Next, I tracked down Cathy Constable, a geophysicist at Scripps Institution of Oceanography at the University of California, San Diego. An international leader in the art of using statistical techniques in geophysics, she and her co-investigators have been tireless in constructing models of the magnetic field that go back in time millions of years. She’s used them to examine what the field looked like during previous reversals and has compared those scenarios to today’s. During a break between sessions, I asked her if we are in the throes of a reversal. “Hell no!” she said. “Not in my lifetime!”
In 2006, Constable and Monika Korte from GFZ German Research Center for Geosciences in Potsdam published a paper meticulously outlining the case for an imminent reversal and doing the math to assess the likelihood. It was like reading a crisp legal brief. The assumption was that the more we know about past reversals, the better we can predict whether one is happening now. So, is a reversal overdue? The last one was 780,000 years ago and reversals have been happening about three times every million years for the past 90 million years. Therefore, some say, it’s time. Constable and Korte crunched the numbers and found that contention statistically suspect. Maybe. Maybe not. Over time, an interval of more than 780,000 years between reversals is not wholly unusual.
What about the argument that the dipole is decaying fast? Constable and Korte pointed to the fact that while, yes, it is waning, its pace while doing so is in line with what’s been going on for the past 7,000 years. Nothing unusual there. At other times during the past 7,000 years, it has decayed just as fast or even faster without prompting the poles to flip. Not only that, but it is still strong compared to the dipole’s intensity at the moments of other reversals. It’s even strong compared to the long-term average over the past 160 million years; it’s nearly twice that figure. The role of the South Atlantic Anomaly in triggering a reversal was a little tougher to analyze. Their best conclusion: There is an absence of evidence to support a looming reversal.
But the fact that so far there is no certainty on the timing of a reversal doesn’t stop geophysicists from trying to figure it out. For example, the north magnetic pole, which has always wandered, has begun moving at a full gallop to the north-northwest at the pace of about 55 kilometers a year. (The south magnetic pole, by contrast, continues to meander sedately.) An animated video of its track just since 1999 shows a breathtaking race across the High Arctic. It’s a clear indication that the field is changing rapidly, that something is afoot in the outer core.
Not only that, but a new batch of papers has used novel methods to come to different conclusions about a reversal. A French study looked at the past 75,000 years of sedimentary and volcanic data as well as ice cores from Greenland, where radioactive isotopes of beryllium and chlorine had been deposited over time. The concentration of those isotopes, created when cosmic rays battered the Earth’s upper atmosphere, is a good measure of the intensity of the Earth’s dipole. The more there are, the lower the dipole’s strength. The authors, one of whom, Carlo Laj, traveled to Pont Farin in 2002 to redo Brunhes’s measurements, found excellent matches in the records for the Laschamp near-reversal, a nod to the precision of the method. Their conclusion: The field is decaying so fast that a reversal may be irreversibly under way, but the poles themselves won’t reverse for at least five hundred years. But since the risks of a reversal lie not in the actual shift of the poles but in the strength of the magnetic shield to protect the Earth from dangerous radiation, this finding implies that the next five hundred years or more could be the ones to watch. Not much comfort.
A study by two Italian researchers took an altogether different, highly controversial approach. They used a theoretical systemics approach to examine the geomagnetic field, looking at how it interacts with other systems governing the planet. The behavior of the South Atlantic Anomaly in particular led them to conclude that today’s field is “rather special” and is approaching a critical transition. They even put a date on the point of no return: 2034, give or take three years. That’s not when the reversal will happen, but when it becomes inevitable.
At the University of Rochester geophysicist John Tarduno and others did an ingenious study examining the burned clay huts of Iron Age Bantu-speakers who lived in villages along Africa’s Limpopo River beginning in about 1000 CE. These are some of the few ancient readings from the southern hemisphere. When the rains failed, these early farmers torched their storage huts in a cleansing ceremony, heating up the magnetite-rich clays well past their Curie point. As they cooled, they took on the magnetic memory of the day. Tarduno, working with archeologists, has discovered that this part of Africa displayed a low field seven hundred years ago. The field then regrew in strength before weakening again to form part of the South Atlantic Anomaly.
