Our planet’s core powers a magnetic field that shields us from a hostile cosmos. But how does it really work?
On the remote Kola Peninsula in northwestern Russia, amid the rusting ruins of an abandoned scientific research station, is the deepest hole in the world. Now covered and sealed with a welded metal plate, the Kola Superdeep Borehole, as it’s called, is a remnant of a largely forgotten Cold War race that aimed not at the stars, but at Earth’s interior.
A team of Soviet scientists began drilling at Kola in the spring of 1970, with the goal of penetrating as far into Earth’s crust as their technology would allow. Four years before the Russians started punching their way into the Kola crust, the United States had given up on its own deep-drilling program: Project Mohole, an attempt to bore several miles through the Pacific seafloor and retrieve a sample of the underlying mantle. Mohole fell far short of its target, reaching a depth of just 601 feet after five years of drilling under more than 11,000 feet of water.
The Soviets were more persistent. Their work at Kola continued for 24 years — the project outlived the Soviet Union itself. Before drilling ended in 1994, the team hit a layer of 2.7-billion-year-old rock, almost a billion years older than the Vishnu schist at the base of the Grand Canyon. Temperatures at the bottom of the Kola hole exceeded 300 degrees Fahrenheit; the rocks were so plastic that the hole started to close whenever the drill was withdrawn.
While the researchers at Kola bored patiently downward, their counterparts in the space race sent dozens of craft heavenward: as far as the moon, Mars and beyond. By the early 1990s, when the Kola effort began to stall, the Voyager spacecraft had already passed beyond Pluto’s orbit. And the depth of the Kola hole after 24 years of drilling? About 7.6 miles — deeper than an inverted Mount Everest and roughly halfway to the mantle, but still a minuscule distance, considering Earth’s 7,918-mile diameter. If Earth were the size of an apple, the Kola hole wouldn’t even break through the skin.
All the mines on Earth, all the tunnels, caves and chasms, all the seas, and all of life exist within or on top of the thin shell of our planet’s rocky crust, which is much thinner, comparatively, than an eggshell. Earth’s immense, deep interior — the mantle and core — has never been directly explored, and probably never will be. Everything we know about the mantle, which begins about 15 miles below the surface, and about Earth’s core, 1,800 miles beneath us, has been gleaned remotely.
While our understanding of the rest of the universe grows almost daily, knowledge of the inner workings of our own world advances far more slowly. “Going into space is just a lot easier than going down for an equivalent distance,” says David Stevenson, a geophysicist at the California Institute of Technology. “Going down from 5 kilometers to 10 is much harder than going from zero to 5.”
What scientists do know is that life on Earth’s surface is profoundly affected by what happens at inaccessible depths. Heat from Earth’s inner core, which is as hot as the surface of the sun, churns an outer core of molten iron and nickel, generating a magnetic field that deflects lethal cosmic and solar radiation away from the planet. For a glimpse of what Earth might be like without its protective magnetic shield, we have only to look at the lifeless surfaces of worlds with anemic magnetic fields, like Mars and Venus.
For a glimpse of what Earth might be like without its protective magnetic shield, we have only to look at the lifeless surface of a world like Venus
The planetary architecture that provides Earth’s sheltering field has been broadly understood for several decades now: a solid-iron inner core roughly the size of the moon, surrounded by a 1,400-mile-thick outer core of liquid iron and nickel, with 1,800 miles of solid mantle above, topped by a crust of slowly drifting tectonic plates. But when it comes to the very center of the planet, this blueprint is sorely incomplete.
“Right at this moment, there is a problem with our understanding of Earth’s core,” says Stevenson, “and it’s something that’s emerged only over the last year or two. The problem is a serious one. We do not understand how the Earth’s magnetic field has lasted for billions of years. We know that the Earth has had a magnetic field for most of its history. We don’t know how the Earth did that. … We have less of an understanding now than we thought we had a decade ago of how the Earth’s core has operated throughout history.”
A Modest Proposal
On a warm summer morning, I met with Stevenson at his Caltech office in Pasadena. He was dressed for the weather, wearing shorts, sandals and a short-sleeved shirt. We talked for a while about how the surfaces of Mars and other planets, despite being tens or hundreds of millions of miles away, are far more accessible than Earth’s core.
