IT IS Earth’s silent defender. Without it, a constant onslaught of charged particles would bombard our planet’s atmosphere, changing its chemistry and disrupting our electronic infrastructure. Assuming any of that stuff was even there to disrupt. In Earth’s infancy, our guardian may have prevented the sun’s action from stripping away the protective bubble of gas surrounding our planet entirely, and so allowed life – and eventually intelligent life – to flourish.
This silent defender is Earth’s magnetic field, a force field whose source lies in the churning molten iron that forms the planet’s core. Electrons flowing through this fluid generate an electric current, which in turn creates a magnetic field. The core is a giant, self-sustaining electromagnet: a dynamo.
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That’s been the general story for decades. But over the last few years, it has run into a problem. Evidence is mounting that the dynamo could only have emerged comparatively recently. At the same time, geological clues show that the magnetic field has existed for most of Earth’s 4.5-billion-year history. This contradiction – an ancient magnetic field without anything to power it – is forcing us to rethink our planet’s insides.
With apologies to Jules Verne, we will probably never journey to the centre of the Earth. Our planet’s core is a hellish domain as hot as the sun’s surface, with pressures several million times greater than at sea level. But that hasn’t stopped us from opening a window onto it. Seismic wave measurements, computer models and lab experiments that mimic the core’s extreme conditions all provide a reasonable picture of how things work in Earth’s deepest reaches.
This picture says that, in the beginning, our planet was a hot agglomeration of smaller rocks and debris captured from the early solar system. Iron, the densest element in the newborn planet, slowly sank to the centre where the high temperatures melted it. Next, thermal convection, the same process that occurs in a pot of roiling water, kicked in. Cooler, denser iron in the outer part of the core sank, while the hotter, lighter stuff rose, kicking the magnetic dynamo into action.
Then came a slight complication. At some point, Earth had cooled sufficiently to allow some of the core’s molten iron to solidify. Because of the way physics works under such extreme pressures, the core began to freeze from the inside out. According to most estimates, this process started about one billion years ago. Today, the inner core is a solid iron ball about 1200 kilometres in radius that is growing ever larger as Earth continues to cool. Surrounding it is a layer of liquid iron 2000 kilometres thick, mixed with some nickel and a smattering of lighter elements like sulphur and oxygen.
Fortunately, the freezing kick-started another effect that kept the magnetic dynamo humming. As the inner core grows, it expels the lighter elements. The same thing happens when salt water freezes. The salt doesn’t fit into the ice’s crystal structure, so it gets pushed out, leaving freshwater ice surrounded by extra-salty water. Similarly, inside Earth, the solid inner core is almost pure iron, surrounded by molten iron with a higher concentration of impurities. These make the liquid less dense, so this lighter stuff nearest the inner core rises, while the heavier, iron-rich material above sinks. This sets the outer core churning in a process known as compositional convection.
Shaken all about
So it seems that one way or another, the dynamo has been kept turning for most of Earth’s history. But it’s here we encounter the most recent twist in this magnetic tale. In the past few years, researchers have begun to doubt whether the first part of the story, thermal convection, could ever have happened – and if it did, whether it would have been strong enough to power the magnetic field. “If you want to rely on thermal convection alone, then we’re in trouble,” says David Stevenson at the California Institute of Technology.
The problem lies in the way heat travels. Convection requires layers of differing temperatures: in a pot of boiling water, the bottom is hotter than the top. This can only happen because water doesn’t conduct heat that well. Conduction quickly equalises temperatures throughout a substance, making convection impossible. And this is where the material in Earth’s core may not conform to expectations. “There’s a growing amount of evidence that the conductivity of the core is higher than we thought,” says Peter Olson at Johns Hopkins University in Baltimore, Maryland. The result is renewed controversy, says Francis Nimmo at the University of California, Santa Cruz. “Five years ago, everyone thought they knew the answer.”
The debate reopened in 2012, when two independent groups used computer models to predict that the liquid iron core was twice as conductive as previously thought. Early last year, researchers in Kei Hirose’s group at the Tokyo Institute of Technology in Japan measured the thermal conductivity of iron under pressures comparable to those in the core. Their results matched the predictions, suggesting that Earth’s magnetic field can only have emerged with the first solidification of the core less than one billion years ago.
