THE PLANET INSIDE
PAUL VOOSEN Science MagazineScientists are probing the secrets of the inner core—and learning how it might have saved life on Earth
Earth’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 Tkal?i?, 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,” Tkali 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.
THE ANCIENTS THOUGHT Earth’s center was hollow: the home of Hades or hellfire, or a realm of tunnels that heated ocean waters. Later, following erroneous density estimates of the Moon and Earth by Isaac Newton, Edmond Halley suggested in 1686 that Earth was a series of nested shells surrounding a spinning sphere that drove the magnetism witnessed at the surface.
Basic tenets of planet formation provided a more realistic picture. Some 4.5 billion years ago, Earth was likely born from the collisions of many asteroid like “planetesimals.” The dense iron in the planetesimals would have sunk to the core of the molten proto-Earth, while lighter silicate rocks rose like oil on water to form the mantle. At temperatures of thousands of degrees and millions of atmospheres of pressure, the core would have remained molten, even as Earth’s mantle and crust cooled and hardened.
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.
A mysterious reflection Earthquake pressure (P) and shear (S) waves refractas they pass through Earth, but the liquid outer core stymies S waves. In 1936, Inge Lehmann discovered P waves in a shadow zone associated with an entirely molten core—only possible if the waves were reflections off a solid sphere rather than refractions. Iron heart Earth’s solid inner core, buried 5000 kilometers below our feet, has remained enigmatic since its discovery nearly 100 years ago. About the size of Pluto and growing several millimeters every year, it helps power Earth’s magnetic field. It also possesses a strange interior structure that is only now coming into view with advances in seismology. Inner core At 6000°C and 3 million atmospheres of pressure, the inner core is solid iron but soft. Crust Life sits on a layer of rock that is vanishingly thin compared with the rest of the planet. Mantle Earth’s thickest layer is made of 3000 kilometers of sticky silicate rock. Outer core The molten iron outer core was born along with Earth4.5 billion years ago. Innermost inner core At the core’s center is an off-center globe with odd seismic characteristics. S waves shadow zone P wave shadow zone P wave Outer core Earthquake S wave Mantle Inner Core Lehmann P wave Magnetic driver Earth’s magnetic field, which protects life from radiation, is driven by convective motions in the molten outer core. Growth of the inner core turbocharges those motions. As iron crystallizes, it spits out light elements like oxygen or silicon, which rise toward the mantle ,dragging iron with them. Inner core Outer core Convection Helical flow Mantle Rotation Magnetic flux Erratic spinner Waves from repeating earthquakes and nuclear tests have shown that the inner core does not rotate in sync with the rest of the planet. Some researchers believe gravitational tugs from dense blobs at the bottom of the mantle could be responsible for the erratic spinning .Detector Mantle spin Inner core spin Detector Nuclear test Repeater earthquake Mantle Outer core Dense blob C.
Nearly 30 years ago, Xuedong Song and Paul Richards—both seismologists then at Columbia University—thought they could use those waves to get a handle on the spin of the inner core, which, suspended in liquid, is under no obligation to rotate in sync with the rest of the planet. 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 super rotation 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 super rotation shocked many geophysicists, who had assumed it spun at the same rate as the mantle. It also tantalized them. The rotation could offer clues to how the inner core couples to the outer core and influences the magnetic dynamo. Some thought it could even help explain why Earth’s magnetic poles wander and flip from time to time.
But almost as rapidly as this picture of the inner core’s spin emerged, it grew more complicated and more mysterious. “What we thought 10 years ago isn’t holding together,” Vidale says.
RECENTLY, SONG, now at Peking University, decided to revisit his rotation work. His postdoc, Yi Yang, compiled the world’s most extensive database of repeating earthquakes, with sources not just in the South Sandwich Islands, but also in places like Chile and Kazakhstan. Analyzing more than 500 source-detector pairs with a range of paths through the core, Song and Yi found that the super rotation 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 super rotation, but sub rotation: 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.
