What scientists didn’t expect to discover is that the length of the day swings 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 millisecond.
A millisecond may not seem like much, but it has left scientists scratching their heads.
Recently, Wei Wang and John Vidale of the University of Southern California discovered a clue as to why this might happen when they looked at old seismic data from Soviet underground nuclear tests. The data revealed strange events in the Earth’s interior – events that may explain why the length of the day changes. Their results were recently published in Science advances.
An explosive search
In the 1940s, nuclear weapons were tested above ground. Fortunately, it only took people a few years to figure out that this was a really bad idea. The radiation and repercussions of these tests spread far and wide, even traveling all the way to New York from Nevada. By the early 1960s, most countries decided to conduct their tests underground, where nuclear radiation could be contained.
In order to detect and monitor these tests, many seismic arrays have been built around the world. A seismic array is a set of seismometers that have been laid out in a pattern to give them increased sensitivity to earthquakes and underground nuclear testing.
Among them is the Large Aperture Seismic Array, or LASA, located in Montana. LASA operated from 1964 until 1978, by which time nuclear testing had gone underground.
Decades later, Wang and Vidal found some unexpected patterns in the data provided by LASA.
Surprise in the data
The data from LASA is unique – recent seismic data derived from earthquakes is nowhere close to the matching sensitivity of LASA. In fact, over the 14 years that LASA has been working, scientists have been able to measure more than just earthquakes and nuclear tests — they’ve been able to capture data about what was happening in the center of the Earth.
The Earth’s inner core is about the size of Pluto. It is a solid ball consisting mostly of iron and nickel. It is difficult, if not impossible, to observe it directly. To understand what Earth’s core is doing, scientists rely on seismic waves. These change speed and direction as they travel across the planet.
Seismic data from nuclear weapons tests is far more telling than seismic data alone. “Very powerful and very fast nuclear explosions are uniquely suited to looking for small variations in the particular waves that we observe,” Vidal explained. Big thought. “The suite of LASA seismographs we used in the 1970s has been decommissioned and will probably never be matched.”
Earthquakes and nuclear explosions emit different types of waves. One of these types is P waves. Also called elementary waves, they are very similar to sound waves, in that they compress and expand the material through which they are moving. When P waves reach the Earth’s inner core, they can propagate backscatter.
Wang and Vidal were able to measure how P waves from Soviet nuclear tests scattered from the inner core. They measured the arrival times of waves scattered from the core for events that occurred between 1969 and 1974. The expected arrival time of the waves varies depending on the speed and direction of rotation of the core. The data showed that the rotation of the inner core actually appears to have changed. Not only that – the direction of rotation, in fact Reverse.
To understand how the researchers might infer that the core’s rotation changed direction, imagine a dolphin using echolocation to detect a fish. It sends out a series of sound waves that bounce off objects in its environment. If the fish were heading toward the dolphin, the sound waves would return to the dolphin in faster succession than if the fish were moving away. In a similar fashion, P waves traveling within the Earth return faster if the core is rotating toward the observer than it is spinning away.
The inner core may be the key to getting through the day
The discovery was a complete surprise. “We expected to see a fairly steady rotation, perhaps changing slowly over decades, rather than a reversal of movement,” said Vidal. Previously, scientists believed that the Earth’s inner core rotates a little faster than the crust.
“Wobbling [we see] It’s like a pendulum, Vidal explained. “It goes in one direction for a while, then reverses, and repeats the cycle for many iterations. There is a favorite position for the inner core, which is the gravity well, and it swings around that position.”
The duration of this oscillation is about six years. It may explain why the length of the day changes so little over this time period.
In order to understand this spin reversal, the authors hypothesized a simple model that includes the gravitational interaction between the mantle and the core, as well as the topography 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, there is an exchange of angular momentum between the two. The “differential rotation angle,” as it’s called, corresponds to the change in the length of the day.
While not the only possible explanation, the researchers suggest that the trend reversal does help explain changes in time. The explanation for this scientific mystery may have been under our noses all along.
This article has been reprinted with permission from Big Think, where it was originally published.