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The 1971 Cannikin Nuclear Weapons Test, the Core of the Earth, and Life on the Planet Earth.
Last edited Fri Apr 22, 2022, 06:40 PM - Edit history (1)
I came across this interesting article in Science a few weeks ago and have meant to post reference to it here but never got around to it:
The Planet Inside.
Subtitle:
Scientists are probing the secrets of the inner core—and learning how it might have saved life on Earth
(Paul Voosen, Science March 31, 2022)
It's not a research paper; it's news, and should be open sourced. The article has some cool graphics and photographs of the set up of the Cannikan nuclear test.
Some excerpts:
arth’s magnetic field, nearly as old as the planet itself, protects life from damaging space radiation. But 565 million years ago, the field was sputtering, dropping to 10% of today’s strength, according to a recent discovery. Then, almost miraculously, over the course of just a few tens of millions of years, it regained its strength—just in time for the sudden profusion of complex multicellular life known as the Cambrian explosion.
What could have caused the rapid revival? Increasingly, scientists believe it was the birth of Earth’s inner core, a sphere of solid iron that sits within the molten outer core, where churning metal generates the planet’s magnetic field. Once the inner core was born, possibly 4 billion years after the planet itself, its treelike growth—accreting a few millimeters per year at its surface—would have turbocharged motions in the outer core, reviving the faltering magnetic field and renewing the protective shield for life. “The inner core regenerated Earth’s magnetic field at a really interesting time in evolution,” says John Tarduno, a geophysicist at the University of Rochester. “What would have happened if it didn’t form?”
Just why and how the inner core was born at that moment is one of many lingering puzzles about the Pluto-size orb 5000 kilo meters underfoot. “The inner core is a planet within a planet,” says Hrvoje Tkalčić, a seismologist at Australian National University (ANU)—with its own topography, its own spin rate, its own structure. “It’s beneath our feet and yet we still don’t understand some big questions,” Tkalčić says.
But researchers are beginning to chip away at those questions. Using the rare seismic waves from earthquakes or nuclear tests that penetrate or reflect off the inner core, seismologists have discovered it spins independently from the rest of the planet. Armed with complex computer models, theorists have predicted the structure and weird behavior of iron alloys crushed by the weight of the world. And experimentalists are close to confirming some of those predictions in the lab by re-creating the extreme temperatures and pressures of the inner core.
Arwen Deuss, a geophysicist at Utrecht University, feels a sense of anticipation that may resemble the mood in the 1960s, when researchers were observing seafloor spreading and on the cusp of discovering plate tectonics, the theory that makes sense of Earth’s surface. “We have all these observations now,” she says. It’s simply a matter of putting them all together...
...Early 20th century seismologists confirmed that view with a key bit of evidence: an earthquake shadow. When an earthquake strikes, the rupture emits primary, or pressure, waves (P waves) that ripple out in all directions. Secondary, or shear, waves (S waves) follow. For large earthquakes, seismologists were able to detect P waves on the other side of the planet, after they were bent and refracted by Earth’s interior layers. But strangely, S waves were missing. That only made sense if the iron core was liquid, because liquids lack the rigidity that allows S waves to sashay through.
It wasn’t until the early 1930s that Inge Lehmann, a pioneering Danish seismologist, noticed another breed of P waves that showed the core was not entirely liquid. These waves arrived at angles that were only possible if they had bounced off something dense. By 1936 she had deduced the existence of a solid inner core, ultimately measured to be about 2440 kilometers in diameter: the planet inside...
What could have caused the rapid revival? Increasingly, scientists believe it was the birth of Earth’s inner core, a sphere of solid iron that sits within the molten outer core, where churning metal generates the planet’s magnetic field. Once the inner core was born, possibly 4 billion years after the planet itself, its treelike growth—accreting a few millimeters per year at its surface—would have turbocharged motions in the outer core, reviving the faltering magnetic field and renewing the protective shield for life. “The inner core regenerated Earth’s magnetic field at a really interesting time in evolution,” says John Tarduno, a geophysicist at the University of Rochester. “What would have happened if it didn’t form?”
Just why and how the inner core was born at that moment is one of many lingering puzzles about the Pluto-size orb 5000 kilo meters underfoot. “The inner core is a planet within a planet,” says Hrvoje Tkalčić, a seismologist at Australian National University (ANU)—with its own topography, its own spin rate, its own structure. “It’s beneath our feet and yet we still don’t understand some big questions,” Tkalčić says.
