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 spin 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.
So, no dynamo, no solution to the Reynolds number to describe the turbulence of the core, no ability to predict what the magnetic field will do, no way to say whether the field is in the process of reversing or, if it is, when it will happen. In fact, no precise idea of what the field looked like during past reversals and no certainty that the dynamo is operating the same way now as it has over the past billions of years it has existed.
And without predictions, Lathrop said, we can’t prepare. Maybe we don’t need to prepare, he mused. But it would be good to know for sure.
Prepare for what? Between Lathrop, Finlay, Constable, and others, there’s quite a list. The concern is what happens as the field is in the process of reversing. That’s when the field protecting the Earth dies down to perhaps only one-tenth of its normal strength. The magnetosphere, that stretchy web of invisible lines surrounding the Earth that makes our planet a galactic sanctuary from radiation, could show up in a more complicated pattern, Lathrop said. What will it look like when the dipole is beaten back so far that other magnetic poles are present? What will its protective force look like then?
While Constable scoffed at the idea that a reversal is imminently on the way, she also said she is far from sanguine. The field is demonstrably not stable. She pointed to the paleomagnetic evidence that we have been living in an unusually strong magnetic field for the past few hundred years, exactly at the same time as we have developed electromagnetic systems of technology and become dependent on them. When the field weakens and solar radiation penetrates closer to the surface of the Earth, those systems could be vulnerable to attack. That’s even without a reversal. When she looks back into the archive of the past seven thousand years, she sees fluctuations that have been large enough to have serious repercussions for society.
As for Finlay, he said he didn’t lie awake at night tossing and turning about fallout from the decaying dipole. In fact, he abhorred the alarmism that sometimes accompanied media discussions of a flip of the poles. His best analysis was that a reversal would be a leisurely process taking many hundreds or a few thousand years. Like Constable, his most immediate concern is how a weakened field will affect technology. He pointed out that the last time the field reversed, an advanced society based on electromagnetic systems wasn’t around. If the field continues to weaken, society is going to have to think about how to modify technology to protect it from surges of solar radiation. When? He gave a scientist’s answer: We would be wise to start preparing as soon as possible.