Why might Earth’s inner core slow down?

The rotation of the Earth’s inner core may have slowed down, as the planet’s core now rotates at a slightly slower clip than the layers above, Discover new research. The slowdown could change the rotational speed of the entire planet, as well as affect how the core evolves over time.

For the new study published in the journal natural geosciences, Scientists used a database earthquakes to investigate behavior Earth’s solid inner core over time. The inner core sits like a ball bearing in the molten metal surroundings of the outer core. Because of this liquid cocoon, the “ball bearing” may not rotate at the same rate as the rest of the planet. Over the years, some researchers have found that the nucleus rotates a bit faster than cloak and cortex, a condition called “supercirculation.” But the studies did not report consistent numbers, however First study to observe the differential core rotation to estimate that the inner core rotates about one degree per year faster than the rest of the planet; Others have found annual acceleration of small fractions of a degree.

These differences are not significant. The discrepancy in rotation time between the inner core and the rest of Earth is very slight. Nor are the differences a threat to life on the surface: unlike the 2003 sci-fi movie Nucleus, There’s no need to call in a crack team of geophysicists and astronauts to drill into the center of our planet and start blowing things up. At most, it may affect the rotation of the inner core Earth’s general rotation It contributes to fluctuations in the planet’s magnetic field. Every year the core expands by about a millimeter, as some of the molten iron solidifies in the outer core. Seismic studies showed. Solidification also causes the outer core to rotate, which in turn creates the planet’s magnetic field. Study author Xiaodong Song, a geophysicist at Peking University in China, says that the rotation of the inner core can influence this solidification process in ways that are not yet fully understood, affecting the magnetic field.

The rotation may also be important for how the inner core has grown over billions of years, says John Vidal, a geophysicist at the University of Southern California who was not involved in the study but has researched the core’s rotation.

But the catch is that no one really knows how fast the inner core is spinning. In the new study, Song and geophysicist Yi Yang, also of Peking University, found that the core appeared to rotate steadily, faster than Earth’s general rotation, between the 1970s and early 2000s. Around 2009, Song says, this rotation slowed abruptly to match Earth’s speed, then perhaps slowed so much that now the rest of the planet is spinning faster.

Song and Yang measured this circulation using pairs of nearly similar earthquakes that originated in the same spots, separated only by time. Because earthquakes are nearly identical, their shock waves should also look identical as they travel through the core and back outward, where they are detected by seismometers around the planet—that is, unless the core itself shifts and changes the trajectory of one earthquake waves for the other. If the core rotates differently than the rest of the planet, identical waves of earthquakes occurring months or years later will strike the core at slightly different points and thus bounce back with only slight variations. The researchers compared earthquake waves going back to 1964 to track changes in how the core moved over time. If they’re right, the core’s rotation now lags the planet’s total rotation by a tiny amount.

“We assume that [slowed rotation] It will continue in the coming years and decades and we should be able to see that in [our] “A relatively short human time frame,” says Song.

The new findings probably won’t end the debate about the inner core. The work is well done, Vidal says, and he does an admirable job of combining the different data. But there are many competing explanations for what is happening. For example, Vidale search hints that the core may change its rotation every six years or so, while researchers Guanning Pang and Keith Koper reported one “totter” in the early 2000s and little change since then. In the 2022 study. “I do not see [the new work] Quite conclusive,” says Vidale.

Lianxing Wen, a geodynamics scientist at Stony Brook University who was not involved in the new study, is also looking at core rotation. He doesn’t think the inner core rotates differently from the rest of the planet. A better explanation for the changes in seismic waves traveling through the core, Wayne says, is that the surface of the inner core is not smooth like a ball bearing but rather uneven and constantly changing. “We believe that the inner core has a variable topography that best explains the observed temporal changes of seismic waves reflecting off the inner core,” he says. Wen says the new research misinterprets these changes as being caused by the core’s rotation rather than its volatile surface.

Fortunately, says Song, Earth’s seismic monitoring is better than ever, yielding richer data about the planet’s interior than in previous decades. By continuing to watch the seismic waves, researchers should be able to show whether they are right about the inner rotation of the core.

“The exciting news is that we don’t have to wait that long,” says Song.