The nuclear tests conducted in the 1960s and 70s by the U.S. and Soviet Union were destructive to the environment and could cause a future where wars could trigger an ice age. However, they have a surprising bright side: scientists used them to study the Earth’s core.
There are not many ways to look at the core, and one of them involves nuclear weapons. Of course, there are no direct ways to see the Earth’s core, at least not without being burned to the ground or blasted through by mole people. The deepest hole we have ever dug — affectionately called “the entrance to hell” — has reached 12,263 meters (40,230 feet), which is still a long way from breaking through the Earth’s crust to the lower layers. However, we can look beneath the surface quite effectively using earthquakes in a technique known as seismic tomography.
When earthquakes occur, waves of energy are sent out in all directions. By measuring tremors at several locations on the surface, scientists can create a map of the Earth’s interior. Because rocks and liquids inside the Earth have different densities, the waves travel through them at different speeds, allowing geologists to figure out what type of material the waves travel through.
In the 1990s, scientists suggested that the Earth’s core was spinning faster than other layers of the planet. In the 2022 study, researchers from the University of Southern California (USC) used wave data from the Montana Large Aperture Seismic Array (LASA) collected during underground nuclear bomb tests conducted by the Soviet Union on the Arctic archipelago of Novaya Zemlya in 1971-74. Using the methods they developed, they found that the core was rotating slower than previously thought, about 0.1 degree per year.
The team then examined data from tests conducted by the United States near Alaska in 1969 and 1971, and found that the inner core reversed direction, rotating (i.e., slower than the surface) by at least a tenth of a degree per year.
“The idea that the inner core was oscillating was a model that existed, but the community was divided on its viability,” John E. Vidale, professor of earth sciences at the University of Southern California, said in a press release. “We came to this expecting to see the same direction and rate of rotation in an earlier pair of atomic tests, but instead we saw the opposite. We were quite surprised to find that it was moving in a different direction.”
According to scientists, the oscillation could explain the fluctuations in the length of Earth’s days, which vary by about plus or minus 0.2 seconds in a six-year cycle.
“From our findings, we can see shifts in the Earth’s surface compared to its inner core, as people have claimed for 20 years,” Vidale said. “However, our latest observations show that the inner core rotated a little slower from 1969 to 1971 and then moved in the other direction from 1971 to 1974. We also note that the duration of the day increased and decreased as predicted. The coincidence of these two observations makes the oscillation a likely interpretation.
Using nuclear tests to study the Earth’s core is an unusual but effective method. Scientists continue to look for new ways to study the core to expand our understanding of what is going on inside our planet.