five times through the innermost core.
“Congratulations to them for uncovering observations that further studies could use to unravel the perplexities of the inner core’s structure,” said George Helffrich of the Tokyo Institute of Technology’s Institute of Earth Life Sciences in Japan, who was not involved in the research. .
There does not appear to be any significant difference in composition between the outer and inner parts of the inner core, and the transition appears gradual rather than abrupt.
Vernon Cormier, a physics professor at the University of Connecticut who was not involved in the research, said that could indicate some change in Earth’s ancient past. The inner core is quite young, in geological terms: Estimates range from 600 million to 1 billion years old, Dr. Cormier said. That’s 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 spin rate of the inner core changes.
“The reason people study the structure of the inner core is that they try to link it to Earth’s magnetic field,” said Dr. Cormier. “People will try to look for any changes in the Earth’s magnetic field that may have occurred at the same time as the change in the crystallization of the inner core.”