For Vidale, the pattern from 1969 to 1974, from slow to fast, may indicate a fundamental rhythm of the inner core. For decades, radio astronomers have tracked minute changes in Earth’s surface rotation—the length of a day—against a cosmic reference frame: the fixed position of distant cosmic beacons called quasars. Although most of the yearly jitter is due to events like hurricanes and earthquakes, a tiny-but-regular 6-year wobble in day length has emerged. “Nobody has been able to say what causes it,” says Benjamin Chao, a geodesist at Academia Sinica. “But everybody bets on the core.”
Chao says one possible explanation for the 6-year cycle is gravitational interactions between the mantle and inner core. The inner core is likely to be lumpy, with hills hundreds of meters high, and at the bottom of the mantle, seismologists have discovered two ultra dense, continent-size blobs. The tugs of the blobs on the hills could create a loose coupling between the mantle and the inner core—enough to “pull the inner core back and forth” in cycles of super rotation and sub rotation, Chao says.
Song, however, only sees a slowdown, with no sign of an oscillation. He ties his record to a longer term trend in the length of a day, which saw the planet spin progressively faster from the 1970s before settling down in the early 2000s. Song thinks gravitational tugs from the mantle might have pulled the inner core along, but with a lag.
Given that neither finding has yet been published, it’s hard to say how they fit together. “Is everybody right? Is everybody wrong?” Romanowicz asks. Either way, varying rotation seems more plausible than constant super rotation, says Miaki Ishii, a seismologist at Harvard University. “It makes more sense than what we have right now.”
The nature of the iron left behind is the subject of ongoing debate. Iron atoms at Earth’s surface—in your cast iron skillet, for example—pack themselves in cubic arrangements. But when tiny samples of iron are compressed between two diamonds to inner core–like pressures, the atoms rearrange into hexagons. The hard question is what happens when iron is simultaneously squashed and heated to thousands of degrees, says Lidunka Voadlo, a computational mineral physicist at University College London. These conditions are difficult to re-create in the lab, because carbon in the diamonds often contaminates the iron when the apparatus is heated. But in computer models, Voadlo says, “There’s no limit to the pressure and temperature you can get.”
Modeling by Vodlo and her collaborators suggests hexagonal packing is the most stable arrangement under inner core conditions. The models also find that pure iron grows soft when it sits at 98% of its melting point, as it may throughout much of the inner core. This “premeeting effect,” as it is called, could explain why S waves travel much slower than expected in the supposedly solid inner core.
The story isn’t closed for cubic iron, however. Just as water must cool below freezing before ice can nucleate, researchers have suggested iron can’t solidify directly into its hexagonal form unless it is nearly 1000 K cooler than the inner core. Atom-scale modeling published early this year by a team led by Yang Sun, a mineral physicist at Columbia, suggests a solution: Iron accreting onto the inner core could first crystallize into its cubic form before transitioning into a hexagonal end state.
Although the cubic versus hexagonal debate may seem academic, the structure may determine how the iron crystals align, how much nickel and other light elements can mix with the iron, how much heat it releases on crystallization, and even its melting point. “The fundamental properties of iron change depending on what phase you’re in,” Voadlo says.
A new wave of lab studies may help settle the question. After years of halting progress, researchers are on the verge of regularly re-creating and observing inner core conditions. One strategy is to press and heat iron in diamond anvil cells, as before—but to glimpse its structure, quickly, before it is contaminated with carbon. New, powerful x-ray light sources such as the Extremely Brilliant Source at the European Synchrotron Radiation Facility, which turned on in 2020, can take that kind of flash photo.
Another is to harness the massive lasers of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), which are typically aimed at pellets of hydrogen isotopes to spark tiny nuclear fusion reactions. In a study published earlier this year, NIF researchers instead turned some of those beams on iron, heating and pressurizing it to levels far beyond those seen in Earth’s core. Each time they examined the iron’s structure with an x-ray, it came out the same—as hexagonal iron, says Richard Kraus, an LLNL research scientist who led the study.
A third tack to re-create the inner core is through shock wave experiments. Jung-Fu Lin, an experimental mineral physicist at the University of Texas, Austin, has partnered with researchers in China who use bursts of gas to fire projectiles into iron at speeds 10 times faster than a rifle bullet, generating core like temperatures and pressures. They are already seeing hints of the pre melting effect identified by Voadlo and predicted by others. If the results hold up, they may suggest the “solid” inner core isn’t so solid after all. “It’s like a smoothie,” Lin says. “Very soft.”
