[Earthquake data hint that the inner core stopped rotating faster
than the rest of the planet in 2009, but not all researchers agree.]
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HAS EARTH’S INNER CORE STOPPED ITS STRANGE SPIN?  
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Alexandra Witze
January 23, 2023
Nature [[link removed]]

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_ Earthquake data hint that the inner core stopped rotating faster
than the rest of the planet in 2009, but not all researchers agree. _

Earth’s inner core is made mostly of solid iron, and can rotate
separately from the outer parts of the planet., Johan Swanepoel/SPL

 

Thousands of kilometres beneath your feet, Earth’s interior might be
doing something very weird. Many scientists think that the inner core
spins faster than the rest of the planet — but sometime in the past
decade, according to a study, it apparently stopped doing so.

“We were quite surprised,” say Yi Yang and Xiaodong Song,
seismologists at Peking University in Beijing who reported the
findings today in _Nature Geoscience_1
[[link removed]].

The results could help to shine light on the many mysteries of the
deep Earth, including what part the inner core plays in maintaining
the planet’s magnetic field and in affecting the speed of the whole
planet’s rotation — and thus the length of a day. But they are
just the latest instalment in a long-running effort to explain the
inner core’s unusual rotation, and might not be the final word on
the matter.

“I keep thinking we’re on the verge of figuring this out,” says
John Vidale, a seismologist at the University of Southern California
in Los Angeles. “But I’m not sure.”

Mysteries of the deep

Researchers discovered the inner core in 1936, after studying how
seismic waves from earthquakes travel through the planet. Changes in
the speed of the waves revealed that the planet’s core, which is
about 7,000 kilometres wide, consists of a solid centre, made mostly
of iron, inside a shell of liquid iron and other elements. As iron
from the outer core crystallizes on the surface of the inner core, it
changes the density of the outer liquid, driving churning motions that
maintain Earth’s magnetic field.

Researchers have learnt about the inner core’s rotation by studying
earthquakes that originated in the same region, such as the Kuril
Islands (shown here), over decades. Getty

The liquid outer core essentially decouples the 2,400-kilometre-wide
inner core from the rest of the planet, so the inner core can spin at
its own pace. In 1996, Song and another researcher reported2
[[link removed]] studying
earthquakes that originated in the same region over three decades, and
whose energy was detected by the same monitoring station thousands of
kilometres away. Since the 1960s, the scientists said, the travel time
of seismic waves emanating from those earthquakes had changed,
indicating that the inner core rotates faster than the planet’s
mantle, the layer just beyond the outer core.

Later studies refined estimates of the rate of that
‘super-rotation’, to conclude that the inner core rotates faster
than the mantle by about one-tenth of a degree per year. But not
everyone agrees. Other work has suggested that super-rotation happens
mostly in distinct periods, such as in the early 2000s, rather than
being a continuous, steady phenomenon3
[[link removed]]. Some
scientists even argue that super-rotation does not exist, and that the
differences in earthquake travel times are instead caused by physical
changes on the surface of the inner core4
[[link removed]].

Last June, Vidale and Wei Wang, an Earth scientist also at the
University of Southern California, threw another spanner into the
works. Using data on seismic waves generated by US nuclear test blasts
in 1969 and 1971, they reported that between those years, Earth’s
inner core had ‘subrotated’, or rotated more slowly than the
mantle5 [[link removed]].
Only after 1971, they say, did it speed up and begin to super-rotate.

A rotational shift

Now, Yang and Song say that the inner core has halted its spin
relative to the mantle. They studied earthquakes mostly from between
1995 and 2021, and found that the inner core’s super-rotation had
stopped around 2009. They observed the change at various points around
the globe, which the researchers say confirms it is a true planet-wide
phenomenon related to core rotation, and not just a local change on
the inner core’s surface.

Related: Earth's magnetic field is acting up and geologists don't know
why. [[link removed]]

The data hint that the inner core might even be in the process of
shifting back towards subrotation. If so, something is probably
happening to the magnetic and gravitational forces that drive the
inner core’s rotation. Such changes might link the inner core to
broader geophysical phenomena such as increases or decreases in the
length of a day on Earth.

Still, many questions remain, such as how to reconcile the slow pace
of the changes that Yang and Song report with some of the faster
changes reported by others. The only way out of the morass is to wait
for more earthquakes to happen. A “long history of continuous
recording of seismic data is critical for monitoring the motion of the
heart of the planet”, say Yang and Song.

“We just have to wait,” Vidale adds.

_doi: [link removed]

References

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Yang, Y. & Song, X. _Nature Geosci_.
[link removed] (2023).

Article [[link removed]] Google Scholar
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Song, X. & Richards, P. G. _Nature_ 382, 221–224 (1996).

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Article [[link removed]] Google Scholar
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Pang, G. & Koper, K. D. _Earth Planet. Sci. Lett._ 584, 117504
(2022).

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Yao, J., Tian, D., Sun, L. & Wen, L. _J. Geophys. Res. Solid
Earth_ 124, 6720–6736 (2019).

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Article [[link removed]] Google Scholar
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Wang, W. & Vidale, J. E. _Sci. Adv._ 8, eabm9916 (2022).

*
Article [[link removed]] PubMed
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Scholar
[[link removed].]

_ALEXANDRA WITZE. I am a science journalist in Boulder, Colorado. I
report on the earth, planetary and astronomical sciences. Find more
recent thoughts at @alexwitze. My book ISLAND ON FIRE, coauthored with
my husband Jeff Kanipe, is about the extraordinary 18th-century
eruption of the Icelandic volcano Laki. Reach me at
[email protected]._

_NATURE is a weekly international journal publishing the finest
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timeliness, accessibility, elegance and surprising
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affecting science, scientists and the wider public._

_First, to serve scientists through prompt publication of significant
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_Nature's original mission statement
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published for the first time on 11 November 1869._

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