But researchers are beginning to chip away at those questions. Using the rare seismic waves from earthquakes or nuclear tests that penetrate or reflect off the inner core, seismologists have discovered it spins independently from the rest of the planet. Armed with complex computer models, theorists have predicted the structure and weird behavior of iron alloys crushed by the weight of the world. And experimentalists are close to confirming some of those predictions in the lab by re-creating the extreme temperatures and pressures of the inner core.
Arwen Deuss, a geophysicist at Utrecht University, feels a sense of anticipation that may resemble the mood in the 1960s, when researchers were observing seafloor spreading and on the cusp of discovering plate tectonics, the theory that makes sense of Earth’s surface. “We have all these observations now,” she says. It’s simply a matter of putting them all together...
...Early 20th century seismologists confirmed that view with a key bit of evidence: an earthquake shadow. When an earthquake strikes, the rupture emits primary, or pressure, waves (P waves) that ripple out in all directions. Secondary, or shear, waves (S waves) follow. For large earthquakes, seismologists were able to detect P waves on the other side of the planet, after they were bent and refracted by Earth’s interior layers. But strangely, S waves were missing. That only made sense if the iron core was liquid, because liquids lack the rigidity that allows S waves to sashay through.
It wasn’t until the early 1930s that Inge Lehmann, a pioneering Danish seismologist, noticed another breed of P waves that showed the core was not entirely liquid. These waves arrived at angles that were only possible if they had bounced off something dense. By 1936 she had deduced the existence of a solid inner core, ultimately measured to be about 2440 kilometers in diameter: the planet inside...
I am a member of a generation that grew up with a very real fear of nuclear war destroying the world. In fact, I am old enough to recall "snow" on television screens that were alleged to have been tied to open air nuclear tests conducted by nations that were not party to the 1963 nuclear test ban treaty.
After 1963, nuclear tests moved underground and were conducted underground. (I discussed some consequences of underground nuclear testing in a long and desultory and quite possibly unreadable post in this space: 828 Underground Nuclear Tests, Plutonium Migration in Nevada, Dunning, Kruger, Strawmen, and Tunnels)
Nobody sensible favors nuclear weapons development or use, but it should be very clear by now to any adult that there are an awfully large number of people who lack sense. Nevertheless, it turns out that the history of nuclear testing has provided some valuable research tools. For example, the distribution of Cesium-137, a long lived radioisotope released in open air bomb tests has allowed for the tracking of soil erosion. (About 25.7% of the Cs-137 released in open air nuclear bomb tests in 1963 is still radioactive.)
(My generation has avoided the destruction of the planet in a nuclear war, but we've been spectacularly successful in destroying the planet by other means. Our visceral misunderstanding of nuclear issues as a whole has led to this tragedy, but that's an off topic other matter.)
In any case, it turns out that the underground nuclear tests have provided information on the planetary core:
...Combing through archival seismic records, they looked for subtle variations in the travel times of P waves for several dozen South Sandwich earthquakes over the course of decades. Their travel times through the outer core and mantle stayed constant, as expected. But with each passing year, P waves going through the inner core sped up a bit. “It was delicate, but you could see the changes,” Song says.
There was only one way he and Richards could account for this puzzling trend: The inner core was rotating faster than the rest of the planet, by about 1° per year. This superrotation was gradually realigning the seismic wave paths with a north-south axis in the inner core known to boost P wave speeds. Every 400 years, they suggested in a 1996 Nature paper, the inner core made an extra revolution inside Earth.
A few years later, John Vidale, a seismologist now at the University of Southern California, validated the result using a slightly different method. Vidale specializes in using records from the Large Aperture Seismic Array (LASA), a U.S. Air Force facility in Montana, closed in 1978, that operated more than 500 sensors in deep boreholes to detect atomic bomb tests. “It’s still the best data, better than the best arrays today,” he says. Seismic waves from nuclear tests were ideal because, unlike earthquakes, the source can be precisely located.
Vidale used the waves from two Soviet underground bomb tests detonated in 1971 and 1974 beneath Novaya Zemlya, a remote Arctic archipelago. Instead of looking for waves that passed through the inner core, as Song and Richards did, Vidale chose ones that ricocheted off it, registering its spin like the beam of a radar gun. “We could see one side of the inner core getting closer, and one side getting further away,” he says.
He found that over the 3 years between the tests, the inner core rotated 0.15° per year faster than the rest of the planet—much less than Song’s first estimate. But subsequent work by Song in 2005, using 18 pairs of South Sandwich earthquakes that repeated in the same spot over the span of decades, lined up with Vidale’s reduced estimate.
The discovery of the inner core’s superrotation shocked many geophysicists, who had assumed it spun at the same rate as the mantle...
