On September 27, 1971, a nuclear bomb exploded on Russia’s Novaya Zemlya islands. The powerful blast sent waves rippling so deep inside Earth they ricocheted off the inner core, pinging an array of hundreds of mechanical ears some 4,000 miles away in the Montana wilderness. Three years later, that array picked up a signal when a second bomb exploded at nearly the same spot.
This pair of nuclear explosions was part of hundreds of tests detonated during the throes of Cold War fervor. Now, the records of these wiggles are making waves among geologists: They have helped scientists calculate one of the most precise estimates yet of how fast the planet’s inner core is spinning.
Surface-dwellers know that Earth spins on its axis once about every 24 hours. But the inner core is a roughly moon-size ball of iron floating within an ocean of molten metal, which means it is free to turn independently from our planet’s large-scale spin, a phenomenon known as super-rotation. And how fast it’s going has been hotly debated.
Capitalizing on the zigzagged signals from those decades-old nuclear explosions, John Vidale, a seismologist at the University of Southern California, now has the latest estimate for this rate. In a recent study published in Geophysical Research Letters, he reports that the inner core likely inches along just faster than Earth’s surface. If his rate’s right, it means that if you stood on a spot at the Equator for one year, the part of the inner core that was previously beneath you would wind up under a spot 4.8 miles away.
“It’s a careful, good piece of work,” says Paul Richards, a seismologist at Columbia University who was a coauthor on a 1996 study that first documented super-rotation of the inner core. “Something is changing down there.”
Better understanding the history and current dynamics of the iron blob nestled within our planet could yield more clues to the processes charging and stabilizing our magnetic field—a geologic force field that protects our world from various kinds of harmful radiation. We don’t yet fully understand how this magnetic dynamo works, but scientists strongly suspect it’s tied to the mysterious motions deep inside the planet. (Learn what really happens when Earth's magnetic field flips.)
“The Earth is this extreme natural lab,” says Elizabeth Day, a deep-earth seismologist at Imperial College London who was not part of the work. Thousands of miles below our feet, pressures are crushing and temperatures are searing. “We can’t easily reproduce all of those in an actual laboratory. But if we can peer into the Earth, we get a bit of insight into this really extreme set of conditions.”
The new work is just one of many attempts to figure out the core’s rate of super-rotation, but offers one of the slowest rates for super-rotation yet suggested. Still, the differences between these studies is not necessarily a bad thing, Day says.
“It doesn’t mean anyone is wrong,” she says. “It just means everyone is looking at slightly different things.”
Previous work, including the paper Richards coauthored, used various properties of earthquake waves traveling through the planet to deliver their estimates for the inner core’s super-rotation, with several sitting around a few tenths of a degree a year. Such measurements aren’t easy to make, though, and the resolution of many of these analyses were low. But unlike earthquakes, which send out juddering waves, nuclear explosions provide a clean signal to work with.
“This is like Earth just got hit with a hammer,” Day says.
The issue was extracting the data, which were encoded on nine-track tapes by the Large Aperture Seismic Array in Montana. By the 1990s, the tapes had made their way to the Albuquerque Seismological Laboratory, where Paul Earle, then a graduate student at Scripps Institute of Oceanography, was tasked with extracting the echoes of Soviet nuclear tests from the deteriorating tapes.
Earle spent two weeks in a room full of boxes laden with discs sporting cryptic labels. Many of the tapes were worn, their magnetic information lost to time. Roughly one in 10 couldn’t be read by a tape-player, says Earle, who is now a seismologist with the U.S. Geological Survey.
But the effort was worth it. Earle, Vidale, and Doug Dodge of Lawrence Livermore National Laboratory used the scattered waves from these nuclear explosions to peer into the planet’s core. By comparing the fingerprint of waves scattered back from explosions at nearly the same location in 1971 and 1974, the team could calculate how much faster the inner core turned relative to the rest of the planet. The process is similar to tracking a moving airplane using radar, Richards notes.
Their initial results, published in a 2000 Nature study, pointed to a rotation rate of 0.15 degrees a year. Vidale then shifted gears and didn’t give the inner core much thought for nearly 15 years.
That changed in December 2018, when he walked through the bustling poster hall at the American Geophysical Union’s annual conference. There, Vidale spotted the work of Jiayuan Yao, now a research fellow in geophysics at Nanyang Technological University.
Yao had combed through tens of thousands of earthquakes in search of pairs that strike at different times in precisely the same location. By comparing the seismic waves that grazed the inner core from 40 of these geologic twins, he hoped to suss out the mysteries held deep in our planet.
“That is really great data,” Vidale recalls thinking. However, Yao’s interpretation of the data didn’t point toward super-rotation, and instead suggested something else seemed to be going on.
Intrigued by this conundrum, Vidale turned back to his dataset on the nuclear explosions, but with the original analysis codes nowhere to be found, he had to start from scratch, digging even deeper into the Cold War-era ripples with an updated method.
His resulting analysis still yielded super-rotation, but it was both slower and more precise than previous estimates, pointing instead toward the newly described rate of 0.07 degree a year between 1971 and 1974.
But while other scientists praise the thoroughness of Vidale’s latest work, the debate seems far from settled.
Yao and his colleagues recently published an intriguing alternative explanation using his data from twin earthquakes. Perhaps, they posit, the inner core is actually rotating at the same speed as the rest of our planet, and the apparent difference could instead be explained by the inner core having a jagged surface that shifts over time, with mountains rising or canyons cutting into the iron orb. (Read about ‘mountains’ taller than Everest that lurk deep inside Earth.)
Vidale finds that analysis intriguing, but while he agrees that there may be more than super-rotation in the mix, he’s skeptical of Yao’s precise explanation.
One possibility, Richards argues, is that blob itself is warping over time.
“It’s like when you throw a pizza up in the air,” he says. “It’s spinning, but it’s flopping around. It’s deforming as it rotates.”
It’s also possible that the rate of inner core rotation varies over time, adds Xiaodong Song, a deep-earth seismologist at the University of Illinois who coauthored the 1996 study first documenting inner core rotation. While Vidale’s latest rate is robust, it’s limited to a single time period, so further confirmation is necessary, he says via email.
“It’s so hard to do these studies,” says Jessica Irving, a deep-earth seismologist at Princeton University. “Every scrap of data becomes valuable, and unfortunately there just aren’t very many scraps of data.” Perhaps more definitive answers may be on the horizon. Analyses are getting better, and data are accruing on seismometers around the world that are constantly listening for our planet’s every tremble.
Solving the puzzle of the inner core, Yao says, “doesn’t need to take another decade.”