New research reveals Earth’s inner core oscillates on a six-year cycle

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Although it is widely accepted today that the inner core of our planet is not fixed, American scientists have recently shown that, contrary to what we previously thought, its movement is not continuous. .

A pivotal study

Over the past three decades, the refinement of seismic detection and analysis techniques has greatly improved our understanding of the inner core of the Terrea solid sphere with a radius of 1,220 kilometers (about the size of the dwarf planet Pluton) mainly composed of iron and nickel.

In the context of work published in the journal Science Advancesa team of researchers from theuniversity of southern california discovered that the latter oscillated on a six-year cycle and rotated more slowly than expected, thus contradicting previously proposed models, which suggested a constant rotation and faster than that of the surface of the planet.

« As has been regularly argued for twenty years, it turns out that the Earth’s surface is moving relative to its inner core “, Explain John Vidale, lead author of the study. ” However, our observations show that the inner core rotated slightly more slowly from 1969 to 1971, before changing direction between 1971 and 1974. The fact that day length has also varied periodically over the past few decades makes the oscillation the most likely interpretation. »

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Two separate analyzes of seismic waves

To reach these conclusions, two separate analyzes were necessary. Made in 2019 and focusing on the seismic waves generated by the Soviet underground nuclear tests from 1971 to 1974 in the Arctic archipelago of Novaya Zemlyathe first had shown that the inner core rotated more slowly than expected (about 0.1 degrees per year).

The recent application of this methodology to two earlier atomic tests carried out under the island ofAmchitkaat the end of the archipelago ofAlaskarevealed that its direction of rotation had also changed.

« The idea that the inner core oscillates had been proposed before, but the community was divided regarding its validity. “, Explain Vidale. « We went into this study expecting to observe the same direction and rate of spin as in the pair of previous atomic tests, but saw precisely the opposite. This confirms the hypothesis according to which the inner core oscillates according to variations in the length of the day. [plus ou moins 0,2 seconde pendant six ans] and geomagnetic fields. »

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