“Of course, the universe above the Earth is mostly transparent! So you have the wonderful opportunity to use photons to tell you about the rest of the universe,” he says. “But you can’t do that inside the Earth. So the methods we have for seeing inside the Earth, if you will, are actually quite limited.”
Eleven years ago, Stevenson published a paper in the journal Nature outlining a wild scheme to get around some of those limitations. His article, “Mission to Earth’s core — a modest proposal,” described a way to send a small probe directly to the center of the Earth. The article’s title was a nod to Jonathan Swift’s 1729 satirical essay, “A Modest Proposal,” which mocked harsh British policies in Ireland by suggesting that the Irish alleviate their poverty by selling their children as meat to the English gentry. Like Swift, Stevenson wasn’t arguing for the actual feasibility of his idea; the paper was a thought experiment, an exercise to show the literally earthshaking scale of effort that would be needed to probe deep into the planet.
The first step in Stevenson’s journey to the center of the Earth: Detonate a thermonuclear weapon to blast a crack several hundred meters deep in Earth’s surface. Next, pour 110,000 tons of molten iron into the crack. (Stevenson told me that he now thinks 110,000 tons is an underestimate. On the plus side, a nuclear explosion might not be necessary — a million tons of conventional explosives might suffice.) Molten iron, being about twice as dense as the surrounding mantle, would propagate the crack downward, all the way to the core. The crack behind the descending blob of iron would quickly seal itself under pressure from the surrounding rock, so there would be no risk of the crack spreading catastrophically and splitting the planet wide open. Carried along with the sinking iron would be a heat-resistant probe about the size of a football. Stevenson estimated that the molten iron and probe would move at a rate of about 10 mph and reach the core in a week.
The probe would record data on the temperature, pressure and composition of the rock it passed through. Since radio waves can’t penetrate solid rock, the probe would vibrate, transmitting data in a series of tiny seismic waves. An extremely sensitive seismometer on Earth’s surface would receive the signals.
It’s within the reach of current technology to build a probe capable of surviving immersion in molten iron and to collect its data, but what about the rest of the plan? Could some version of Stevenson’s idea possibly work?
“The particular scheme I proposed is probably impractical,” he tells me, mostly because of the enormous quantities of molten iron that would be needed. “But it was not physically ridiculous. Engineering it may have been ridiculous, but in terms of physical principles, I wasn’t violating any laws of physics. I was showing that in a world unrestricted by concerns about how much money you would spend, you could contemplate doing what I described.”
Proposing a realistic mission wasn’t the point of the paper, Stevenson says. He wanted to highlight the limits of what can be known by constructing theories about Earth’s interior from our perch on the planet’s surface. “I wanted to remind people that the history of planetary exploration has told us the importance of going there. Time after time, we have learned things when we arrive at a planet that we had not suspected by looking at that planet from afar. I believe very strongly in this aspect of science.
“There is a danger that we will compartmentalize our understanding of an aspect of the universe by saying to ourselves, ‘OK, we know we can’t go there, so we’re going to build this elaborate story of what’s there based on remote observations.’ And this is what we do for the Earth,” Stevenson continues. “We don’t even know whether the material immediately adjacent to the core is entirely solid or partly solid. We don’t know the character of the core-mantle boundary. There are a lot of questions that would only be answered with precision by going there.”
Seeking the Center
Lacking direct access to anything beyond a few miles beneath Earth’s surface, Stevenson and other geophysicists are forced to rely on indirect methods, at least for now. Educated guesswork — and not-so-educated guesswork — has a long history in geology. While Kepler, Galileo and others were establishing the foundations of modern astronomy in the 17th century, the study of Earth itself remained a medieval science, mired in myth and fantastic imaginings.
A map published in 1664 by Jesuit scholar Athanasius Kircher depicts a cavernous Earth riddled with chambers — some filled with air, some with water, some with fire. Hell occupied Earth’s blazing center; purgatory lay a bit farther out. Ducts flowing with flames warmed hot springs, fed volcanoes and tormented the damned. Whatever his faults as a theoretician, Kircher was no armchair scholar. He once had an assistant lower him into the active and smoking crater of Mount Vesuvius so he could take temperature measurements.
