If either ingredient had been absent, the Earth would not have a magnetic field. For example, if the core were not iron and nickel or other superb conductors of electricity. If the core had cooled down enough so that it did not have to transport heat energy. (As it happens, the silicate mantle that Kornprobst loved is like a blanket, helping to keep enough heat in so that the core sheds heat relatively slowly, keeping the dynamo alive.) Today, the innermost solid part of the core is about two-thirds the size of the moon and is growing slowly. It is still at a high temperature—somewhere in the range of 5,000 degrees Celsius—but because it is under tremendous pressure, its “freezing point,” or point of solidification, is also high. The outermost core, which is under less pressure, is like a runny liquid—at about 4,200 degrees Celsius—sustaining the dynamo. In billions of years, when the outer core finally cools enough to solidify, the Earth’s magnetic field will die. It will be like Mars. Mars had an internal magnetic field until, a leading theory suggests, its core cooled enough that the dynamo, and therefore the magnetic field, sputtered out. It still has a weak magnetic field in the rocks of its crust. But without the strong internal field, solar wind ripped away Mars’s atmosphere, which may be one reason there’s no life on that planet. Likewise, our moon once had a dynamo and now has a weak magnetic field that resides in its crust. Tantalizingly, rock records on both the moon and Mars retain a chronicle of the history of their internal magnetic fields, if only we could get at them and read them the way Brunhes once read ours.
Other planets still have magnetic fields. Mercury, the tiniest rocky planet. The gas giants, Jupiter and Saturn. Both ice giants, Uranus and Neptune. So does the sun. The sun’s dynamo is driven by the extreme heat given off by nuclear fusion going on within its interior. The heat is so intense that it knocks electrons out of their orbitals and they move in electrically conducting plasmic waves, creating a magnetic field. The sun’s magnetic field and ours are constantly interacting. When the sun has episodes of high magnetic activity, our field lets more solar wind into the upper reaches of our atmosphere and we often get brighter auroras that show up at lower latitudes. When our field weakens, our magnetosphere compresses, again, allowing the voracious solar wind to penetrate closer to the Earth’s surface.
All these celestial bodies rotate. A rotating body reinforces a two-pole, or dipole, magnet, roughly akin to a bar magnet running through the center of the planet or the sun, aligned with the angle of rotation. The dipole is the default position. Sometimes the direction of the field needs to change. But because the field must run from north to south, when a field changes direction, it means the poles have to switch places too.
In the case of the sun, the poles change places every eleven years as the magnetic field is annihilated and reborn. It’s a highly volatile system. The sun feeds on the change that a reversal causes in order to keep going. Scientists know this because the sun has no rocky crust. It is naked. They can see inside it, tracking reversals in the direction of the field and the shifting position of the poles. Solar pole flips coincide with more sunspots, which are small patches of the sun’s surface that are cooler than the rest and therefore look darker. We have precise records of their patterns going back five hundred years, including a whole month’s worth drawn by Galileo in 1612.
The Earth’s rotation gives it a bias toward a two-pole magnetic field too, which we implicitly refer to every time we talk about our North and South Poles. As with the sun, our magnetic poles have switched places many times through our planet’s life. But unlike the eleven-year cycle of the sun, our planet’s poles take eons between reversals: roughly every 300,000 years in recent eras. The last time they flipped was 780,000 years ago, leading some physicists to ask whether the next reversal is overdue. Is the core plotting to shift the direction of the Earth’s magnetic field sooner rather than later?
Recent satellite findings have given them new cause to wonder. A more complex picture of the field within the core has emerged. Yes, the Earth has a dipole. Yes, the dipole is dominant. But the field has more submissive and more complex structures too. The triplet of satellites in the atmosphere now monitoring the Earth’s magnetic field—the Swarm satellites launched in 2013 by the European Space Agency—are tracking an epic battle going on inside the Earth’s core. Possibly fed by a gyre in the outer core, non-dipole magnetic fields are struggling to topple the governing dipole. Their power is reaching into the dipole, striving for insurrection. It is like the ancient battle of the titans, taking place in the underworld. Already, a chunk of the dipole field in the southern Atlantic Ocean, ranging roughly from Africa to South America, below the equator, has succumbed. There, the field is running in the opposite direction from the way it is supposed to be running. The protective capacity of the magnetic field overtop that part of the Earth has decayed dramatically enough that communication satellites circling overhead shut off as solar radiation attacks them. The field is not strong enough to repel the sun’s dangerous radiation in the atmosphere—it remains strong at the surface—in that part of the world. The field has waned in unexpected ways.
