Nuclear explosions helped scientists triangulate the rate and extent of the oscillation.
Scientists have long postulated that Earth’s core doesn’t just spin—it spins fasterthan the surface does. But in new research published last week in Science Advances, a pair of experts from the University of Southern California (USC) say the core travels more slowly than the outer surface of Earth, and even changes directions about every six years. This movement pattern indicates that Earth’s core actually oscillates, turning decades of science on its head.
“The inner core is not fixed—it’s moving under our feet, and it seems to going back and forth a couple of kilometers every six years,” John Vidale, a USC earthquake researcher involved in the new work, explains in a press release.
Earth is a rocky planet, positioned right in the sweet spot where that rock is solid on the surface, including beneath all of the world’s oceans. Underneath its surface, Earth is layered with regions of molten rock, from the mantle all the way down to the core. And this swirling molten rock is essential to Earth’s functioning. At the surface, it spews from volcanoes, but also from seams that run through the oceans, providing new surface area that replenishes what is lost during continental drift. In the core, the spinning molten iron-based rock creates our magnetic field, helping to defend the surface from solar radiation.
It makes intuitive sense that Earth’s core would spin fast, because that mechanism is what creates the “electromagnet” effect that helps to enliven and protect Earth. For that reason, it’s a tough piece of conventional wisdom to push back on. To do that, Vidale and Ph.D student Wei Wang worked together on the new paper. To study Earth’s core in more detail, they focused on a dataset gathered over a six-year period in the 20th century in order to build a numerical model.
That data came from 1969-1975. In the past, this research duo has studied the same period of data in other papers, but this is the first to put forward the claim that the core has at times spun backward, and demonstrated the claim using direct seismological research. You may wonder why they’re studying data from decades ago when Earth’s core is presumably continuing to behave in some way now. It’s because the late 60s into the early 70s was a peak time for certain tests of nuclear explosions, including infamous ones on the Alaskan island of Amchitka.
Why were these tests done underground? Well, for a certain time period beforehand … they weren’t. And those above-ground tests released a huge amount of radiation and damaged the nearby environments, even when those environments were “remote” islands; Nowhere is really remote enough for scientists to safely conduct an above-ground nuclear test. That’s why testing eventually moved underground, and has been required by law to take place there since 1963.
All of that brings us back to this research. Basically, these huge, exceptional explosions taking place underground generate seismological data that ripples, literally, around and through the world. It creates a pinpoint that can be tracked for how and where it travels. And how that data is perceived depends, in a way, on the framing. It’s like that old joke: six people are touching an elephant in the dark; The person holding the tail thinks it’s a broom, the person holding the trunk thinks it’s a rope, and so on.
If you have two explosions that show a certain rotation speed, you might assume a constant rotation speed. But with three, four, or more explosions, you can contextualize the rotation speed as well as the rate of change of that speed. That’s where this new paper helps to create a dialogue. “While we cannot answer the question of what other processes are also driving changes in the [inner core] seismograms, the separation of those changes from the changes due to oscillation will aid in their elucidation,” the pair report in the paper.
In other words, knowing that rotation speed can itself be changing will help scientists isolate other factors they want to study—without being misled by overly specific data.
Source: Popular Mechanics