Even the best astronomers of the day stumbled when they turned their attentions Earthward. In a paper published in 1692, Edmond Halley, later famed for charting the orbit of his eponymous comet, argued that Earth was mostly hollow, consisting of three concentric shells rotating around a core. He estimated that the outermost shell — the one we live on — was 500 miles thick. (Halley based his calculations on a mistaken result by Isaac Newton regarding the relative masses of the moon and Earth, leading Halley to grossly underestimate Earth’s mass.) Atmospheres of glowing gas separated the shells, each of which had its own magnetic poles. Halley believed the inner shells might even be inhabited and lit by subterranean suns.
A detailed picture of Earth’s structure began to emerge only after the invention of the time-recording seismograph in 1875. North America’s first seismograph was installed at Lick Observatory near San Jose, Calif., in the late 19th century; it recorded the San Francisco earthquake of 1906. By the early 20th century, a global network of the instruments allowed researchers to record seismic waves that had traveled from one side of the planet to the other.
An earthquake powerful enough to be felt occurs somewhere in the world about once every 30 minutes. Each releases a variety of seismic waves. In addition to the waves that distort Earth’s surface and cause so much destruction, earthquakes spawn two other types of seismic energy that ricochet through the body of the entire planet. Primary waves, or P-waves, compress the layers of rock or liquid they pass through. They move at more than 16,000 feet per second through granite. Secondary waves, or S-waves, pull rocks apart as they undulate through the planet, creating what scientists call shear forces. Traveling at about half the speed of P-waves, they’re the second type of wave to reach seismographs, hence their name.
Secondary waves move only through solids; shear forces don’t exist in liquids (since liquids can’t be torn apart). The speeds and paths of both types of waves vary with the density and elasticity of the materials they encounter. Whenever the waves reach a boundary between regions differing in density or other properties, they are deflected from their trajectories. By analyzing these sorts of data from seismic waves, scientists can identify the rocks and metals that make up Earth’s mantle and core.
Until well into the 20th century, most scientists believed Earth had a liquid iron core. The evidence seemed clear: Seismic maps of Earth’s interior revealed an absence of S-waves at the center of the Earth, presumably because the waves hit a liquid zone through which they could not travel. Seismic studies also revealed that all earthquakes created a P-wave “shadow zone” on Earth’s surface where primary waves didn’t arrive at some seismic stations; the location of the P-wave shadow zone varied with the point of origin of the earthquake. To explain the shadow zone, scientists reasoned that Earth’s presumed liquid core deflected P-waves from their expected trajectories, so they wouldn’t be recorded at all seismographic stations.
The first hint that Earth actually had a solid iron core beneath a liquid layer came in 1929, after a magnitude-7.8 earthquake shook New Zealand. Such large temblors provide a wealth of data, and researchers around the world pored over seismograph recordings in the quake’s aftermath. But only one scientist noticed anything unusual. Inge Lehmann, a Danish seismologist, made meticulous notes on seismic activity, including the arrival time of P-waves, at various seismographic stations. (Lehmann kept her notes on cards that she stored in empty oatmeal boxes.) She found P-waves in what should have been P-wave shadow zones. If Earth’s core were completely liquid, P-waves should have been deflected away from the shadow zones. In a paper published in 1936, she argued that the anomalous P-waves must have been deflected from some denser structure within the liquid core, sending them on trajectories into the shadow zones. Lehmann concluded that Earth must have a solid inner core. It wasn’t until 1970 that instruments became sensitive enough to prove beyond a doubt that she was right. Lehmann, who published her last scientific paper when she was 98, died in 1993 at the age of 104.
Earth’s Burning, Churning Engine
With the discovery of the nature of the inner core, the basic components of Earth’s composition — and even the planet’s evolution from its molten origins — were in place. Or so it seemed until recently. New research has uncovered a flaw in our understanding of the core — specifically, about the manner in which heat energy flows from the core and through the overlying mantle. The problem raises important questions about the age of the inner core, and about how Earth generates its magnetic field, a phenomenon crucial for the existence of life.
Based on the radioactive dating of ancient rocks, scientists estimate that Earth formed about 4.5 billion years ago. As the molten proto-Earth cooled, its outermost layer hardened into a thin crust. Earth’s mantle also solidified over time, though even now the temperature at the lower mantle is about 4,000 F.
