Research provides clear evidence that the rotation of the Earth's inner core has been slowing down since about 2010 | Forbes Japan Official Website (Forbes Japan)

The latest research results show that the rotation of the Earth's inner core has been slowing down for 14 years, around 2010, and as a result, the length of the day may be slightly longer.

In June in the scientific journal NatureadvertisementThe new study contradicts previous research, which found that the inner core rotates faster than the Earth's surface.

nucleus

The Earth's inner core is at its deepest point, about 5,000 kilometers below the surface, and is a solid ball of iron and nickel. It is about the size of the Moon, and is surrounded by an outer core of liquid iron and nickel. The outer core generates the Earth's magnetic field (the geomagnetic field). Above the outer core is a rocky mantle, and finally the crust.

The Earth's inner core is very difficult to study because it is impossible to observe or sample directly. The easiest way to examine the core is to collect data from seismic waves generated by earthquakes. In this case, the researchers used seismic wave data from earthquakes and nuclear tests to analyze the movement of the inner core.

convincing result

A joint research team from the University of Southern California (USC) and the Chinese Academy of Sciences has discovered that the Earth’s inner core is slowing down relative to the surface. This change in the inner core’s motion occurred around 2010. The paper suggests that the slowdown is caused by disturbances in the liquid iron outer core and the gravitational pull of parts of the mantle.

This research revealed that the rotation speed of Earth's inner core had changed from slightly faster to slower than the rotation speed of Earth's mantle for the first time in about 40 years. “The inner core was slowing down for the first time in decades,” said study co-author John Beddall, professor and chair of the Department of Earth Sciences in the USC Dornsife College of Arts, Letters and Sciences.He says“Other scientific teams have recently proposed a variety of similar models, but this latest study provides the most compelling conclusion.”

An imaginary illustration depicting how Earth's inner core began to slow down around 2010, rotating more slowly than Earth's surface (USC Graphic/Edward Sotelo)

buried in noise

The slowing of the inner core is expected to affect the length of the day for the simple reason that the slower the inner core moves, the more resistance it has to the speed of Earth’s rotation. Since it takes 24 hours to make up one day, the slowing of the inner core means that the Earth takes longer to complete one rotation. But the expected change will be a fraction of a second, less than a fraction of a second. “It’s on the order of a thousandth of a second, and it’s very hard to notice because it’s almost lost in the noise of the ocean and the turbulence of the atmosphere,” Bedell said.

Seismic wave data

The study used seismic data from 121 earthquakes that occurred between 1991 and 2023 near the South Sandwich Islands, an uninhabited volcanic archipelago in the South Atlantic Ocean. They also used data from nuclear tests conducted by the United States, France and the former Soviet Union between 1971 and 1974.

“When I first saw the seismic records that showed this change, I was puzzled,” Vidal said. “But the discovery of 20 other observations showing the same pattern made it impossible to ignore this finding.”

(Forbes.com Original Text)