When Jules Verne wrote “A Journey to the Center of the Earth” over 150 years ago, he envisioned 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 we do about the interior of our planet.
However, in the past 30 years, 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 constantly rotating faster than the planet’s surface, a new study shows it oscillates, going back and forth for more than a mile every six years.
The cycle could explain variations in the length of days, which have been shown to fluctuate continuously over the decades.
USC researchers identified a six-year cycle of super- and sub-rotation in Earth’s inner core, contradicting previously accepted models that suggested it consistently rotates faster than the planet’s surface
FOUR LAYERS OF THE PLANET EARTH
Crust: Up to a depth of 70 km, this is the outermost layer of the earth, covering both the ocean and the land.
Coat: Descending to 2,890 km with the lower mantle, this is the planet’s thickest layer and made of silicate rocks richer in iron and magnesium than the crust above it.
outer core: This region runs from a depth of 2,890 to 5,150 km and is made of liquid iron and nickel with trace elements.
inner core: Descending to a depth of 6,370 km in the center of planet Earth, this area is thought to be made of solid iron and nickel. But this new study suggests it contains both mushy and hard iron.
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 must rely on indirect measurements to explain the pattern, speed and cause of the movement and changes.
The US team used seismic data from 1969 to 1974 to create a computer model of the core’s motion.
Simulations confirmed the shifts of the Earth’s surface compared to the inner core, as scientists have been claiming for 20 years.
However, it contradicted previous theories that suggested the rotational speed was consistently faster than the planet’s surface.
“The inner core is not fixed — it moves under our feet and seems to go back and forth a few miles every six years,” said lead author Professor John Vidale of the University of Southern California.
Research published in 1996 was the first to suggest that the inner core is rotating faster than the rest of the planet — known as superrotation — at about 1 degree per year.
Subsequent findings from Prof. Vidale reinforced the idea that the inner core is superrotating, albeit at a slower rate.
Using data from the LASA (Large Aperture Seismic Array), a United States Air Force facility in Montana, Prof. Vidale found that the inner core rotates about 0.1 degrees per year.
Lab workers developed a new beam-forming technique to analyze waves generated by Russian underground atomic bomb tests from 1971 to 1974 in the Arctic archipelago of Nova Zemlya.
The latter results emerged when they applied the same methodology to a few previous nuclear tests under Amchitka Island at the tip of the Alaskan archipelago – Milrow in 1969 and Cannikin in 1971.
By measuring the compression waves that resulted from the nuclear explosions, they found that the inner core had changed direction, sinking at least a tenth of a degree a year.
“Our latest observations show that the inner core rotated slightly more slowly from 1969-71 and moved in the opposite direction from 1971-74,” said Professor Vidale.
“We also note that the length of the day grew and shrank as would be predicted.
“Since those two observations coincide, oscillation is the most likely interpretation.”
Because the Earth’s inner core is so inaccessible, researchers have had to rely on the only means available to probe Earth’s innermost core – 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 the predicted time shifts
This is the first time the six-year oscillation has been indicated by direct seismic observation.
Using seismological data from atomic tests in previous studies, the researchers were able to determine the exact location and time of the seismic event.
“The idea that the inner core oscillates was a model out there, but the community was divided on whether it was viable,” said Professor Vidale.
“We went into this expecting to see the same rotational direction and speed in the previous pair of atomic tests, but instead we saw the opposite.
“We were quite surprised to find it was going the other way.”
LASA closed in 1978, and the era of underground nuclear testing in the US is now over, meaning that future research in this area will require researchers to rely on relatively inaccurate earthquake data, 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 Earth’s magnetic fields, both of which agree with the theory in both amplitude and phase.
Vidale said the findings provide a compelling theory for many questions from the research community.
“One of the questions we tried to answer is: does the inner core move gradually or is it mostly locked in the long run compared to everything else?
“We’re trying to understand how the inner core is formed and how it moves over time — this is an important step to better understand this process.”
The study is published in Science Advances.
THE LIQUID IRON CORE OF THE EARTH CREATES THE MAGNETIC FIELD
It is believed that our planet’s magnetic field is generated deep in the Earth’s core.
No one has ever traveled to the center of the Earth, but by studying shock waves from earthquakes, physicists have been able to figure out the likely structure.
At the heart of the Earth is what was thought to be the solid inner core, two-thirds the size of the moon, and made primarily of iron. However, this new study disputes this.
At 5,700°C, this iron is as hot as the sun’s surface, but the crushing pressure caused by gravity prevents it from liquefying.
Surrounding this is the outer core, which is a 1,242-mile (2,000 km) thick layer of iron, nickel, and small amounts of other metals.
The metal is liquid here, because of the lower pressure than the inner core.
Differences in temperature, pressure and composition in the outer core cause convection currents in the molten metal as cool, dense matter sinks and warm matter rises.
The ‘Coriolis’ force, caused by the Earth’s rotation, also creates swirling eddies.
This flow of liquid iron generates electric currents, which in turn create magnetic fields.
Charged metals passing through these fields continue to create their own electrical currents, and so the cycle continues.
This self-perpetuating loop is known as the geodynamo.
The spirals caused by the Coriolis force cause the individual magnetic fields to align roughly in the same direction, and their combined effect creates one massive magnetic field engulfing the planet.