The Spinning Magnet
Page 18
By the mid 1970s, plate tectonics was a foundation of geology. Wegener’s reputation was resurrected. By 1980, the Germans had named an eminent research institute after him. It focuses on the oceans and polar regions. Even Wegener’s reconstruction of the existence of the vanished supercontinent of Pangaea, with some massaging and math from Bullard, had become doctrine. Recently, using calculations from seismographic data, geophysicists have been able to reconstruct long-vanished crustal plates that were subducted into the mantle as much as 250 million years ago, like a record of a ghost Earth. Despite the wide acceptance of the theory of plate tectonics and the evidence to support it, some scientists continued to repudiate the idea. The most vociferous holdout was Sir Harold Jeffreys of Cambridge, the same distinguished theoretical geophysicist who had initially ignored Lehmann’s finding that the Earth had an inner core. Jeffreys said the mechanics of moving plates were impossible. He died in 1989 at age ninety-seven, unconvinced.
Nevertheless, the theory of plate tectonics also proved the theory of pole reversals. Relying on readings from the seafloor, geophysicists have firmly established a calendar of the Earth’s magnetic chronology going back 252 million years to the boundary between the Permian and Triassic geological periods. (Models taking the calendar back even further also exist, supplemented with the rock record.) That date coincides with the biggest extinction spasm in the Earth’s history, when about 95 percent of the species on the planet went extinct. The trigger for the mass extinction was an influx of carbon dioxide into the atmosphere from the volcanic eruptions that created the modern-day Siberian Traps formation. Over the past 252 million years, the record shows that reversals generally happen two or three times every million years, with at least two longer periods known as “superchrons” when they did not happen at all. In the past 90 million years, reversals have steadily become more frequent. Near-reversals, or excursions, happen about ten times as often. That’s when the shield wastes away to a small fraction of its usual strength, the dipole is destabilized, and the poles move erratically as far as the equator before going back home. The last excursion happened 40,000 years ago, coinciding with the extinction of Neanderthals.
Once geophysicists knew for sure that the reversals happened and when they had happened, the focus turned to trying to figure out where the field was headed.
CHAPTER 21
at the outer edge of the dynamo
Oddly, since we were in the land of the café au lait, morning coffee was hard to come by in Nantes, at least in the quantities required for alertness at a scientific conference. So Kathy Whaler and I took a detour one morning and stopped off at the bustling train station’s take-out coffee stand.
Like most at the conference, Whaler, a professor of geophysics at the University of Edinburgh, was a luminary. You couldn’t move through the crowd at the conference center without bumping into people who have done the defining modern-day work on the inner Earth. The level of expertise was so high that when I would ask someone a question, I almost invariably got redirected. “Check with so-and-so, who wrote a big paper on that and is standing right over there.” And then, when I would find that person—the one who knew the answer better than anyone else on Earth—there was always demurring and more redirecting for another fragment of evidence from someone else.
It was the culture of a meeting like this not to be definitive. Because the discipline itself is still evolving, the meeting was less about certainties than about tracking the slow progress toward certainties. That meant it was the place for presenting careful summaries of what was agreed on, exploring new findings, detailing methodologies to figure out more and more precise ways of reading what the data were telling you, comparing contradictory interpretations. It was where you wore a sackcloth to admit errors, accepted criticism, basked in praise, defended your approach, and, maybe most important, found out what others were looking at. The focus was on the challenges that remain, from the arcane to the urgent. So, for example, are there radioactive elements in the core? What is the chemical composition of the deep mantle? How can you look into the heart of the Earth’s magnetic field without interference from the thick mantle and the crust? How is the field changing over time?
Reading what’s going on within the core itself was where Whaler came in. She was at Cambridge working on her doctorate under the supervision of David Gubbins when an astounding batch of new satellite data came in. These were figures from MAGSAT, the first satellite that could read the whole magnetic vector—both direction and strength—across the whole planet. Operated by the National Aeronautics and Space Administration (NASA) and the US Geological Survey, it collected data for about half a year, ending in the late spring of 1980. (An earlier set of satellites, POGO, launched from 1965 to 1969, was the first to do a general map of the field, but only looked at its strength, not its direction.)
The MAGSAT data were superb. For the first time, researchers could look at the global structure of the field. Whaler was hooked, she told me as we walked across a bridge over the Loire with our coffees to rejoin the others. All of a sudden, you could compare these precise figures from the satellite with data from modern observatories on the ground and try to reconcile the two sets of numbers. And not only the modern numbers but the whole archive. It meant putting together data stretching back to sixteenth-century sailors’ measurements of declination and inclination with the nineteenth-century measurements captured by Gauss’s magnetic union and Sabine’s magnetic crusade. It meant combining those with the record in the rocks that Brunhes and others were beginning to read in the twentieth century and with the new findings from the seabed floor. Finally, a big picture of the field over time was emerging. It was intoxicating.
