A novel study suggests that Earth's inner core oscillates

Causing variation in the length of a day.
Loukia Papadopoulos
3d illustration of a cross-section and the structure of the earth.Christoph Burgstedt/iStock

Our planet may not function as we previously believed. 

The University of Southern California scientists have provided evidence that the Earth’s inner core oscillates, according to a press release by the institution published Friday. This finding contradicts previously accepted models that assumed the Earth consistently rotates at a faster rate than the planet’s surface.

It may also help explain the variation in the length of a day, which has been shown to oscillate persistently for the past several decades.

A 20-year-old assertion

“From our findings, we can see the Earth’s surface shifts compared to its inner core, as people have asserted for 20 years,” said John E. Vidale, co-author of the study and Dean’s Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences. “However, our latest observations show that the inner core spun slightly slower from 1969-71 and then moved the other direction from 1971-74. We also note that the length of the day grew and shrank as would be predicted.

“The coincidence of those two observations makes oscillation the likely interpretation.”

The researchers used data from the Large Aperture Seismic Array (LASA), a U.S. Air Force facility in Montana, to suggest that the inner core of our Earth rotated slower than previously predicted, approximately 0.1 degrees per year. This was not what the scientists expected to find.

“The idea the inner core oscillates was a model that was out there, but the community has been split on whether it was viable,” Vidale explained. “We went into this expecting to see the same rotation direction and rate in the earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in the other direction.”

Now, the team needs to find sufficiently precise observations to compare against their current results in order to continue future research. They hope to do that by using seismological data from atomic tests used in previous studies.

An inner core that oscillates 

However, their current data does support the speculation that Earth's inner core oscillates based on variations in the length of day — plus or minus 0.2 seconds over six years — and geomagnetic fields. 

“The inner core is not fixed — it’s moving under our feet, and it seems to go back and forth a couple of kilometers every six years,” Vidale said. “One of the questions we tried to answer is, does the inner core progressively move, or is it mostly locked compared to everything else in the long term? We're trying to understand how the inner core formed and how it moves over time — this is an important step in better understanding this process.”

As sensor technology advances, the intrepid team of researchers may soon find the tools they need to prove their theory. This next step would forever change how we look at our planet's inner core.

The results of the study were published in the journal Science Advances.

We investigate the differential rotation of Earth’s inner core relative to the mantle using pairs of precisely located nuclear explosions. We find that the inner core subrotated at least 0.1° from 1969 to 1971, in contrast to superrotation of ~0.29° from 1971 to 1974. These observations contradict models of steady inner core rotation and models that posit much faster rotation rates. The reversal of polarity, timing, and rotation rates is consistent with a model of oscillations about an equilibrium with gravitational locking of the mantle and inner core due to lateral density variations. The model, which has a 6-year period, can explain the variation in the length of day, which has oscillated fairly steadily for the past decades. Inner core oscillation would also allow interpretations of causal connections between inner core and mantle lateral variations, which are problematic if the inner core consistently superrotates.

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