Science

Why Earth’s Inner Core May Be Slowing Down

Why Earth's Inner Core May Be Slowing Down


The spin of Earth’s inner core may have slowed, with the heart of the planet now rotating at a slightly more sluggish clip than the layers above, new research finds. The slowdown could change how rapidly the entire planet spins, as well as influence how the core evolves with time.

For the new study, published in the journal Nature Geoscience, scientists used a database of earthquakes to probe the behavior of Earth’s solid inner core over time. The inner core sits suspended like a ball bearing in the molten-metal ocean of the outer core. Because of this liquid cocoon, the “ball bearing” may not spin at the same rate as the rest of the planet. Over the years, some researchers have found that the core rotates slightly faster than the mantle and crust, a condition called “super rotation.” But studies have not returned consistent numbers, with the first study to observe differential core rotation estimating that the inner core rotates up to one degree faster per year than the rest of the planet; others found an annual speedup of just tiny fractions of a degree.

These differences aren’t dramatic. The variation in rotation time between the inner core and the rest of Earth is very minor. Nor are the differences a threat to life on the surface: In contrast to the 2003 science-fiction movie The Core, there’s no need to call in a crack team of geophysicists and astronauts to drill to the center of our planet and start blowing things up. At most, the inner core rotation might influence Earth’s overall spin and contribute to fluctuations in the planet’s magnetic field. Each year the core expands by about a millimeter, as some of the molten iron in the outer core solidifies, seismic studies have shown. The solidification also drives the circulation of the outer core, which, in turn, creates the planet’s magnetic field. The rotation of the inner core could influence this solidification process in ways that are not yet fully understood, thus impacting the magnetic field, says study author Xiaodong Song, a geophysicist at Peking University in China.

The rotation might also matter for how the inner core grows over billions of years, says John Vidale, a geophysicist at the University of Southern California who was not involved in the study, but who has researched core rotation.

The catch, however, is that no one really knows how fast the inner core spins. In the new study, Song and geophysicist Yi Yang, also at Peking University, found that the…

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