At an initial pass, Whaler, Gubbins, and others put together a 380-year record of the field and its variation over time, Gubbins explained in an article in Scientific American in 1989. Some of the findings were expected. At the surface of the Earth, the field looked like that of a bar magnet lying along the same axis as the one on which the planet spins. These are the endless looping lines that flow from the south magnetic pole out into space and then back into the Earth at the magnetic north pole. The denser the lines are, the stronger the field is.
Their reconstructed 380-year record also catalogued what’s known as the “westward drift” in the field over the past few centuries. The idea emerged in the late seventeenth century, when Halley suspected that the field was listing to the west, for which he found evidence in London’s shifting measurements of declination. Gubbins’s team tested that idea by tracking the field line on the planet where declination is 0, or where the compass points to both magnetic and geographic north. Its official name is the agonic line, after the Greek phrase for “without angle.” In 1700, for example, the line ran midway through the Atlantic Ocean, curved over the Gulf of Mexico, and ran straight through the Great Plains. By 2017, it had drifted so far west that it was on the Pacific side of South America, careening up through the middle of Minnesota. This was the longitudinal prime meridian that Halley and so many others had sought hundreds of years ago, believing it would bisect the Earth into two neat halves and solve the problem of navigation at sea. Gubbins’s model showed that the agonic line is wildly unpredictable. Back in the early seventeenth century, for example, it ran up through Africa, looped over Norway, slid down past Greenland across the top of South America and out into the Pacific before scooting up to the North Pole via Southern California.
And then there was the strength of the field. Scientists have kept a continuous record of the field’s strength since 1840, after Gauss worked out how to measure it. And while it’s possible to figure out the field’s strength from proxies going back further in time—like ancient terracottas and lavas and mariners’ measurements—that’s not considered to be as precise as direct measurements. So to geophysicists, 1840 is a key line between observed, indisputable measurements and those that are derived from other evidence. When Gubbins looked at hi
s maps going back over time, it was absolutely clear that the dipole had declined since those first measurements in 1840. Looking at terracottas magnetized two thousand years ago during the Roman era, the team could work out a sustained and remarkable weakening from that time.
But why was it weakening? Every single reading taken until then had measured the magnetic field as it manifests itself at the surface of the Earth. But the field, as Gauss showed mathematically in 1838, is generated within the Earth. Between the outer edge of the Earth’s core and the surface was nearly 3,000 kilometers of mantle and crust, potentially interfering with the magnetic signal from the core. What if the field looked different closer to its source?
The trick there was to work out how to strip away any magnetic interference from the crust and the thick mantle and to see what was happening to the field as close as possible to where it originated—at the outer edge of the dynamo. Gubbins and his team wanted to know more precisely what drove the Earth’s magnetic power, how it had evolved, and where it was headed. They were convinced that looking at the field at the core–mantle boundary would give them some clues.
By 1985, Whaler, Gubbins, his graduate student Jeremy Bloxham (now at Harvard), and others had figured it out, at the same time as a separate group working independently at the Scripps Research Institute in California. They used the mathematical methods devised by James Clerk Maxwell in the nineteenth century to project their readings from the Earth’s surface down to the bottom of the mantle, to where the mantle hugged the outer core. The Gubbins group started with the 1980 data and then reached back in time to 1777, making maps that captured both the direction of the field and its intensity at the core. The maps showed the number of field lines and where they exited and entered the core’s surface. That’s known as the magnetic flux over an area. Then they colored the outgoing flux shades of red depending on how intense it was, and the incoming flux, blue.
This picture of Earth’s core was a revelation, a bewildering hodgepodge of swirls and colors that betrayed a far more complicated field than anyone had imagined. As Gubbins explained, if the maps had been describing a simple two-pole system, the northern portion of the image ought to have been blue and the southern, red. It would have been deepest blue at the magnetic north pole, aligned with the Earth’s axis of spin, and deepest red at the magnetic south. That would reflect the fact that the lines converge at the poles, increasing intensity. In addition, the core’s magnetic equator, close to the geographic equator, would have shown up as a boundary between red and blue where no flux penetrated the surface.
Instead, the map of the core showed elements of a two-pole system but also other anatomy deep within the outer core. It was like being able for the first time to see inside the human body with a magnetic resonance imaging machine, discerning the shape of the liver and heart and lungs. For one thing, while the north was mainly blue, and the south mainly red, that was not absolute. There was red where blue was expected and blue where red was expected. Also, several blobs showed up where the flux was either greater or less than expected. Two of the low-flux patches were around the poles, the opposite of what researchers would expect. And then there were the two curious patches below the southern Atlantic Ocean and Africa. They were strong, running in the opposite direction from the way the dipole would demand: in instead of out. Not only that, but the one under Africa was moving westward at the astonishing pace of about one-third of a degree of longitude every year. As for the dipole itself, the map showed what Gubbins and his team believed to be evidence of two columns of liquid spinning in the outer core, separated from the rest of the molten metal making up the outer core. Those columns of movement seemed to be supporting the dipole. And the patch moving under Africa appeared to be what was undermining the dipole.