The findings are significant, Tarduno argued, because the weak patch in the magnetic field, then and now, lies over the edge of an unusual formation in the mantle at the boundary of the core. It’s a place millions of years old with steep sides, where seismic waves move with unusually low velocity. Tarduno posited that this piece of mantle affects the movement of molten iron in the outer core, changing its magnetic flow. In turn, this shifts the field’s direction, producing the reversed-flux patches now visible, and saps the field of strength on the surface. Tarduno’s proposal was that rather than being triggered by random phenomena in the core—or related to Finlay’s gyre—reversals might be triggered by this oddity in the mantle, particularly if several reversed-flux patches link up. While Tarduno stopped short of saying a reversal is nigh, he stressed the dramatic decay of the dipole over the past 160 years, calling it “alarming.”
And new findings keep emerging, questioning the basic understanding of how the Earth’s field works. A fascinating paper published in 2017 examined the handles of clay jars made in the Levant near Jerusalem between 750 BCE and 150 BCE. The handles were stamped with royal Judean seals when they were still wet, which means the date of their firing can be pinpointed with a high degree of precision and therefore so can the date of the magnetic field they record. This degree of exactitude in the rock record is rare. Just before 700 BCE, the field’s intensity spiked up by about 50 percent. At that time, the field was already strong compared to today, so with the spike it became nearly twice as strong as it is now. The odd thing is that it decayed abruptly too, waning by more than 25 percent in just thirty years. That’s far more rapid change than geophysicists have believed the outer core is capable of. If it’s real, it suggests a degree of volatility previously unimagined.
For their part, French geophysicists Jean-Pierre Valet and Alexandre Fournier adjured their colleagues to keep heart. In an exhaustive review paper, they argued that the answer to understanding what happens during a reversal lies in closer examination of sedimentary rocks. They argued for better techniques in studying rocks’ magnetic memory, especially to track the field in the throes of a transition. Perhaps greater use of new magnetometers that test rock samples so small as to be nearly microscopic. Perhaps the beryllium isotope readings. “Despite many unresolved questions we are far from pessimistic and consider the quest for a proper description of polarity transitions to not be hopeless,” they wrote.
Back and forth. Back and forth. Like so much of the lengthy investigation of the Earth’s fickle magnet, the questions outstrip the techniques that could provide definitive answers.
So where to go from here? Finlay had his eyes trained on what might emerge from a whole new angle of investigation. Not squeezing more information from more ancient rocks. Not building ever more detailed analysis of what rocks are saying. Not calculating ever more realistic numerical models of the field. Instead, trying to physically reproduce the self-sustaining dynamo in the heart of the Earth. That means rather than being stuck on the surface, or at the boundary between the mantle and the core (which is as deep as the math will allow maps to be constructed), geophysicists could go even deeper, right into the mysteries of the outer core
itself. One of the promising experiments is in a lab in Maryland run by Daniel Lathrop. The whole geophysical world, Finlay confided, is holding its breath waiting to see if Lathrop’s experiment will “dynamo”—that is, make a dynamo on its own. Perhaps a plot for The Core II, starring Hilary Swank?
CHAPTER 24
the great hazardous spinning sphere of sodium
Torrential rain, thunder, lightning, and hail had pummeled the University of Maryland in College Park the evening before I was to visit Daniel Lathrop’s lab. The expansive greens of the Georgian-style campus were sodden the next morning. The air was heavy. But the leaves of the chestnut trees were open and full of promise. Lathrop entered his office at a canter, skidded to a stop, and immediately started talking fast. Tall and rangy, dressed in khakis, he seemed as kinetic and nonlinear as his subject. To wit: the machine in the core of the Earth that continually creates and destroys the planet’s magnetic field, also known as the geodynamo.
