Earth’s inner core began to solidify within the past 565 million years, a study finds. That could explain why the planet’s magnetic field did not collapse.
The inner core of planet Earth was once molten. But it solidified within the past 565 million years, a new study suggests. That would have been just in time to save the planet’s protective magnetic field from imminent collapse. It also would have kick-started that field into its current, powerful phase, the study says.
Scientists shared their new analysis in the February Nature Geoscience.
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It supports something previously suggested by a computer model. That model had proposed that Earth’s inner core is relatively young.
Earth formed some 4.54 billion years ago. The new study offers clues to how — and how quickly — Earth has been losing heat since its formed. And that is key to learning how the planet’s magnetic shield formed. It also offers insight into heat-related motions within the mantle (between Earth’s crust and core), as well as into plate tectonics.
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“We don’t have many real benchmarks for the thermal history of our planet,” says Peter Olson. He’s a geophysicist at Johns Hopkins University in Baltimore, Md. He was not involved in the new study. “We know [Earth’s] interior was hotter than today, because all planets lose heat. But we don’t know what the average temperature was a billion years ago, compared with today.” Pinning down when iron in the inner core began to crystallize could offer a window into how hot the interior of the planet was back then, he says.
The planet’s core is made of iron and nickel and has two layers. The inner core is solid. Around it sits a molten outer core. When that solid inner core formed has been a long-standing mystery. People have suggested it could have been anywhere from 500 million years ago to more than 2.5 billion years ago, says coauthor John Tarduno. He’s a geophysicist at the University of Rochester in New York.
The core’s two layers interact. And that drives Earth’s geodynamo — the circulation of an iron-rich fluid that powers the planet’s magnetic field. That field surrounds the planet. And that’s a good thing. It protects Earth from being battered by the solar wind, a constant flow of charged particles from the sun.
As the inner core cooled and crystallized, the chemical recipe of its surrounding fluid changed. The more buoyant liquid would have risen like a plume. The cooling crystals would have sunk. That density-driven circulation was self-sustaining. And it has driven a strong magnetic field with two opposing poles, north and south.
