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Earth's core may have hardened just in time to save its magnetic field



Earth's inner core solidified around 565 million years ago – just in time to not only save the planet's protective magnetic field from imminent collapse, but also to kick-start it in its current, powerful phase, and new study suggests. ] The finding, reported online January 28 in Nature Geoscience supports an idea previously proposed by simulations that Earth's inner core is relatively young. It also provides insight into how and quickly, Earth has been release heat since its formation 4.54 trillion years ago —key to understanding not only the generation of the planet's magnetic shield but also convection within the mantle and plate tectonics.

We don't have many real benchmarks for the thermal history of our planet, ”says Peter Olson, a geophysicist at Johns Hopkins University who was not involved in the new study. “We know the interior was hotter than today, because all the 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 started to crystallize could offer a window in how hot the interior of the planet was at the time, Olson says the planet's iron-nickel core is made up of two layers: a solid inner core and a molten outer core. When that solid inner core is formed is a long-standing mystery ( SN: 9/1

9/15, p. 18 ). "Proposed images have been anywhere from 500 million years ago to over 2.5 billion years," says coauthor John Tarduno, a geophysicist at the University of Rochester in New York.

of the magnetic field. That field, surrounding the planet, protects Earth from being battered by the solar wind, a constant flow of charged particles ejected by the sun. As the inner core cools and crystallizes, the composition of the remaining fluid changes; more buoyant liquid rises like a plume while the cooling crystals sink. That self-sustaining, density-driven circulation generates a strong magnetic field with two opposing poles, north and south, or polarity.

Traces of magnetism in ancient rocks suggest that Earth had a magnetic field as back as 4.2 billion years ago . This may have been generated by heat within the planet driving circulation within the molten core. But over time, computer simulations suggest, the heat-driven circulation would not have been strong enough to continue to power a strong magnetic field. Instead, the field began to shut down, signaled in the rock record by weakening intensities and rapid polarity reversals over millions of years. And then, at some point, Earth's inner core began to crystallize, jump-starting the geodynamo and generating a strong magnetic field.

Feeling stable

planet's magnetic field for billions of years. New evidence suggests that about 565 million years ago, was weak and increasingly unstable (left). Sometime after that, the inner core began to solidify (red, right), which stabilized and strengthened the field, giving it relatively consistent north and south poles (right).

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Now scientists think they've found evidence of when that magnetic field breakdown was happening. led by geophysicist Richard Bono, now at the University of Liverpool in England, examined magnetic inclusions within a suite of rocks in Quebec, Canada, dating to about 565 million years ago Analyzes of the inclusions – needless iron-rich grains that align themselves with the orientation of the magnetic field that existed when the rocks formed – the planet's magnetic field was extremely weak at that time, the researchers report.

"These paleo-intensity values ​​were 10 times less than the present magnetic field, lower than anything observed previously, "Tarduno says." It suggests there's something fundamental going on in the core. ”

Combined with previous studies that found that the magnetic field was also rapidly reversing polarity during that time period, the new result indicates that Earth's field may have been on the point of collapse about 565 million years ago. That suggests that the inner core was yet solidified. Fortunately for life on Earth, it eventually did.

"Presumably things worked out well for our planet," Tarduno says. "But that doesn't necessarily mean it."

The new finding is "potentially very important," Olson says. Because the rocks bearing the magnetic grains did not cool instantaneously but over a long time, the data represented an average field intensity of about a 100,000-year period. That means the scientists have just captured a snapshot in time of a fluctuating field, but have found a true, persistent signal, he says. Computer simulations have suggested that the weak field may have been much longer, from about 900 million to 600 million years ago. More pale-intensity data from within that time span, as well as from other locations, would help to confirm that the observed weak phase really signaled the final throes of that pre-inner core field.

Peter Driscoll, a geophysicist at the Carnegie Institution for Science in Washington, DC, was one of the theoreticians who estimated how long the weak phase might have loaded. Driscoll, whose commentary accompanies the study in Nature Geoscience notes that a young solid inner core also highlights lingering conundrums about how quickly Earth cooled. For example, "the core is cooling quickly, which means it was very hot in the recent past, and that the lower mantle was very hot in the recent past" – so hot that both were just 1 billion to 2 billion years ago . "We absolutely do not see that in the rock record."

Driscoll adds that he hopes the new study garners attention to the glaring gap in paleomagnetic data from this time period. "There is a lot more time here that we could fill in." Function (f, b, e, v, n, t, s) {if (f.fbq) return; n = f.fbq = function () {n.callMethod?
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