The elements were now in place to begin to see the kinetic workings of the molten inner Earth, to glimpse its very heart pumping. The trail of discoveries to get there had hopscotched from the development of the simplest compass by the Chinese in the centuries before the common era to Gauss’s proof in the nineteenth century that the Earth’s magnetic force was within the planet itself, from the realization in the seventeenth-century garden near London that the field was changeable to the twentieth-century seismic-wave data proving that the core was both liquid and solid. All these pieces of the magnetic puzzle so arduously assembled over so many centuries had fallen into place to produce a series of maps that showed not just what was in the core, and how that core gyrated, but also how it gyrated over time.
Since then, new data have poured in. By 1999, the Danes had launched the Ørsted satellite, named after Hans Christian Ørsted. It remains in orbit. Originally, it captured the whole field vector, but since 2006 has been recording only intensity. The Germans launched the CHAMP satellite in 2000, which orbited for ten years until it reentered the atmosphere and burned up. The SAC-C, launched by a robust international coalition including NASA, orbited from 2000 to 2013. In 2013, the European Space Agency began the Swarm mission—a trio of satellites measuring the whole field at the same time. Together, the satellite data represent an unbroken, high-grade record of the Earth’s magnetic field, as seen from above, for the better part of two decades.
By 2000, the geophysicist Andrew Jackson, another of Gubbins’s graduate students, had developed what is now a widely used, more precise computer model allowing researchers to see what was going on at the boundary between the mantle and core stretching back four hundred years. Between his model and others, critical changes in the field at that boundary have become clear. The blue reversed-flux patch Gubbins and his team discovered has kept growing and has kept moving westward. In 1984, it joined forces with a similar, smaller patch underneath Antarctica. By 1997, this formation had attached itself to the field in the northern hemisphere, meaning that a massive blob of blue now runs through the red of the southern hemisphere almost from the magnetic equator to the south pole. The non-dipole part of the magnetic field is getting stronger, a dramatic shift for a single human generation.
As for the dipole’s overall intensity, it is waning too. Since the critical year 1840, it has decayed by about 10 percent, measured from the surface of the Earth. And it’s the part of the field that is changing most slowly—because it is the biggest—while other wily structures in the field, led by a sinuous gyre, strain ever more wildly to break free.
CHAPTER 22
anomaly to the south
Christopher Finlay talked about the gyre in the heart of the Earth as if it were alive. It was eccentric, he said. It had limbs. It twisted and stretched. The gyre’s actions were coercing the Earth’s magnetic dipole to decay. To me, it sounded as if this mysterious gyre were an organism secretly sucking energy from the dipole, feeding enemy factions within the core, destabilizing the regime. Far from stable and orderly, this was anarchy. It was a breathtaking peek at a covert drama taking place within the invisible magnetic force.
If anybody could say what the gyre looked like, it was Finlay. Tall and lanky, with curly brown hair and a quick smile, he was one of a small number of people on the planet with the prowess to work it out. He grew up near Belfast, Ireland, ambling around the countryside with a compass in hand, fascinated with the heroic tales of Edmond Halley and the magnetic crusade. By the time I met him, he was a geophysicist at the National Space Institute at the Technical University of Denmark in Copenhagen, the university Hans Christian Ørsted set up in 1829. Along with institutes in Potsdam and Paris, Finlay’s was one of the three European scientific centers monitoring data from Swarm, the triplet of satellites now tracking the magnetic field from space. His boss in Copenhagen, Nils Olsen, was known as the dean of Swarm data. The bottom line: If you’re a scientist working on the Earth’s evolving magnetic field, you know the work that Finlay and Olsen are producing.
It was through Finlay that I had come to be at the meeting in Nantes. When I saw him in Copenhagen, he had told me about it, under questioning. It would, he s
aid carefully, be highly technical. Nevertheless, I had written to the organizers and received enthusiastic permission to attend as a journalist guest. Once there, Finlay was one of my guides to the goings-on, good-naturedly pointing out people to consult and explaining basic concepts. As the conference progressed, I ended up with a couple of pages of questions at the back of my notebook under the heading: Ask Chris!
In Copenhagen, I had caught him in the midst of preparing teaching materials and getting ready for another meeting. He was on the run. Every now and then, he would jump up and dash over to his computer to show me what he was talking about. He would click through PowerPoint presentations filled with notes for the classes he taught, and pull up colored maps showing the Earth’s magnetic field. As for Gubbins before him, and then Jackson, who supervised Finlay’s PhD at the University of Leeds, those maps represented an important way of describing his understanding of the field and how it is changing. Above his desk, nine printed-out maps were carefully thumbtacked to a large bulletin board. Each map save one had the same color tacks on each of the four corners. A great whiteboard dominated the wall beside it, covered with mathematical formulae and calculations in tidy blue, green, and black markings.
All this work is part of a quest to understand the dynamo inside the Earth. To do that, Finlay and his colleagues have created computerized numerical simulations to see if they can replicate the one that generates our magnetic field. The idea is to understand today’s field but also predict its future movements. It turns out that a key component in the model seems to be the gyre.