Imagine Earth’s inner core — the dense center of our
planet — as a heavy, metal ballerina. This iron-rich dancer is capable of
pirouetting at ever-changing speeds.
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That core may be on the cusp of a big shift. Seismologists
reported recently in the journal Nature Geoscience that after brief but
peculiar pauses, the inner core changes how it spins — relative to the motion
of Earth’s surface — perhaps once every few decades. And, right now, one such
reversal may be underway.
This may sound like a setup for a world-wrecking,
blockbuster movie. But fret not: Precisely nothing apocalyptic will result from
this planetary spin cycle, which may have been happening for eons. The
researchers who propose this speculative model instead aim to advance
understanding of Earth’s innermost sanctum and its relationship with the rest
of the world.
‘Planet within a planet’The inner core is like “a planet within a planet, so how it
moves is obviously very important,” said Xiaodong Song, a seismologist at
Peking University in Beijing and an author of the study.
In 1936, Danish seismologist Inge Lehmann discovered that
Earth’s liquid outer core envelops a solid metal marble — and it has bamboozled
scientists ever since.
“It’s weird that there’s a solid iron ball kind of floating
in the middle of the Earth,” said John Vidale, a seismologist at the University
of Southern California who was not involved with the study.
Scientists think the core crystallized out of a molten metal
soup at some point in Earth’s not-too-distant past, after the planet’s internal
inferno had sufficiently cooled.
The inner core cannot be directly sampled, but energetic
seismic waves emanating from potent earthquakes and Cold War-era nuclear weapon
tests have ventured through the inner core, illuminating some of its
properties. Scientists suspect this ball of mostly iron and nickel is 2,446km long
and as about as hot as the sun’s surface.
But these waves also created a conundrum. If the core was
inert, the voyages of core-diving waves coming from near-identical quakes and
nuclear explosions would never change — yet, over time, they do.
This may sound like a setup for a world-wrecking, blockbuster movie. But fret not: Precisely nothing apocalyptic will result from this planetary spin cycle, which may have been happening for eons.
One explanation: The inner core is spinning, deflecting
these waves. In the mid-1990s, Song was one of the first scientists to suggest
that the inner core may be rotating at a different speed than Earth’s surface.
Since then, seismologists have found evidence implying the inner core’s spin
can both speed up and slow down.
So, what is going on?One idea is that two titanic forces are battling for control
over the world’s heart. Earth’s magnetic field, generated by swirling iron
currents in the liquid outer core, is pulling at the inner core, causing it to
spin. That impulse is countered by the mantle, the mucilaginous layer above the
outer core and below Earth’s crust, the immense gravitational field of which
grasps the inner core and slows its spin.
By studying core-diving seismic waves recorded from the
1960s to the present day, Song and Yi Yang, another Peking University
seismologist and a co-author of the study, posit that this tremendous tug of
war causes the inner core to spin back and forth on a roughly 70-year cycle.
In the early 1970s, relative to someone standing on Earth’s
surface, the inner core was not spinning. From then, the inner core has
gradually spun faster eastward, eventually overtaking the speed of rotation of
Earth’s surface. Afterward, the inner core’s spin decelerated until its
rotation appeared to have stopped at some point between 2009 and 2011.
The inner core is now starting to gradually spin westward
relative to Earth’s surface. It will likely accelerate then decelerate once
again, reaching another apparent standstill in the 2040s and completing its
latest eastward-westward spin cycle.
This 70-year rhythm, if it exists, could have a tangible
effect on parts of Earth’s deeper viscera. But it may only be capable of
stirring up comparatively minor turbulence closer to the surface — perhaps by
causing subtle shifts in the planet’s magnetic field, or even by very slightly
tweaking the length of a day, which is known to increase and decrease by a
fraction of a millisecond every six years.
This is just one of several competing models explaining the
erratic voyages of waves that reach the core. It is also possible that Earth’s
innermost layer is wobbling about. Conversely, Earth’s ferrous nucleus may have
a metamorphosing surface, twisting any seismic waves that pierce it.
“No matter which model you like, there’s some data that
disagrees with it,” Vidale said.
Because of its inaccessibility, this abyssal realm may
forever elude explanation.
“It’s certainly possible we’ll never figure it out,” Vidale
said. But, he added, “I’m an optimist. The pieces are going to fall into place
someday.”
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