A question for Philippe Cardin and the other experts here at the conference in Nantes is whether that waning will continue and spread or whether the dipole will fend off the interlopers. The Earth’s dipole is already weakening, although slowly. When it gets weak enough—whether it’s in this go-around or later—and when the rearguard non-dipole magnetic factions in the core challenge the dipole’s dominance strongly enough, the two poles will falter, as they have hundreds of times before. The poles will be thrust from their current positions. Other poles from the non-dipole fields will gain strength. The Earth will have perhaps four or eight magnetic poles during that time of transition. Our protective shield will wither to only one-tenth of its normal strength during the time the poles are traveling. That process could take thousands of years—meaning, here on Earth, we could be exposed to more radiation for those same thousands of years.
At some point in the future, the two poles will find themselves on opposite sides of the Earth from where they are now. The default dipole field, reinforced by the rotation of the Earth and the Coriolis force, will regrow itself, snapping back into place. But the direction of the field will have changed. What we think of as our current north pole will be in the south. South will be north.
We don’t know when the reversal will happen or how long it will take to complete. We don’t know exactly what will happen to life on Earth during the process. But we are gathering hints about how this unpredictable system in the planet’s core works, and glimpses of why it does what it does.
CHAPTER 18
shocks inside the earth
The monsoon rains had been falling for two days when the earthquake hit, and the ground of the Shillong Plateau in northeastern India was saturated. By 5:15 on the afternoon of June 12, 1897, when the earth began shaking, the land was so wet that much of it melted away beneath people’s feet, a process geologists refer to as “extensive liquefaction.” Land slid. Bridges sank. Sand boils and mud volcanoes erupted. Every single building in an area about the size of Louisiana was reduced to rubble. A cleft that ran for miles cracked open in the subterranean crustal plate. More than 1,500 people died. Known as the Assam earthquake, it has been estimated at a magnitude of 8.7 and is considered one of the largest in modern history.
A dozen primitive seismographs in Europe captured the movement of the Earth as the crust ruptured, tracking the waves of shocks and aftershocks as they flowed from one side of the planet to the other, through the center. Seismology was experimental at that time and geologists were only beginning to be able to read the story the graph lines could tell. But the Irish geologist Richard Dixon Oldham happened to be in India working with the Geological Survey of India just then. All corners of society dissuaded him from investigating the huge earthquake. They had different priorities, being fixated not on death and destruction but on the Diamond Jubilee celebrations of Queen Victor
ia, just eleven days away. Nevertheless, Oldham went to the site, looked at the seismographic records, and produced a carefully written report. But he kept thinking about those waves, and by 1906, the year of Brunhes’s paper on the terracotta and the year after Einstein’s on the special theory of relativity, Oldham put out a journal article for which he continues to be remembered: the first description of the internal structure of the Earth based on measured observations.
Oldham’s revelation was to be able to look at the seismographs of the Assam earthquake and separate out two different types of waves—P waves and S waves. Then, he could see that they had traveled at different speeds. P, or primary, waves are the fast ones, moving at thirty times the velocity of sound. S, or secondary, waves are slower. Not only that, but Oldham found that some of the waves had passed straight from one side of the Earth to the other, while others had taken a detour. The only way he could make sense of what he saw in the waves was to deduce that the Earth had a core made of a different material from its surroundings, and that the different material was changing the path of the waves.
