Our units of time are not as constant as we think. The length of a day depends on how long it takes the earth to rotate on its axis. But all sorts of other things affect how long it takes our planet to do that. Gravitational interactions with other planets, activity in the oceans and atmosphere, and the exchange of angular momentum between the Earth’s core and mantle all have a small effect on how long it takes the Earth to rotate.
What the scientists didn’t expect was that the length of a day would fluctuate back and forth. In 2013, scientists discovered a six-year cycle in the length of the day, increasing and decreasing by just over 0.1 milliseconds.
A fraction of a millisecond might not seem like much, but scientists were scratching their heads.
Recently, Wei Wang and John Vidale of the University of Southern California found a clue as to why this might be happening when they looked at old seismic data from underground Soviet nuclear tests. The data revealed strange things happening inside the Earth – events that could explain why the length of a day changes. Their results were recently published in scientific advances.
In the 1940s, nuclear weapons were tested above ground. Luckily, it only took a few years for people to realize that this was a really bad idea. The radiation and fallout from these tests were widespread, even reaching from Nevada to New York. By the early 1960s, most countries had decided to conduct their tests underground where nuclear radiation could be contained.
To detect and monitor these tests, several seismic arrays have been built around the globe. A seismic array is a series of seismographs arranged in a pattern to give them increased sensitivity to both earthquakes and underground nuclear tests.
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Among them was the Large Aperture Seismic Array (LASA) in Montana. LASA operated from 1964 to 1978, when underground nuclear testing had declined.
Decades later, Wang and Vidale found some unexpected patterns in the provided LASA data.
A surprise in the data
LASA’s data is unique – modern seismic data derived from earthquakes falls far short of the sensitivity of the matching LASA. In fact, in the 14 years that LASA ran, scientists were able to measure more than earthquakes and nuclear tests — they were able to collect data on what was going on at the center of the Earth.
The inner core of the earth is about the size of Pluto. It is a solid sphere composed mostly of iron and nickel. It is difficult, if not impossible, to observe directly. To understand what the Earth’s core is doing, scientists rely on seismic waves. These change speed and direction as they travel through the planet.
Seismic data from nuclear weapons tests is much more powerful than earthquake data alone. “Very powerful and very fast nuclear explosions are great for looking for small differences in the individual waves that we observe,” explained Vidale think big. “The LASA set of seismometers that we use was shut down in the 1970s and will probably never be reached.”
Earthquakes and nuclear explosions emit a variety of wave types. One of these types are P waves. Also known as primary waves, they behave in a similar way to sound waves, compressing and stretching the material they travel through. When P waves reach the Earth’s inner core, they can backscatter.
Wang and Vidale were able to measure how the P-waves from Soviet nuclear tests were backscattered from the inner core. They measured the arrival times of backscattered waves from the core for events that took place between 1969 and 1974. The expected arrival time of the waves changes depending on the speed and direction of rotation of the nucleus. The data showed that the rotation of the inner core did appear to change. Not only that – the direction of rotation actually turned around.
To understand how the researchers were able to infer that the core’s rotation had changed direction, imagine a dolphin using echolocation to spot a fish. It emits a series of sound waves that bounce off objects around it. When a fish moves toward the dolphin, the sound waves return to the dolphin faster than when the fish moves away. Similarly, P-waves propagating within the Earth return faster when the nucleus rotates toward the observer than away.
The inner core can be the key to the length of a day
The discovery was a total surprise. “We expected a fairly steady rotation, changing slowly perhaps over decades, not a reversal of motion,” Vidale said. Scientists used to believe that the inner core of the earth rotates slightly faster than the crust.
“The swing [we see] looks like a pendulum,” explained Vidale. “It goes one way for a while, then reverses and repeats the cycle for many iterations. There is a preferred position for the inner core, a gravity well, and it oscillates around that position.”
This oscillation has a duration of about six years. And it could explain why the length of the day changes slightly during this period.
To understand this rotation reversal, the authors posit a simple model that includes the gravitational interaction between the mantle and core and the topographies where the core meets the mantle and where the inner core meets the outer core. If the inner core rotates at a different speed than the mantle and crust, an exchange of angular momentum takes place between the two. The so-called “differential rotation angle” corresponds to the change in day length.
While not the only possible explanation, the researchers suggest that the reversal in direction does help explain the changes over time. It may well be that the explanation of this scientific mystery has been under our noses all along.
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