Except it can’t have done. “We know Earth’s magnetic field existed long before then,” Nimmo says. In the past, magnetic minerals in molten rock aligned with the field. When the molten rock froze, it recorded the field’s presence. In 2015, Nimmo was part of a team that found such prehistoric compasses inside our planet’s oldest rocks, implying that a field existed as far back as 4.2 billion years ago. That would predate the first evidence of life on Earth, consistent with the idea that a magnetic field is essential not only for life to get started but also for a habitable planet to arise at all (see “Some foreign field“). Their results are still disputed, but the evidence is stacking up that the magnetic field is at least 3.45 billion years old. That points to a substantial gap when neither type of convection could have sustained the dynamo.
So what did? Stevenson and others think they have identified the culprit. It’s a process similar to compositional convection, but one that doesn’t involve solidification of the inner core. As the outermost layers of the liquid core cooled, the lighter elements that were dissolved in the iron would have precipitated out. After rising out of the liquid core, these elements would have been absorbed into the mantle, the largely solid region that forms the bulk of Earth, leaving behind denser liquid iron. This would sink, triggering convection.
Stevenson and Joseph O’Rourke, also at Caltech, argue that the most important light element in this process is magnesium. It’s abundant and doesn’t readily dissolve in iron. Because of its insolubility, magnesium would be the first element to precipitate out once the molten iron in the outer core started to cool, either forming like frost at the boundary with the mantle, or else gently snowing up from deeper in the outer core.
But magnesium’s insolubility in iron also poses a problem: how did it get into the core in the first place? To achieve this you need heat, says Stevenson. Lots of it. One way to generate this heat is via collisions with other space rocks. Such impacts were common in the early solar system – it’s likely that a particularly massive one gouged a chunk out of Earth that eventually became the moon.
Hirose has a different idea. Instead of magnesium, he favours silicon. It’s even more abundant, he says, and would therefore be likely to dominate deep in the planet. His experiments also show that silicon dioxide crystallises easily in the core, without the need for any external processes. “So far, the silicon dioxide story is most feasible,” he says. Hirose’s lab is now including magnesium in their experiments to better determine its role.
Some researchers have even suggested that convection may not drive the dynamo at all. Instead, Earth’s wobbling rotation could jostle the molten iron. Or the moon’s gravity could tug the liquid core in the same way it causes ocean tides. “There’s a group of people that are enthusiastic about the idea, but I would say it’s probably not mainstream,” says Bruce Buffet at the University of California, Berkeley.
“Magnesium could snow up towards the mantle from inside Earth’s core”
At the moment, everything’s up in the air. Even the thermal conductivity calculations could be wrong. In fact, a study contradicting Hirose’s measurements ran alongside his in the same scientific journal. “This is a fast-moving field,” Nimmo says. “I don’t think we have a completely satisfactory answer.”
Whatever process turns out to fit the bill could also apply to other worlds. We have already discovered thousands of planets outside the solar system, many of them Earth-sized and, presumably, with similar chemical compositions. And massive temperature-raising impacts could also play a part, as young planetary systems are rife with collisions. If convective mechanisms do power magnetic fields on other planets, then that could facilitate alien life. “Of course,” says Buffet, “it depends on the details.”
Some foreign field
WHILE our magnetic field appears to predate life on Earth, the claim that planets need magnetism to be habitable is far from clear-cut.
We’ve long assumed that the field blocks the solar wind, which is full of charged particles that can strip Earth of its atmosphere. But some researchers have suggested almost the opposite. The magnetic field, they say, might instead act as a huge sail that catches and absorbs these charged particles, which then harm the atmosphere.
And even if the magnetic field were essential for life on Earth, what does that mean? Many scientists describe a planet as habitable just because it’s Earth-sized and the right distance from its star for liquid water to exist. It’s tempting to add a magnetic field to that list of requirements. But that’s too simplistic.
“Planets are extremely diverse,” says David Stevenson at the California Institute of Technology. “In the landscape of possibilities, I see extraordinary richness.” This may be promising when it comes to searching for extraterrestrial life, but makes it difficult to reduce our understanding of planets to just a few parameters. “The magnetic field,” he says, “is just one of the tunable knobs in this rich landscape.”
This article appeared in print under the headline “Earth’s ghostly guardian”
