When Jules Verne wrote ‘A 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 about the interior of our planet.
Over the past 30 years, however, our understanding of the Earth’s inner core has expanded dramatically and has been shown to move and change over the decades.
But while it was previously thought to spin at a steadily faster rate than the planet’s surface, a new study shows it oscillating, going back and forth more than a mile every six years.
The cycle could explain the variations in the length of the days, which have been shown to fluctuate persistently over the past few decades.
USC researchers identified a six-year cycle of super- and sub-rotation in the Earth’s inner core, contradicting previously accepted models that suggested it was constantly rotating at a faster rate than the planet’s surface.
FOUR LAYERS OF PLANET EARTH
Crust: At a depth of up to 70 km, this is the outermost layer of the Earth, which covers the oceanic and terrestrial areas.
Mantle: Going down to 2,890 km with the lower mantle, this is the thickest layer on the planet and is made up of silicate rocks richer in iron and magnesium than the upper crust.
Outer core: From a depth of 2,890 to 5,150 km, this region is made of liquid iron and nickel with lighter elements.
Inner core: Going down to a depth of 6,370 km in the very center of planet Earth, this region has been thought to be made of solid iron and nickel. But this new study suggests it contains hard iron and cake.
The inner core of the Earth is a hot, dense ball of solid iron the size of Pluto, and as hot as our sun.
It is impossible to observe directly, that is, researchers must rely on indirect measures 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 movement of the nucleus.
The simulations confirmed changes in the Earth’s surface compared to its inner core, as scientists have claimed for 20 years.
However, he contradicted previous theories suggesting that the speed of rotation was consistently faster than the surface of the planet.
“The inner core is not fixed, it moves beneath our feet and it seems to go back and forth a couple of kilometers (1.25 miles) every six years,” said lead author John Vidale, a professor at the University of Southern California.
Research published in 1996 was the first to suggest that the inner core rotates faster than the rest of the planet, also known as super-rotation, about 1 degree per year.
Professor Vidale’s later findings reinforced the idea that the inner core rotates super, albeit at a slower pace.
Using data from LASA (Large Aperture Seismic Array), a U.S. Air Force facility in Montana, Professor Vidale found that the inner core rotates approximately 0.1 degrees per year.
Laboratory staff developed a new beam-forming technique to analyze the waves generated from Soviet underground nuclear bomb tests between 1971 and 1974 in the Arctic archipelago Novaia Zemlya.
The last results came when they applied the same methodology to a couple of previous atomic tests under the island of Amchitka at the tip of the Alaska archipelago: Milrow in 1969 and Cannikin in 1971.
By measuring the compression waves resulting from the nuclear explosions, they discovered that the inner core had reversed direction, subtracting at least one-tenth of a degree per year.
“Our latest observations show that the inner core spun a little slower between 1969 and 1971 and then moved in the other direction between 1971 and 1974,” said Professor Vidale.
“It simply came to our notice then.
“The coincidence of these two observations makes the oscillation the probable interpretation.”
Because the Earth’s inner core is so inaccessible, researchers had to rely on the only means available to probe the innermost Earth: 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 changes
This is the first time that the six-year oscillation has been indicated by direct seismological observation.
Using seismological data from atomic tests in previous studies, researchers have been able to identify the exact location and time of the seismic event.
“The idea that the inner core oscillates was a model that existed, but the community has been divided on whether it was viable,” Professor Vidale said.
“We set about hoping to see the same direction and speed of rotation in the previous pair of atomic tests, but instead we saw the opposite.
“We were very surprised to see that it was moving in the other direction.”
LASA closed in 1978 and the era of underground atomic testing in the United States is over, which means that researchers will have to rely on comparatively inaccurate earthquake data for future research in this area, even with advances. recent in instrumentation.
However, the study admits the speculation that the inner core oscillates depending on variations in the length of the day (about 0.2 seconds over six years) and the geomagnetic fields, both agree with the theory both in amplitude as in phase.
Vidale said the findings provide a compelling theory for many of the questions raised by the research community.
“One of the questions we’ve tried to answer is whether the inner core is moving progressively or is mostly blocked compared to everything else in the long run?
“We are trying to understand how the inner core was formed and how it moves over time; this is an important step in better understanding this process.”
The study is published in Science Advances.
THE EARTH’S LIQUID IRON NUCLEI CREATES THE MAGNETIC FIELD
It is believed that the magnetic field of our planet is generated in the depths of the Earth’s core.
No one has ever traveled to the center of the Earth, but by studying the shock waves of earthquakes, physicists have been able to find out their probable structure.
At the heart of the Earth is what was thought to be its solid inner core, two-thirds the size of the moon, and made mostly of iron. However, this new study denies it.
At 5,700 ° C, this iron is as hot as the surface of the Sun, but gravitational pressure caused by gravity prevents it from becoming liquid.
Surrounding it is the outer core, which is a layer of iron, nickel, and small amounts of other metals 1,242 miles (2,000 km) thick.
The metal here is fluid, due to 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 cold, dense matter sinks and hot matter rises.
The force ‘Coriolis’, caused by the rotation of the Earth, also causes eddies.
This flow of liquid iron generates electric currents, which in turn create magnetic fields.
The charged metals that pass through these fields create their own electric currents, and so the cycle continues.
This self-sustaining loop is known as a geodynamic loop.
The spiral caused by the Coriolis force means that the separate magnetic fields are aligned in approximately the same direction, and their combined effect accumulates to produce a vast magnetic field surrounding the planet.