Something Strange is Occurring in Earth’s Inner Core

A new study seems to support the contentious idea that the Earth’s inner core’s rotation has decelerated. Decades of earthquake data indicate that since around 2010, the inner core has been rotating more slowly than the mantle and surface, according to research published on June 12 in Nature.

This study appears to confirm a controversial finding from last year suggesting that the inner core might have reversed its rotation relative to the mantle and surface, a shift that could occur approximately every 35 years.

The study also implies that an interference has slowed this recent reversal, says John Vidale, a geophysicist at the University of Southern California. “It’s returning more slowly than it advanced.”

The Inner Core Rotates in the Same Direction as the Mantle and Surface but at a Different Rate

The inner core still rotates in the same direction as the mantle and surface, but at a different rate. To illustrate, think of a bus and a truck traveling side by side in the same direction. If the truck slows down, the bus pulls ahead. To someone on the bus, the truck seems to move backward, though both vehicles are moving forward from a pedestrian’s perspective.

Similarly, the new study suggests that if someone on Earth’s surface could observe the inner core — like the bus driver watching the truck — it would appear to be rotating opposite to its direction a few decades ago.

The 2023 study made headlines but received mixed reactions from the scientific community.

Some researchers, such as seismologist Lianxing Wen from Stony Brook University, argued that the inner core doesn’t rotate independently and that the observed data could be due to changes in the shape of the inner core’s surface.

Others believed the rotation fluctuated over shorter intervals. An analysis from the 2023 study suggested a 20-to-30-year oscillation, while a previous study coauthored by John Vidale suggested a 6-year cycle.

For the new study, Vidale and his colleagues analyzed repeating earthquakes in the South Sandwich Islands near Antarctica from 1991 to 2023. These seismic waves traveled through the planet’s interior, including the inner core, and were recorded as waveforms by instruments in Alaska.

The Team Searched for Matching Waveforms from Different Times

The team looked for matching waveforms from different times. If the inner core rotates independently, waves from repeating quakes should traverse different parts of it, resulting in distinct waveforms. If the inner core reversed its rotation relative to the surface, as suggested in the 2023 study, there should be identical waveforms from before and after the reversal, indicating the inner core returned to a previous path.

Out of 200 waveform comparisons, the team found 25 matches. The data suggest the inner core changed its rotation direction relative to the mantle around 2008, then continued rotating in the new direction at less than half the previous speed.

Vidale suggests this slower rotation might be due to the gravitational pull of the mantle, which holds about 70% of Earth’s mass. Denser mantle areas may distort the inner core’s motion, affecting its oscillation. “The inner core’s surface is near the melting point, so it’s likely soft on the outermost part,” says Vidale.

Vidale now agrees with the 2023 study’s conclusion: the inner core’s rotation probably oscillates on a roughly 70-year cycle. However, Wen argues that changes in the inner core’s surface can fully explain the data, with rising or subsiding patches altering quake waveforms.

Geophysicist Hrvoje Tkalčić believes the truth may lie somewhere in between. Seismologists are moving toward the idea that the inner core’s rotation is distinct and fluctuates but more data is needed for a definitive understanding. Observing the inner core’s rotation over the next five to ten years could provide further insights, especially if it reenters a period of rapid rotation, as it did around 20 years ago.

Read the original article on: Science News

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