When Jules Verne wrote “Journey to the Center of the Earth” more than 150 years ago, he imagined a land of glowing crystals, turbulent seas, prehistoric animals and giant mushrooms.
But what really lies beneath our feet remains a mystery – even today we know more about Saturn’s rings than our planet’s interior.
Over the past 30 years, however, our understanding of the Earth’s inner core has expanded dramatically, and it has been shown to shift and change over decades.
But whereas it was previously thought to spin at an ever faster rate than the planet’s surface, a new study shows that it oscillates, spinning back and forth a mile every six years.
The cycle could explain variations in the length of days, which have oscillated persistently for several decades.
USC researchers have identified a six-year cycle of super-rotation and sub-rotation in Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the surface of the planet
Earth’s inner core is a solid, hot iron ball the size of Pluto — and as hot as our sun.
It is impossible to observe directly, which means researchers must rely on indirect measurements to explain the pattern, speed, and cause of its movement and changes.
The US team used seismic data from 1969 to 1974 to create a computer model of core motion.
The simulations confirmed the shifts of the Earth’s surface relative to its inner core, as scientists have claimed for 20 years.
However, this contradicted previous theories suggesting that the rate of rotation was consistently faster than the surface of the planet.
“The inner core is not fixed – it moves under our feet and seems to go back and forth a few kilometers (1.25 miles) every six years,” said lead author Professor John Vidale of the University of Southern California.
Research published in 1996 was the first to propose that the inner core rotates faster than the rest of the planet – also known as super-rotation – at around 1 degree per year.
Professor Vidale’s later findings reinforced the idea that the inner core super-rotates, albeit at a slower rate.
Using data from the Large Aperture Seismic Array (LASA), a US Air Force facility in Montana, Professor Vidale found that the inner core rotates about 0.1 degrees per year.
Laboratory staff have developed a new beamforming technique to analyze the waves generated by Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago Novaya Zemlya.
The latest results emerged when they applied the same methodology to a pair of earlier atomic tests beneath Amchitka Island at the tip of the Alaskan archipelago – Milrow in 1969 and Cannikin in 1971.
Measuring the compression waves resulting from nuclear explosions, they discovered that the inner core had an inverted direction, rotating underrotated by at least a tenth of a degree per year.
‘Our latest observations show that the inner core rotated slightly slower from 1969 to 1971 and then moved in the other direction from 1971 to 1974,’ Prof Vidale said.
“We also note that day length has increased and decreased as might be expected.
“The coincidence of these two observations makes oscillation the likely interpretation.”
Because Earth’s inner core is so inaccessible, researchers have had to rely on the only means available to probe the deepest Earth – seismic data (stock image)
Map A shows the location of LASA (triangle) and the two nuclear test pairs (stars). B and C show the distribution of predicted time lags
This is the first time the six-year oscillation has been indicated by direct seismological observation.
Using seismological data from atomic tests in previous studies, researchers were able to determine the exact location and time of the seismic event.
“The idea that the inner core oscillates was a model that existed, but the community was divided on its viability,” Prof Vidale said.
“We set out hoping to see the same direction and rate of spin in the previous atomic test pair, but instead we saw the opposite.
“We were quite surprised to find that it was moving the other way.”
LASA closed in 1978 and the era of underground atomic testing in the United States is now over, meaning that researchers will have to rely on relatively inaccurate seismic data for future research in this area, even with recent advances in instrumentation.
However, the study supports speculation that the inner core oscillates with variations in day length – plus or minus 0.2 seconds over six years – and geomagnetic fields, both of which match the theory in amplitude and phase.
Vidale said the findings provide compelling theory for many questions posed by the research community.
“One of the questions we’ve tried to answer is: Is the inner core moving gradually or is it mostly locked to everything else in the long run?
“We are trying to understand how the inner core formed and how it moves through time – this is an important step in understanding this process better.”
The study is published in Science Advances.