Scientists at the University of Southern California have discovered evidence suggesting the Earth’s inner core oscillates, defying widely held beliefs that it rotates at a higher rate than the planet’s surface. According to seismic data analysis, the inner core changed direction in the six-year period from 1969 to 1974, according to their findings, which was published today in Science Advances. The researchers claim that their concept of inner core movement also explains the change in day duration, which has been seen to vary for decades.
“We can observe the Earth‘s surface movements relative to its inner core, as people have claimed for 20 years,” said John E. Vidale, co-author of the study and Dean’s Professor of Earth Sciences at the University of Southern California’s Dornsife College of Letters, Arts, and Sciences. “However, our most recent data suggest that the inner core rotated slightly slower from 1969 to 1971 before moving in the other direction from 1971 to 1974.” We also notice that the length of the day has grown and shrunk as expected.”Because of the synchronicity of those two findings, oscillation is the most plausible explanation.”
The rate and direction of rotation are determined by atomic testing
In the last 30 years, our understanding of the inner core has vastly improved. Over decades, the inner core—a hot, compact ball of solid iron the size of Pluto—has been demonstrated to move and/or alter. It’s also impossible to witness directly, so researchers must rely on indirect data to explain the movement and changes’ pattern, pace, and reason.
The initial study, published in 1996, proposed that the planet’s inner core rotates at a rate of around 1 degree per year faster than the rest of the world, a phenomenon known as super-rotation. Vidale’s subsequent findings confirmed that the inner core super-rotates, albeit at a slower rate.
Wei Wang and Vidale used data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, to discover that the inner core spun at a slower rate than previously thought, about 0.1 degrees per year. The study used a novel beamforming technique developed by Vidale to evaluate waves created by Soviet underground nuclear bomb explosions in the Arctic archipelago Novaya Zemlya from 1971 to 1974.
Wang and Vidale discovered the new results by using the same methods to two previous atomic experiments conducted beneath Amchitka Island, near the tip of the Alaskan archipelago—Milrow in 1969 and Cannikin in 1971. They determined the inner core has reversed direction, sub-rotating at least a tenth of a degree every year, by measuring the compressional waves caused by nuclear explosions.
The well-known six-year oscillation was first detected through direct seismological observation in this newest study.”The idea that the inner core oscillates was out there,” Vidale adds, “but the community was split on whether it was realistic.” “We expected to witness the same rotation direction and rate as in the previous two atomic tests, but we got the opposite result. We were taken aback when we discovered it was travelling in the opposite direction.”
Future studies will delve deeper into the reasons for the formation of the inner core
Future study, according to Vidale and Wang, will be contingent on finding sufficiently accurate observations to compare against these findings. They were able to establish the specific position and time of the relatively simple seismic event using seismological data from earlier investigations, according to Wang. However, the Montana LASA closed in 1978, and the period of underground atomic testing in the United States is past, so even with recent developments in technology, the researchers would have to rely on rather imprecise earthquake data.
The findings back up the notion that the inner core oscillates due to variations in day length—plus or minus 0.2 seconds over six years—and geomagnetic fields, which both match the theory in amplitude and phase. According to Vidale, the data provide a persuasive theory for many of the scientific community’s questions.
“The inner core isn’t fixed—shifting it’s beneath our feet every six years, and it seems to go back and forth a couple of kilometres,” Vidale explained. “One of the problems we wanted to answer was whether the inner core moves over time or is mostly locked in comparison to everything else.We’re trying to figure out how the inner core formed and travels through time—this is a critical step toward a greater understanding of the process.”