His work began with a conundrum. The Earth’s core is not permanently magnetized, he explained, plopping down in a chair, a well-used espresso machine to his right and a backpack tossed onto the floor at the other end of the room. He was lining up a trip to take his whole family to Calgary, Alberta, in the Rocky Mountains, on summer vacation, where he planned to rent a big RV and see as many of Canada’s national parks as humanly possible in two weeks. The core can’t be permanently magnetized, he continued, because its temperature is far past the Curie point. Yet the Earth has a magnetic field. So where does it come from? And how do you understand the magnetic core well enough to describe it mathematically, bring the math to life in the lab, and then make predictions about its behavior that you can apply to the real world?
The wild card is its turbulence, he told me. Math has long been able to describe how a liquid flows in an enclosed space. Sometimes it’s calm; sometimes more agitated. That agitation is called turbulence. The more turbulent something is, the harder to predict, the more nonlinear. Lathrop, ever the geophysics professor, pointed to the Earth’s atmosphere. That huge storm we had last night with the hail and lightning? That was turbulence in the fluid medium of the atmosphere. But in the air, you can see what’s going on. And if you create a model to predict next week’s weather and you’re wrong, you can adjust the model to reflect what happened. That makes the model more and more accurate over time. In the core, it’s much tougher to see the storms in the first place and therefore it’s tougher to create a good model for predictions. And it takes longer to find out whether you’re right. It could take tens to thousands of years, which doesn’t exactly fit into a scientist’s career plans, Lathrop noted wryly. Plus, the core is far, far bigger than the atmosphere and therefore has a lot more turbulence, making it even harder to forecast.
Why is the fluid in the outer core turbulent? What purpose does it serve?
He leapt up and wrote a simple equation on the board beside his desk. (“This is the only equation I’ll write for you,” he promised, chuckling.) It’s a formula for what’s known as the Reynolds number, which predicts how fluids will flow using the variables of velocity times size divided by viscosity. It shows that anything really big has a nonlinear flow. Not only is it turbulent, but it must be turbulent. It’s the way nature works. More important, it’s how physics works. For example, blood flowing through capillaries has a small Reynolds number. It flows in a relatively calm, predictable way, with little turbulence, because capillaries are small. Clouds have a high Reynolds number and therefore their flow is nonlinear, sometimes resulting in storms or hurricanes. But the flow in the Earth’s core is almost incomprehensibly nonlinear because the core is so large.
“You’ve got to expect it’s gonna have weather,” Lathrop deadpanned. In fact, the equation to get the Reynolds number for the core can be written, but not solved, although scientists don’t like to admit that, he said. What that means is that there is at present no scientific way—either theoretical or mathematical—to predict the future of the Earth’s magnetic field beyond the five years that Finlay and his colleagues can calculate for the International Geomagnetic Reference Field. It’s like the weather. Forecasters can give us a pretty good idea of what the weather will be like tomorrow and next week and even two weeks from now. But ask what the temperature will be like on New Year’s Day in a decade, and meteorologists resort to generalities.
And that brought Lathrop to his current experiment, the latest in a string in which he has tried to replicate the Earth’s dynamo. Joseph Larmor, an Irish mathematician, wrote a two-page paper in 1919 suggesting that both the Earth and the sun might have a self-sustaining fluid moving inside them. This was before Harold Jeffreys had discovered that the core was fluid and before Inge Lehmann had found the inner core. But Larmor used Michael Faraday’s experiments in the basement of the Royal Institution as his leaping-off point to create the vision of the Earth as an electrical generator. The Earth’s interior was shedding heat through convection into rotating molten metal whose atoms had unpaired spinning electrons. The convection of heat produced a system of electrical currents flowing in the liquid, which, as Faraday had shown, produced a magnetic field. Larmor’s idea was hotly contested by “anti-dynamo” researchers and mainly ignored until after the Second World War. A series of brilliant numerical models using some of the world’s first supercomputers finally produced a full-scale simulation of the geodynamo in 1995. Several times, the dynamo’s field spontaneously reversed direction. This model by geophysicists Paul Roberts of UCLA and Gary Glatzmaier, now at the University of California at Santa Cruz, showed that the outer core often tried to trigger reversals but that the inner core usually blocked them. That suggested the enigmatic inner core held the key to reversals. To Lathrop, the next step was to see if he could create a real-life dynamo in a lab.
