“Australian scientists argue that within the Earth’s core, based on measurements of waves reverberating along the Earth’s diameter, is an innermost inner core, about 800 miles wide”

What’s Inside the Earth’s Core?
by Kenneth Chang   /  Feb. 23, 2023

“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.

“The cross-sections of the Earth in the right column contain ray path segments sampling the OIC and IMIC. Yellow stars are sources, blue inverted triangles are receivers.”

“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.

“Red colors indicate slower than average velocity, blue colors indicate faster than average velocity. The cross section is made along a great circle between Africa and the southwestern Pacific (crossing under Eurasia).”

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.

“Contours (four red and four blue curves) and background color (eight red and eight blue colors) have constant extreme values (±1 s).”

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.”

Earth’s Inner Core Paused, Then Reversed Its Spin
by Robin George Andrews  /  Jan. 23, 2023

“Imagine Earth’s inner core — the dense center of our planet — as a heavy, metal ballerina. This iron-rich dancer is capable of pirouetting at ever-changing speeds. That core may be on the cusp of a big shift. Seismologists reported Monday in the journal Nature Geoscience that after brief but peculiar pauses, the inner core changes how it spins — relative to the motion of Earth’s surface — perhaps once every few decades. And, right now, one such reversal may be underway.

This may sound like a setup for a world-wrecking, blockbuster movie. But fret not: Precisely nothing apocalyptic will result from this planetary spin cycle, which may have been happening for eons. The researchers who propose this speculative model instead aim to advance understanding of Earth’s innermost sanctum and its relationship with the rest of the world. The inner core is like “a planet within a planet, so how it moves is obviously very important,” said Xiaodong Song, a seismologist at Peking University in Beijing and an author of the study.

In 1936, the Danish seismologist Inge Lehmann discovered that Earth’s liquid outer core envelops a solid metal marble — and it has bamboozled scientists ever since. “It’s weird that there’s a solid iron ball kind of floating in the middle of the Earth,” said John Vidale, a seismologist at the University of Southern California who was not involved with the study. Scientists think the core crystallized out of a molten metal soup at some point in Earth’s not-too-distant past, after the planet’s internal inferno had sufficiently cooled.

“Raw palaeointensity versus inclination data shown with a best-fitting dipole for the four studied time intervals.”

The inner core cannot be directly sampled, but energetic seismic waves emanating from potent earthquakes and Cold War-era nuclear weapon tests have ventured through the inner core, illuminating some of its properties. Scientists suspect this ball of mostly iron and nickel is 1,520 miles long and as about as hot as the sun’s surface. But these waves also created a conundrum. If the core was inert, the voyages of core-diving waves coming from near-identical quakes and nuclear explosions would never change — yet, over time, they do.

One explanation: The inner core is spinning, deflecting these waves. In the mid-1990s, Dr. Song was one of the first scientists to suggest that the inner core may be rotating at a different speed than Earth’s surface. Since then, seismologists have found evidence implying the inner core’s spin can both speed up and slow down. What’s going on? One idea is that two titanic forces are battling for control over the world’s heart. Earth’s magnetic field, generated by swirling iron currents in the liquid outer core, is pulling at the inner core, causing it to spin. That impulse is countered by the mantle, the mucilaginous layer above the outer core and below Earth’s crust, the immense gravitational field of which grasps the inner core and slows its spin.

By studying core-diving seismic waves recorded from the 1960s to the present day, Dr. Song and Yi Yang, another Peking University seismologist and a co-author of the study, posit that this tremendous tug of war causes the inner core to spin back and forth on a roughly 70-year cycle. In the early 1970s, relative to someone standing on Earth’s surface, the inner core was not spinning. From then, the inner core has gradually spun faster eastward, eventually overtaking the speed of rotation of Earth’s surface. Afterward, the inner core’s spin decelerated until its rotation appeared to have stopped at some point between 2009 and 2011.

“Reverse of the length of day variations (-LOD)”

The inner core is now starting to gradually spin westward relative to Earth’s surface. It will likely accelerate then decelerate once again, reaching another apparent standstill in the 2040s and completing its latest eastward-westward spin cycle. This 70-year rhythm, if it exists, could have a tangible effect on parts of Earth’s deeper viscera. But it may only be capable of stirring up comparatively minor turbulence closer to the surface — perhaps by causing subtle shifts in the planet’s magnetic field, or even by very slightly tweaking the length of a day, which is known to go increase and decrease by a fraction of a millisecond every six years.

“time range between 1968-1977 with illustration of sub- and super-rotation.”

This is just one of several competing models explaining the erratic voyages of waves that reach the core. It is also possible that Earth’s innermost layer is wobbling about. Conversely, Earth’s ferrous nucleus may have a metamorphosing surface, twisting any seismic waves that pierce it. “No matter which model you like, there’s some data that disagrees with it,” Dr. Vidale said. Because of its inaccessibility, this abyssal realm may forever elude explanation. “It’s certainly possible we’ll never figure it out,” Dr. Vidale said. But, he added, “I’m an optimist. The pieces are going to fall into place someday.”



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