25 Jan 2023 06:15 am
Two Chinese geoscientists claim the Earth’s innermost core of iron and nickel has slowed its rotation. But don’t panic: it’s probably not the first time, but the latest results are fueling debate about how the Earth’s core influences some fundamental properties of our planet.
Deep in the center of the earth, at a depth of about 5,000 kilometers, begins the so-called inner core. Unlike the liquid outer core that surrounds it, the inner core consists of iron and nickel and is solid due to the high pressure despite the temperatures prevailing there. Just like the overlying, partly liquid layers of earth, the solid innermost core also rotates around the earth’s axis – according to current knowledge, however, a little slower or faster than the earth’s mantle. Now, geoscientists analyzing earthquake data have found that Earth’s solid inner iron-nickel core, rotating almost smoothly within a molten outer shell, has slowed its spin a tiny bit.
What at first sounds like a 2003 science fiction film à la “The Core” predicting such a scenario is probably not the first time, not even rare, to happen. Although no one can examine the core directly, geophysicists can analyze the core through seismic waves, such as those caused by natural earthquakes or even nuclear weapons tests. That’s exactly what geoscientists Yi Yang and Xiaodong Song, seismologists at Peking University, did for their latest study. The study published in the journal NatureGeoscience was published, describes the investigations and results of the movement of the inner core of the earth.
The scientists now say they have found that the movement of the inner core relative to the outer shell has recently slowed, leading them to describe this as “paused,” which “with a gradual reversal [der Relativbewegung] of the inner core as part of an oscillation lasting about seven decades”. “We made surprising observations that indicate that the inner core has almost stopped rotating in the last decade and may be undergoing a reversal in its direction of rotation”, explained to Yang and Song Nature. The last reversal of this type was probably registered in the early 1970s.
Our knowledge of the interior of the earth is still very limited, despite the technical achievements of recent decades. In ancient times, people even believed that the core of the earth was possibly hollow. Then came the notion that the Earth’s core was composed of iron and a molten mixture, coupled with the theory that the core also rotated independently relative to the Earth’s mantle. It was Song who confirmed the rotation theory in 1996. Since then, geoscientists have been trying to figure out how fast or slow the innermost core rotates.
At first it was suggested that the inner core makes a full revolution every 400 years relative to the earth’s crust, driven by an electromagnetic torque and balanced by gravitational forces between the core and the earth’s mantle. However, other scientists soon theorized that it rotates much more slowly, taking 1,000 years or more for a complete revolution relative to Earth’s crust. Both the speed of this rotation and the question of whether and why it actually varies are still controversial today. The only thing that is now undisputed is that the rotational speed of the Earth’s core has actually changed over the decades. The authors of the new study also assume that an interaction between the rotation of the core and that of the Earth’s mantle needs to be elucidated.
“This multidecadal periodicity coincides with changes in several other geophysical observations, particularly daylength and magnetic field,” write Yang and Song. “These observations provide evidence for dynamic interactions between the Earth’s layers, from the deepest interior to the surface, which may be due to gravitational coupling and the exchange of angular momentum between the core and mantle and the surface.”
To test his theory, Song used the same method that he and Paul G. Richards used to infer the rotation of the inner core in the 1990s. In 1996, this pair of scientists tracked seismic wave measurements of repeated earthquakes — so-called doublets — that traversed the inner core between 1967 and 1995, roughly from the South Atlantic to Alaska. If the inner core had not moved at all relative to the earth’s crust, the shock waves would always have to follow the same path and thus remain at the same speed. However, Song and Richards recognized that seismic waves had accelerated by a fraction of a second over the relatively long period between the 1960s and 1990s.
Now, in the latest study Song and Yang conducted, this old data was compared to more recent patterns of similar seismic wave records. The new data suggest that the paths of seismic waves, which previously showed measurable changes over time, have changed little since around 2009. “Any transit time difference was gone. This globally consistent pattern suggests that the rotation of the core of the Earth’s interior has recently paused,” Yang and Song conclude.
From their observations, the two scientists also conclude that the slowing down of the inner core could lead to a rotation reversal. The rotation of the solid iron-nickel core oscillates back and forth faster or slower than the rotation of the earth’s crust in an oscillation lasting about seven decades. According to the researchers’ calculations, this process is apparently already triggered when there is a slight imbalance between the electromagnetic and gravitational forces between the core and the mantle. But that’s not all. The researchers point out that the seven-decade change coincides with other periodic changes on Earth, including detectable climate fluctuations. Changes in global average temperature and sea level rise may also be related to processes deep inside our earth in ways that are not yet fully understood.
For Yang and Song, this Earth-scale, but relatively slow for the human life cycle relatively slow, barely perceptible oscillation, which oscillates back and forth every 60 to 70 years, seems to Yang and Song to indicate “a system of resonance across different strata of the earth” – as if the whole planet were becoming internal hum with an extremely deep melody. Since it can and must be assumed that the inner core interacts dynamically with the outer layers of the earth – both through electromagnetic coupling with the outer core and through gravitational forces between the core and the mantle – the study could also help to better understand how processes inside our planet affect what is happening on its surface.
“These observations provide evidence of dynamic interactions between the Earth’s layers, from the deepest interior to the surface.”
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