At that time, six competing theories about the structure of the inner Earth were in play among geologists, mathematicians, and physicists. Oldham’s finding torpedoed five of them. The inner Earth theorists fell into the same two eighteenth-century factions that had fought over the source of the magma that had erupted from the volcanoes of the Chaîne des Puys in France: the Vulcanists and the Neptunists. At that time the Vulcanists believed that the crust of the Earth had been formed by heat, while the Neptunists thought it was the result of Noah’s flood covering the Earth. While their passionate disagreement ostensibly centered on fire versus water, it really came down to a dispute over how old the Earth was. And that was really about when the Earth would end.
And while the Ussher calculation that the Earth had been born in 4004 BCE was losing credibility as evidence stacked up about the planet’s far older origin, the primary geological textbook for both camps was still the Bible. Geology was theology. That’s why people thought the planet’s time of death was inextricably tied to its time of birth. The Old Testament Book of Genesis was their particular guide to interpret their findings in the planet’s rock record. By the late nineteenth century, the two groups of theorists had turned their attention to the structure of the inner Earth and had become known as the solidists and the fluidists. Among the players in this pitched century-long discussion were Ampère, Davy, and the Irish-Scottish physicist William Thomson, who became Lord Kelvin; the scale of absolute temperature measurement is named after him. A disciple of Faraday, Thomson died in 1907.
Some of the fluidists were convinced that the Earth was filled with a central primitive heat that had melted everything inside, leaving only a thin crust overtop. In that model, volcanoes and earthquakes were a direct conduit to the seething cauldron below. Others in the same bloc said the crust was thick, but still enclosed a bubbling liquid that was a by-product of the formation of the Earth, an “ejectum from the solar furnace.” Still another analysis was that the planet held all three states of matter. Deep inside it was gas, surrounded by liquid and then crusted over by a solid.
And then there was the camp of the hard-boiled egg: The Earth was solid from core to crust. This theory’s most famous proponent was Thomson, who declared that the whole planet must be tougher than steel and immovable within. Otherwise, he argued, “its figure must yield to the distorting forces of the moon and sun.” In other words, any liquid within the Earth would be shaped by the violence of tides, just as the ocean was, throwing the planet out of balance throughout the course of every day. Thomson even delivered a lecture in Baltimore in 1884 on the topic, in which he twirled a raw egg and a hard-boiled one to demonstrate his theory. The raw egg wobbled a great deal; the hard-boiled spun like the Earth. It was good theater, if questionable science. A variation on Thomson’s theory was that the Earth was very nearly solid, with a thin liquid layer just under the crust.
The sixth idea was that the Earth had a thick crust, liquid interior, and solid core. This was closest to what Oldham’s interpretation of the Assam earthquake seismological data supported. The other theories soon withered. The fallout was akin to that from J. J. Thomson’s discovery in 1897 of the electron, the first subatomic particle, for which he got the Nobel Prize in 1906, leading to widespread adoption of the theory of atomism, even by previous skeptics, and Bohr’s model of the atom.
The magnitude of Oldham’s finding is difficult to overstate. Geophysics had evolved from Aristotle’s idea that the Earth was an immutable dullard in a glorious heaven, to Gilbert’s contention that the Earth had a magnetic soul, to the surprising finding in the English garden of John Welles in 1634 that the magnetic field was constantly on the move, to Brunhes’s finding that the entire direction of the field had switched at least once, to Oldham’s charting of the shape of part of the Earth’s interior. Oldham’s finding began to get at the heart of why the magnetic field was so kaleidoscopic. This new information held out the possibility that the magnetic signals that people had been measuring for hundreds of years were a proxy for the architecture and even the strategy locked within that hidden place. Seismometers could finally pierce the crust, allowing scientists to peer within the heart of the planet for the first time. The key was to understand that the speed and direction of the waves contained information about the chemical composition and state of the matter they were traveling through.