It was set up in a neighboring building on the campus and as we trotted there, he explained how it worked. Lathrop has spent a lot of time with journalists. In fact, he was dashing into a second hour-long interview right after he finished with me. He’s perfected the art of explaining what he’s doing in the lab without agonizing about what he’s finding. To him, science is an endlessly fascinating exercise in slaking curiosity, endpoint uncertain. “I try not to have very strong personal desires about what the science shows us, because that could lead to bias,” he said. In fact, he is so nonchalant about outcomes that he’s fond of deconstructing the whole idea of scientific certainty: “All science is provisional,” he told me, shrugging.
It took him eight years to work out the details of this latest experiment: a stainless-steel sphere three meters in diameter containing a hollow inner sphere one meter across, roughly the proportion of the Earth’s inner core to its outer core. Each sphere could rotate independently and was hooked up to a motor. The outer sphere was bound with magnetic coils. The space between the two spheres was filled with 12.5 tons of sodium. Sodium is a silvery-white metal so soft you could cut it with a knife. It has one unpaired electron in its outermost filled orbital. Sodium is one of several elements that Humphry Davy discovered in the early 1800s as he experimented with his voltaic piles and the then new process of electrolysis, which uses electrical current to tear molecules apart. It is the best liquid conductor of electricity on Earth, a proxy for the molten iron and nickel in the outer core.
It is also lethally explosive, including at room temperature. Any water touching the sodium, even a drop of sweat, can cause it to react. At higher temperatures, it can combust on its own, producing sodium peroxide smoke caustic enough to burn skin and damage lungs. Sodium is used to cool nuclear reactors and its unusually high volatility has led to an extensive history of serious sodium fires in those reactors.
There was so much sodium in Lathrop’s sphere that it took his team a day and a half to get it above its melting point of 98 degrees Celsius—nearly the boiling point of water—before they could run the experiment. They started the melt on a Monday morning each month and began to sp
in the spheres by Tuesday afternoon, letting them run until the end of the day on Friday. After that, they spent three weeks crunching the data and tweaking the experiment. The spheres were enclosed in a huge metal box, centered in a cavernous laboratory space. Stairs alongside the sphere reached a platform on the top, where lab assistants had set up a computer. When the sphere was twirling, no visitors were allowed in the lab and the team members were in a safety control room a few meters away with their computer terminals. As we walked in, I asked: “Is this dangerous?” Lathrop replied: “I prefer ‘hazardous.’”
The question behind running the experiment—apart from Lathrop’s stated determination to have “no fires and no fatalities”—was whether one could make a self-sustaining dynamo as similar as possible to the Earth’s within the liquid sodium in the sphere. And then see how it behaves. How is the turbulence shaped by the rotation? How does turbulence affect the sodium’s ability to conduct electricity, if at all? Over the longer term, if the sodium “dynamos” on its own, Lathrop and his team may be able to witness a reversal within it. They may even be able to figure out how to predict what the field will do. So the team spins the spheres fast to drive turbulence in the sodium, a proxy for the spin of the Earth. At the same time, the team imposes a small magnetic field onto the spheres, like sowing seeds in a furrow, to see if the sodium will produce its own larger, self-sustaining magnetic field. So far, the flow of the sodium is able to amplify the imposed magnetic field by a factor of ten. But so far, there’s no self-sustaining dynamo and no reversal.
The Spinning Magnet Page 20