... Analyzing more than 500 source-detector pairs with a range of paths through the core, Song and Yi found that the superrotation stopped all at once a decade ago, and since then the inner core has rotated at the same speed as the mantle. The changes “all disappear at the same time,” says Song, who presented the work at a meeting of the American Geophysical Union (AGU) late last year.
Meanwhile, Vidale was trying to push his trend further back in time using LASA data. He focused on two bomb-induced earthquakes, both set off by the U.S. government underneath the far end of Alaska’s Aleutian Islands, in 1969 and 1971. The tests were controversial; the second, Cannikin, at 5 megatons, was the largest ever U.S. underground test, and it faced opposition from environmental activists who chartered a fishing ship, christened it Greenpeace, and sailed it to the island in protest. Despite appeals to the Supreme Court, the test went as planned, creating a crater lake at the island’s surface even though the detonation was 1800 meters down.
The two tests created another, much delayed splash last year at the AGU meeting. Vidale reported that waves from the detonations revealed not superrotation, but subrotation: During the time between the two U.S. tests, the inner core rotated more slowly than the rest of the planet, by some 0.05° per year. Yet by the time of the Soviet tests, the inner core had somehow reversed course and sped up. The “observations are really amazing,” says Barbara Romanowicz, a seismologist at the University of California (UC), Berkeley...
There was only one way he and Richards could account for this puzzling trend: The inner core was rotating faster than the rest of the planet, by about 1° per year. This superrotation was gradually realigning the seismic wave paths with a north-south axis in the inner core known to boost P wave speeds. Every 400 years, they suggested in a 1996 Nature paper, the inner core made an extra revolution inside Earth.
A few years later, John Vidale, a seismologist now at the University of Southern California, validated the result using a slightly different method. Vidale specializes in using records from the Large Aperture Seismic Array (LASA), a U.S. Air Force facility in Montana, closed in 1978, that operated more than 500 sensors in deep boreholes to detect atomic bomb tests. “It’s still the best data, better than the best arrays today,” he says. Seismic waves from nuclear tests were ideal because, unlike earthquakes, the source can be precisely located.
Vidale used the waves from two Soviet underground bomb tests detonated in 1971 and 1974 beneath Novaya Zemlya, a remote Arctic archipelago. Instead of looking for waves that passed through the inner core, as Song and Richards did, Vidale chose ones that ricocheted off it, registering its spin like the beam of a radar gun. “We could see one side of the inner core getting closer, and one side getting further away,” he says.
He found that over the 3 years between the tests, the inner core rotated 0.15° per year faster than the rest of the planet—much less than Song’s first estimate. But subsequent work by Song in 2005, using 18 pairs of South Sandwich earthquakes that repeated in the same spot over the span of decades, lined up with Vidale’s reduced estimate.
The discovery of the inner core’s superrotation shocked many geophysicists, who had assumed it spun at the same rate as the mantle...
... Analyzing more than 500 source-detector pairs with a range of paths through the core, Song and Yi found that the superrotation stopped all at once a decade ago, and since then the inner core has rotated at the same speed as the mantle. The changes “all disappear at the same time,” says Song, who presented the work at a meeting of the American Geophysical Union (AGU) late last year.
Meanwhile, Vidale was trying to push his trend further back in time using LASA data. He focused on two bomb-induced earthquakes, both set off by the U.S. government underneath the far end of Alaska’s Aleutian Islands, in 1969 and 1971. The tests were controversial; the second, Cannikin, at 5 megatons, was the largest ever U.S. underground test, and it faced opposition from environmental activists who chartered a fishing ship, christened it Greenpeace, and sailed it to the island in protest. Despite appeals to the Supreme Court, the test went as planned, creating a crater lake at the island’s surface even though the detonation was 1800 meters down.
The two tests created another, much delayed splash last year at the AGU meeting. Vidale reported that waves from the detonations revealed not superrotation, but subrotation: During the time between the two U.S. tests, the inner core rotated more slowly than the rest of the planet, by some 0.05° per year. Yet by the time of the Soviet tests, the inner core had somehow reversed course and sped up. The “observations are really amazing,” says Barbara Romanowicz, a seismologist at the University of California (UC), Berkeley...
The Cannikin tests represented the birth of an organization to which our "but her emails" media repeatedly calls an "Environmental Organization," Greenpeace. Personally I object to this delusional characterization of this organization I personally regard Greenpeace as an anti-science ignorance factory that has led to vast environmental destruction, but that's another matter, not for discussion here.
The article is written by a Science Staff Journalist, Paul Voosen, who has received a journalism award from the American Geophysical Union. However a note posted to the article by Lianxing Wen of SUNY Stony Brook indicates that the contents of the article are not settled science and thus the claims in the article are subject to controversy.
It's worth a read.
Have a nice weekend.
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