Around the time that Oldham was working up his 1906 paper, Inge Lehmann felt her first earthquake. She was in her teens, she recalled, at home in Copenhagen, when the lamp swayed and the floor began to move. She didn’t reveal whether that earthquake, whose epicenter was never discovered, sparked her lifelong love affair with seismic waves. But three decades after Oldham’s great revelation, she published one of the most important discoveries ever made about the composition of the Earth’s deep interior and therefore about how our planet came to be. The Cambridge physicist Sir Harold Jeffreys had already concluded, in 1929, that because S waves could not pass through the core, it must be completely fluid. It was the first evidence to support Halley’s idea from the late seventeenth century that the core was liquid. It was a huge breakthrough and richly symbolic: The underworld of myth and Old Testament was now laid bare. Jeffreys, writing to Lehmann, then a seismologist in Copenhagen, about the reaction of his American colleague, the Jesuit priest James Macelwane, said: “I should have thought a good Jesuit would have jumped at the discovery of hell, but he reacts all wrong.”
But then Lehmann took a closer look at the seismic shocks that traveled through the Earth and saw a slightly different picture. There was a discrepancy in the waves that could only be explained if Jeffreys’s liquid core had another core nestled within it, somehow different from what surrounded it. Famously, the name of Lehmann’s 1936 paper that explained the idea was simply “P’,” after the P waves her seismometer was reading. (P’ represents the type of P wave that passes through the mantle into the core and then into the mantle again.)
The tale of Lehmann’s discovery is another confluence of unlikely events that characterize so much of the exploration of the Earth’s electromagnetic field and interior. The only woman in the emerging international field of seismology, she was born in 1888, the child of an eminent Danish family that included artists, politicians, scientists, and a surgeon. Her father, Alfred, was a professor of psychology at the University of Copenhagen who launched the practice of experimental psychology in Denmark. He was so immersed in his work that his family only saw him when they ate together and occasionally when he took them on walks on Sundays. Lehmann’s parents sent her to one of the first coeducational schools in Denmark, run by Hanna Adler, whose sister was Niels Bohr’s mother. Bohr, who was three years older than Lehmann, occasionally taught there. Adler, one of the first women to get a university degree in physics, famously traveled around the United States gaining entrée into the best society by trading on her abi
lity to explain Maxwell’s equations, then new. It was similar to the tack Ørsted had taken to get into influential company in Europe at the beginning of the 1800s by carting around a brand-new voltaic pile.
In what turned out to be a boon for the field of geophysics, Adler not only believed in educating girls and boys together, but she also believed in treating them as equals. Each of her students, male and female, studied academic subjects as well as woodworking, soccer, and needlepoint. Lehmann, who died at age 104 in 1993, wrote later in life that Adler recognized no difference in the intellectual ability of boys and girls. Neither did the teachers she hired. Lehmann loved mathematics, and as a treat, her math teacher gave her tougher problems to solve, much to her parents’ dismay. They felt she was too weak to take on the extra work. Lehmann later wrote that she had simply been bored.
Lehmann hit up hard against a different philosophy from Adler’s when she entered Newnham College at the University of Cambridge in 1910 after a stint at the University of Copenhagen. At Cambridge, Lehmann experienced “severe restrictions” on her movements as a woman, she later wrote, “restrictions completely foreign to a girl who had moved freely amongst boys and young men at home.” And while Newnham College was established for female students, the university itself didn’t allow women to earn degrees until 1948, Cambridge being the last university in the United Kingdom to do so.
Lehmann had a breakdown in 1911, which has been put down to too much work. She returned to Copenhagen, honing her mathematical skills in an actuarial office, where she calculated risks of death for insurance policies. At the age of thirty-two, she finally got the equivalent of an advanced degree in physical sciences and mathematics from the University of Copenhagen. She remained in actuarial work for a couple of years until she happened on the geophysicist Niels Erik Nørlund, director of the Danish geodetic institution Den Danske Gradmaaling. (Nørlund was married to the sister of Niels Bohr. Denmark’s intelligentsia was small and well connected, then as now, and Bohr, the superstar physicist, was the node around which some of it rotated.) Nørlund recognized Lehmann’s mathematical genius and in 1925 asked her to become his assistant and to set up a network of seismological observatories in Denmark and Greenland. She had never seen a seismograph before and taught herself how to interpret its squiggles before finally being sent on a three-month training trip to study with European experts. Seismology became her passion.
The Spinning Magnet Page 15
