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


The Earth's inner core solidified around 565 million years ago – just in time to not only save the planet's protective magnetic field from impending collapse, but also to place it in its current and powerful phase, suggests a new study .

The finding, reported online January 28 Natural Geosciences, supports an idea previously proposed by simulations that the inner core of the Earth is relatively young. It also provides insight into how and how quickly Earth has been losing heat since its formation 4.54 billion years ago – fundamental to understanding not only the generation of the planet's magnetic shield, but also convection within the mantle and tectonic plates.

"We do not have many real references to the thermal history of our planet," says Peter Olson, a geophysicist at Johns Hopkins University who did not participate in the new study. "We know that the interior was warmer than today, because all the planets lose heat. But we do not know what the average temperature was a billion years ago compared to today. "Setting down when the iron in the inner core began to crystallize could offer a window on how hot the inside of the planet was at that time, Olson says.

The iron-nickel core of the planet is composed of two layers: a solid inner core and a fused outer core. When this solid inner core formed is a long-standing mystery (SN: 19/9/15, p. 18). "The proposed ages ranged from 500 million years to over 2.5 billion years," says co-author John Tarduno, a geophysicist at the University of Rochester in New York.

The interaction of the two layers drives the geodynamic, the circulation of the iron-rich fluid that feeds the magnetic field. This field, around the planet, protects the Earth from being struck by the solar wind, a constant stream of charged particles ejected by the sun. When the inner core cools and crystallizes, the composition of the remaining fluid changes; more floating liquid rises like a plume as the cooling crystals sink. This density-driven self-sustaining circulation generates a strong magnetic field with two opposite poles, north and south, or polarity.

Traces of magnetism in ancient rocks suggest that the Earth had a magnetic field from 4.2 billion years ago. This previous field was probably generated by heat inside the planet, which drives the circulation inside the molten core. But over time, computer simulations suggest, the heat-driven circulation would not have been strong enough to continue to feed a strong magnetic field. Instead, the field began to shut down, signaling in the rock record by weakening intensities and rapid reversals of polarity over millions of years. And then, at some point, the Earth's inner core began to crystallize, initiating the geodinam and generating a new strong magnetic field.

Feeling Stable

Convection conduction of heat inside the hot and molten core of the Earth (orange) has fed the planet's magnetic field for billions of years. New evidence suggests that, about 565 million years ago, this field was weak and increasingly unstable (to the left). Some time later, the inner core began to solidify (red, to the right), which stabilized and strengthened the field, giving it relatively consistent north and south magnetic poles (right).

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Now scientists think they have found evidence of when the collapse of the magnetic field was happening. Researchers led by geophysicist Richard Bono, now at the University of Liverpool in England, examined magnetic inclusions within a set of rocks in Quebec, Canada, dating to about 565 million years ago. Inclusion analyzes – iron-like, needle-like grains that align with the orientation of the magnetic field that existed when rocks formed – show that the planet's magnetic field was extremely weak at that time, the researchers report.

"These paleo-intensity values ​​were 10 times smaller than the current magnetic field, lower than anything previously observed," says Tarduno. "He suggested that there is something fundamental going on at the core."

Combined with previous studies that found that the magnetic field was also rapidly reversing polarity during that time, the new result indicates that the Earth's field may have been about to fall about 565 million years ago. This suggests that the inner core had not yet solidified. Fortunately for life on Earth, it did.

"Presumably, things worked well for our planet," says Tarduno. "But that does not necessarily mean I needed it."

The new finding is "potentially very important," says Olson. Because the rocks containing the magnetic grains do not cool instantly, but for a long time, the data represent a mean field strength for a period of about 100,000 years. That means scientists have not captured just one snapshot of a floating field, but they have found a true and persistent signal, he says. Computer simulations have suggested that the weak field phase may have lasted much longer, from about 900 million to 600 million years ago. Further paleo-intensity data within that time interval, as well as from other sites, would help confirm that the observed weak phase actually signaled the final spasms of that pre-internal central field.

Peter Driscoll, a geophysicist at the Carnegie Institution for Science in Washington, D.C., was one of the theorists who estimated how long the weak phase could have lasted. Driscoll, whose commentary accompanies the study in Natural Geosciences, notes that a young solid core also highlights persistent puzzles about how quickly the Earth has cooled. For example, "if the core is cooling rapidly, that means it was very hot in the recent past, and that the low mantle was very hot in the recent past" – so hot that they both merged only from 1 billion to 2 billion years ago. "We absolutely do not see it in the record of rock."

Driscoll adds that he hopes the new study will draw attention to the glaring gap in the paleomagnetic data of this time period. "There's a lot more time here that we could be filling."


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