When Jules Verne wrote ‘A Journey to the Center of the Earth’ more than 150 years ago, he imagined a land of glittering 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.
In the last 30 years, however, our understanding of the Earth’s inner core has expanded dramatically, and it has been shown to move and change over decades.
But while it was previously thought to be rotating at a rate consistently faster than the planet’s surface, a new study shows that it oscillates, going back and forth by more than a mile every six years.
The cycle can explain the variations in the length of days, which have oscillated persistently in recent decades.
USC researchers have identified a six-year cycle of super and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates at a faster rate than the planet’s surface.
Earth’s inner core is a hot, dense ball of solid iron the size of Pluto – and as hot as our sun.
It is impossible to observe directly, which means that researchers have to rely on indirect measurements to explain the pattern, speed, and cause of their movement and changes.
The US team used seismic data from 1969 to 1974 to create a computer model of the core’s motion.
The simulations confirmed the changes in the Earth’s surface compared to its inner core, as scientists have claimed for 20 years.
However, it contradicted previous theories suggesting that the rotation rate was consistently faster than the planet’s surface.
“The inner core is not fixed – it’s moving 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 – by approximately 1 degree per year.
Subsequent discoveries by Prof Vidale reinforced the idea that the inner core super-rotates, albeit at a slower rate.
Using data from LASA (Large Aperture Seismic Array), a US Air Force facility in Montana, Professor Vidale found that the inner core rotates approximately 0.1 degrees per year.
The lab team developed a new beamforming technique to analyze the waves generated by Soviet underground nuclear bomb tests from 1971 to 1974 in the Arctic archipelago of Novaya Zemlya.
The most recent results came when they applied the same methodology to a pair of earlier atomic tests under Amchitka Island at the tip of the Alaskan archipelago – Milrow in 1969 and Cannikin in 1971.
By measuring the compression waves resulting from the nuclear explosions, they found that the inner core had reversed direction, sub-rotating at least a tenth of a degree per year.
“Our most recent observations show that the inner core rotated a little slower from 1969-71 and then shifted in the other direction from 1971-74,” said Professor Vidale.
‘We also noticed that the length of the day waxed and waned as would be expected.
“The coincidence of these two observations makes the wobble the likely interpretation.”
Because the Earth’s inner core is so inaccessible, researchers had to rely on the only available means to probe the Earth’s interior – seismic data (stock image)
Map A shows the location of LASA (triangle) and the two pairs of nuclear tests (stars). B and C show the distribution of predicted time shifts
This marks the first time the six-year oscillation has been indicated through direct seismological observation.
Using seismological data from atomic tests in previous studies, the researchers were able to pinpoint the exact location and time of the seismic event.
“The idea that the inner core oscillates was a model that was out there, but the community was divided on whether it was viable,” said Professor Vidale.
“We went into this expecting to see the same direction and rate of rotation in the previous pair of atomic tests, but we saw the opposite.
“We were quite surprised to find that he was moving in the other direction.”
LASA closed down in 1978, and the era of US underground atomic testing is over, meaning researchers will need to rely on comparatively inaccurate earthquake data for future research in this area, even with recent advances in instrumentation.
However, the study supports speculation that the inner core oscillates based on variations in day length — plus or minus 0.2 seconds over six years — and geomagnetic fields, which match the theory in amplitude and phase.
Vidale said the findings provide a compelling theory for many questions posed by the research community.
“One of the questions we’re trying to answer is, is the inner core progressively moving or is it locked up compared to everything else in the long run?
“We are trying to understand how the inner core formed and how it moves over time – this is an important step towards better understanding this process.”
The study was published in Science Advances.