The inner core, once entirely liquid, is slowly solidifying from the inside out, increasing its diameter by about half a millimeter per year, according to some estimates. Iron’s melting point is greater at higher pressure, and as the planet cooled, the extreme pressures at the very center of Earth eventually prevented the iron there from continuing to exist as a liquid. Despite sunlike temperatures, the inner core began to solidify, and it’s been growing ever since. Under slightly less pressure, the outer core — a 1,400-mile-deep, 8,000-degree ocean of iron and nickel — is still hot enough to be fluid. “It would flow through your hands like water,” says Bruce Buffett, a geophysicist at the University of California, Berkeley.
All of Earth’s layers, from core to crust, are in constant motion, caused by the flow of heat. Heat moves through Earth’s interior in two fundamentally different ways: convection and conduction. Convection occurs when heat from below creates motion in the layers above — heated material rises, then falls back again as it cools, only to be heated once more. Convection is what roils a pot of boiling soup. Deep inside Earth, slow-motion convection of rocky minerals in the mantle and heat loss from the cooling solid inner core cause convection in the liquid outer core.
Heat also makes its way through the Earth by conduction — the transfer of thermal energy by molecules inside a material from hotter areas to colder ones — without causing any motion. To continue the soup example, heat is conducted through the bottom of the metal pot. The metal in the pot doesn’t move; it simply transmits, or conducts, heat to the pot’s contents. The same is true inside the Earth: In addition to convection currents moving heated material through the outer core and mantle, heat is conducted through liquids and solids without roiling them.
Researchers have known for many decades that the slow, convective sloshing of liquid iron in the outer core, aided by Earth’s rotation, generates the planet’s magnetic field. As the molten iron flows, it creates electric currents, which generate local magnetic fields. Those fields in turn give rise to more electric currents, an effect that results in a self-sustaining cycle called a geodynamo. Evidence from ancient rocks reveals that Earth’s geodynamo has been up and running for at least 3.5 billion years. (When rocks form, their magnetic minerals line up with Earth’s field, and that orientation is preserved when the rocks solidify, providing geophysicists with a record, written in stone, of the planet’s magnetic past.)
But here is the fundamental problem with our understanding of the geodynamo: It can’t work in the way geophysicists have long believed. Two years ago, a team of scientists from two British universities discovered that liquid iron, at the temperatures and pressures found in the outer core, conducts far more heat into the mantle than anyone had thought possible. “Earlier estimates were much too low,” says Dario Alfè, a geophysicist at University College London, who participated in the new research. “The conductivity is two or three times higher than what people used to think.”
The discovery is vexing: If liquid iron conducts heat into the mantle at such a high rate, there wouldn’t be enough heat left in the outer core to churn its ocean of liquid iron. In other words, there would be no heat-driven convection in the outer core. If a pot of soup conducted heat into the surrounding air this effectively, convection would never start, and the soup would never boil. “This is a big problem,” Alfè says, “because convection is what drives the geodynamo. We would not have a geodynamo without convection.”
Alfè and his colleagues used supercomputers to carry out a “first principles” calculation of heat flow in liquid iron at Earth’s core. By first principles, they mean that they solved a set of complex equations that govern the atomic states of iron. They weren’t estimating or extrapolating from lab experiments — they were applying the laws of fundamental quantum mechanics to derive iron’s properties at extreme pressures and temperatures. The British researchers spent several years developing the mathematical techniques used in the equations; only in recent years have computers become powerful enough to solve them.
“It was exciting and scary because we found values that were very different from what people have used,” Alfè says about the discovery. “The first thing you think is, ‘I don’t want to be wrong with this.’ ”
No Impacts, No Magnetic Field, No Life?
The work has gained wide acceptance since its publication in Nature two years ago, especially since their first principles calculations now have some experimental backing. A team of Japanese researchers recently found that small samples of iron, when subjected to high pressures in the laboratory, displayed the same heat-transfer properties that Alfè and his colleagues predicted. Stevenson, the Caltech geophysicist, says the new values for liquid iron’s conductivity will probably stand the test of time. “It’s possible that the numbers might come down a little bit, but I’d be surprised to see them come all the way down to the conventional value,” he says.
