In the center of the Earth, a giant sphere of solid iron is slowly inflating. This is the inner core and scientists have recently discovered intriguing evidence suggesting that its birth half a billion years ago may have played a key role in the evolution of life on Earth.
At the time, our planet’s magnetic field was faltering, and that would have had critical consequences, they argue. Normally this field protects life on the surface by repelling cosmic radiation and charged particles emitted by our sun.
But 550 million years ago, it had fallen to a fraction of its current strength, before abruptly regaining its power. And as a result of this planetary restart, the Earth witnessed the sudden proliferation of complex multicellular life on its surface. This was the Cambrian explosion, when most major groups of animals first appeared in the fossil record. Scientists have now linked it to events in the very center of the Earth.
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Our planet is made up of spheres. There is a 5-70 km thick layer of rock that covers the Earth like an eggshell. This is called the crust and below it is the mantle, formed by a layer of silicates of 3,000 km. Further down, there is the outer core, made of cast iron, and inside there is another sphere: solid iron. It is over 2,000 km in diameter and grows about a millimeter a year.
“The Earth’s magnetic field is generated by an iron vortex in the outer core,” said John Tarduno, a professor of geophysics at the University of Rochester, New York. “Before the Cambrian explosion, the nucleus was completely melted and its ability to generate a magnetic field was collapsing.”
Analysis of crystals in Quebec rocks by Tarduno’s team has shown that the Earth’s magnetic field was less than 10% of its current strength and would have provided poor protection against cosmic and solar radiation. It is said that the dynamo that drives the Earth’s magnetic field was probably losing energy due to the rapid heat loss from the core.
Then the nucleus began to solidify at its center, which had profound consequences. Essentially, it turbocharged the movements in the outer core, restoring strength to the planet’s magnetic field. “Our research indicates that the formation of the inner core began about 550 million years ago and this happened just before the Cambrian explosion occurred,” Tarduno said.
Why and how the inner core was born had been a mystery. Since its small inception half a billion years ago, it has grown to become a solid iron sphere the size of the moon. It is the most metallic place on Earth and has had a major impact on surface conditions.
View of the snow-covered Stolby Nature Reserve in Krasnoyarsk, Russia. These rock pillars date from the Cambrian period, more than 600 m ago, to the Carboniferous period. Photography: Anadolu / Getty Images Agency
Most importantly, it provided our world with a magnetic field. Observations from other worlds – where these fields have disappeared – reveal the dramatic consequences of this loss. An example is provided by Mars, which lost its magnetic field 4 billion years ago. Without protection from the solar wind, the direct current of protons and electrons coming out of the surface of the sun, the Martian atmosphere was expelled into space, leaving its surface dead and without water.
“Earth would not have evolved like Mars, but it would certainly have lost more water than it has today if it had not restarted its magnetic field,” Tarduno added. “It would certainly have been a much drier planet than the one we live in today.”
However, the geophysicist was reluctant to speculate exactly how the rebirth of the Earth’s magnetic field would have influenced the evolution of life. “I don’t think the return of the Earth’s magnetic field and the subsequent explosion of life on Earth can be disconnected. But we still can’t say what the exact pattern of events was. That needs further study.”