If the inner core is a mystery, then the “innermost” inner core is a riddle wrapped in a mystery. Since the 1980s, seismologists have known that seismic waves run faster through the inner core along a north-south axis, perhaps because the iron crystals have a common alignment, presumably along the prevailing direction of Earth’s magnetic field. But in 2002, Ishii and Adam Dziewoski, also at Harvard, discovered that within a sphere roughly 600 kilometers across, that fast lane is tilted by 45°. Ishii says that anomaly could be a relic of an ancient, tilted magnetic field or a kernel of cubic rather than hexagonal iron. No matter what, she says, “There’s something different going on at the center of the Earth.”
Researchers are poised to turn these hints into something more rigorous. Over the past decade, a clutch of high-quality seismometers has been erected in Antarctica, allowing researchers to catch far more earthquake waves that pass through the inner core’s north-south fast lanes. Armed with the improved resolutions provided by these waves and many others globally, Utrecht’s Deuss and her graduate student Henry Brett used a supercomputing-based technique to create the first 3D view of the inner core—a bit like a CT scan in the hospital.
This work, set for publication soon, confirms the existence of the innermost core, but finds it is slightly offset from the planet’s center. It also reveals speed differences between the fast lanes seen in the inner core’s western and eastern hemispheres. That suggests the story of the fast lanes is more complicated than iron crystals aligning with the dominant magnetic field, which would have a more uniform signal. It’s still early days, similar to where imaging of the mantle was in the 1980s, but Brett says more detailed models are coming soon. “We’re going to be able to ask more interesting questions.”
ALL THIS COMPLEXITY appears to be geologically recent. Scientists once placed the inner core’s birth back near the planet’s formation. But a decade ago, researchers found, using diamond anvils at outer core conditions, that iron conducts heat at least twice as fast as previously thought. Cooling drives the growth of the inner core, so the rapid heat loss combined with the inner core’s current size meant it was unlikely to have formed more than 1 billion years ago, and more than likely came even later. “There’s no way around a relatively recent appearance of the inner core,” says Bruce Buffett, a geodynamicist at UC Berkeley.
The dynamo could have been close to dying.
Tarduno realized rocks from the time might record the dramatic magnetic field changes expected at the inner core’s birth. Until recently, the paleomagnetic data from 600 million to 1 billion years ago were sparse. So Tarduno went searching for rocks of the right age containing tiny, needle-shaped crystals of the mineral titanomagnetite, which record the magnetic field’s strength at the time of their crystallization. In a 565-million-year-old volcanic formation on the north bank of the St. Lawrence River in Quebec, his team found the crystals—and convincing evidence that the magnetic field of the time was one-tenth the present day strength, they reported in 2019. The fragility of the field at the time has since been confirmed by multiple studies.
It was probably a sign that rapid heat loss from the outer core was weakening the convective motions that generate the magnetic field, says Peter Driscoll, a geodynamicist at the Carnegie Institution for Science. “The dynamo could have been close to dying,” he says. Its death could have left Earth’s developing life—which mostly lived in the ocean as microbes and proto jellyfish—exposed to far more radiation from solar flares. In Earth’s atmosphere, where oxygen levels were rising, the increased radiation could have ionized some of this oxygen, allowing it to escape to space and depleting a valuable resource for life, Tarduno says. “The potential for loss was gaining.”
Just 30 million years later, the tide had turned in favor of life. Tarduno’s team went to quarries and roadcuts in the Wichita Mountains of Oklahoma and harvested 532-million-year-old volcanic rocks. After analyzing the field strength frozen in the tiny magnetic needles, they found that its intensity had already jumped to 70% of present values, they reported at the AGU meeting. “That kind of nails it now,” Tarduno says. He credits the growth of the inner core for the field jump, which he says is “the true signature of inner core nucleation.”
Around the same time, life experienced its own revolution: the Cambrian explosion, the rapid diversification of life that gave rise to most animal groups and eventually led to the first land animals, proto millipedes that ventured onto land some 425 million years ago.
It just may be that the clement world they found owes much to the inner iron planet we’ll never see, 5000 kilometers below.
No comments:
Post a Comment