So how can the new findings be reconciled with the undeniable existence of the planet’s magnetic field? Stevenson and other researchers have previously proposed a second mechanism besides heat flow that could produce the required convection in the outer core. The inner core, although composed almost entirely of pure iron, is thought to contain traces of lighter elements, primarily oxygen and silicon. As the iron in the inner core cools and solidifies, the researchers hypothesize, some of those light elements would be squeezed out, like the salt extruded from ice crystals when seawater freezes. Those light elements would then rise into the liquid outer core, creating convection currents. This so-called compositional convection would be another way to power the geodynamo.
Much of the impact energy of primordial collisions would have been converted into heat, liquefying Earth’s interior
But compositional convection would work only once an inner core had already formed. In a purely liquid core, the light elements would be evenly distributed throughout the liquid, so there would be no compositional convection. Based on how fast Earth’s core is cooling and solidifying now, it’s likely that the inner core formed relatively recently, perhaps within the past billion years.
How did the geodynamo manage to function for at least a couple of billion years before the inner core existed? “The problem is actually in Earth’s past,” not in the present, says Alfè. “This is where new hypotheses are coming in. Some people are saying maybe the Earth was a lot hotter in the past.”
If the young Earth contained more heat than current theories account for, there might have been enough left over to power the requisite convection, even given the new findings about liquid iron’s higher conductivity. What could have provided the extra heat? One of the leading explanations would have beggared the imaginations of even the most inventive medieval mapmakers: Primordial collisions between the young Earth and other protoplanets forced mantle material into the core, providing the heat that kick-started Earth’s geodynamo.
The idea that a Mars-size body smashed into Earth roughly 4.5 billion years ago was first proposed in the 1970s, in an effort to explain the uncanny resemblance of moon rocks to terrestrial ones. Moon rocks are unique in that regard. Meteorites, for example, have chemical and elemental profiles that mark them as distinctly otherworldly. “But rocks from the moon and the Earth look identical,” says Buffett.
Stevenson, Buffett and other researchers suspect that the theory of the moon’s fiery birth might also solve the problem of how Earth kept its geodynamo running before the inner core formed: Much of the impact energy of primordial collisions, like the one that may have caused the moon to form, would have been converted into heat, liquefying Earth’s interior. Some of the debris from one of these impacts eventually coalesced to form the moon. Earth itself was left so hot that it would have glowed like a small star for a thousand years. “Most people think that these impacts probably set the initial thermal conditions of the planet,” says Buffett. “And the core has been tapping that heat over geological time.”
Were it not for that store of excess heat, Earth’s geodynamo might never have started. And without a protective magnetic field around the planet, solar radiation would have stripped Earth’s atmosphere and bombarded the surface, which was apparently the fate of Mars. It may be that several seemingly disparate phenomena were essential in making Earth a habitable world: the formation of the moon, the planetary magnetic field, plate tectonics and the presence of water. Without the collision that created the moon, there wouldn’t have been enough heat for convection to start in Earth’s core and power the magnetic field. Without water, Earth’s crust might have remained too strong to be broken up into tectonic plates; and without a tectonically fractured crust, too much heat would have been trapped inside Earth. Without Earth being able to cool, there would have been no convection and conduction.
“Are these things connected, or are they just happy coincidences?” asks Buffett. “We don’t know for sure. These correspondences are intriguing. You can look at Venus: no plate tectonics, no water, no magnetic field. The more you look at this and think about it, the more you think it can’t be a coincidence. The thought that these things might all be connected is kind of wondrous.”
Is Earth unique, then? Does life require more than oxygen, water and suitable temperatures? Are a fortuitous primordial collision and a moon also necessary, along with a churning liquid core? How repeatable might the circumstances be that gave rise to our world, with its crust filmed with life, shielded from a hostile cosmos by a 3.5-billion-year-old internal engine of heat and iron?
“It’s still not clear how unusual our solar system is,” says Stevenson. “It’s certainly clear that planets are extremely common — there is absolutely no doubt about that. But the formation of planets is not a deterministic process. It is a chaotic process that has a variety of outcomes. In our solar system alone, there are striking differences between Earth and Venus. I think it’s a matter of chance, just how the game played out, how the dice were thrown.”
The answers may come as we learn more about the sorts of worlds that orbit other stars, Stevenson says. Perhaps a handful of those worlds will resemble our own, or maybe thousands will. And maybe one will have inhabitants dwelling on a thin mutable crust, drilling, monitoring tremors, building theories, seeking to understand what lies beneath them, and wondering if their world is miraculous or mundane.