Without Earth's magnetic field, migrating animals lose their way and navigation for everything from ships to Boy Scouts is rendered useless. But despite its importance, the process that powers the planet's magnetic field remains a mystery. Ideas abound, but none of them can account for the age of Earth's magnetic field. Now, a new study may have the key to this inconsistency: humble magnesium.
The churning of Earth's molten core generates electrical currents that produce the planet's magnetic field in a process called a dynamo.
"If you didn't have these churning motions, the magnetic field of Earth would decay, and it would die in about ten million years," says Joseph O'Rourke, a postdoctoral researcher at the California Institute of Technology in Pasadena.
But what powers this motion is unclear. Slow solidification of Earth's inner core and radioactive decay—two of the leading hypotheses—don't produce enough energy to power the magnetic field for as long as it's been around.
Rock records indicate the Earth's magnetic field is at least 3.4 billion years old, and perhaps as old as 4.2 billion years. Cooling the inner core would only provide about a billion years worth of energy for the magnetic field. And there just isn't enough radioactive material in Earth's core for the decay hypothesis to work, says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz.
In a new study, published in this week's issue of the journal Nature, O'Rourke and David Stevenson, a planetary scientist at Caltech, propose a new chemical mechanism for setting up buoyancy differences in Earth's interior to drive the geodynamo.
Using computer models, the pair showed that in the aftermath of giant impacts that bombarded early Earth, a small amount of the element magnesium could have become dissolved in the iron-rich core.
"Earth formed in a series of really violent, giant collisions that could have heated the mantle to temperatures as high as 7,000 Kelvin [12,140 degrees Fahrenheit]," O'Rourke says. "At those temperatures, elements that don't normally [mix with] iron, like magnesium, will go into iron."
But because magnesium is only soluble in iron at high temperatures, as Earth's core cools, the magnesium will precipitate, or "snow out," of the outer core as magnesium-rich alloys. Those alloys get transported up to the core-mantle boundary.
"When you pull magnesium-rich alloy out of the core, what's left behind is denser," O'Rourke says. Concentrating mass like that releases gravitational energy that could serve as an alternative power source for the dynamo, he explains.
According to O'Rourke and Stevenson, their magnesium precipitate mechanism could have powered the geodynamo for billions of years until the inner core began to cool and solidify, which current estimates suggest happened about a billion years ago. At that point, the two processes could have begun working in tandem to power Earth's magnetic field, O'Rourke says.
"Magnesium precipitation could drive [iron] convection from the top of the core, whereas the release of light elements from the inner core [from solidification] could drive convection from the bottom," he says.
Planetary scientist Nimmo, who was not involved in the study, says he likes the magnesium precipitation hypothesis because it makes only two assumptions: That Earth gets hot during a giant impact, and that during a giant impact, the metallic core of the impactor gets exposed to silicate mantle material.
"Assumption one is hard to argue with, though exactly how hot it gets is uncertain," says Nimmo. Assumption two is a bit less secure, he says, but most scientists agree that as rocky bodies collided with early Earth, some elements from those impactors, such as magnesium, would get transferred to the mantle. "Once you make those two assumptions, everything else follows naturally."
Now, Nimmo says, all we need are experiments to test O'Rourke and Stevenson's ideas. "Their study is based mainly on computational predictions of how magnesium should partition as a function of temperature," Nimmo says.
Some researchers are already working on those experiments, so it may only be a matter of time before scientists zero in on what makes Earth's magnetic field tick.
“Our process could explain not only how the dynamo worked in the past,” O’Rourke says, “but [how] it could still be operating today."