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.”
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