At the center of the Earth, a gigantic sphere of solid iron slowly swells. This is the inner core, and scientists have recently found 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 that point, our planet’s magnetic field faltered — and that would have had critical consequences, they claim. Normally, this field protects life on the surface by repelling cosmic rays and charged particles emitted from our sun.
But 550 million years ago, it had fallen to a fraction of its current strength — before abruptly regaining its strength. And in the wake of this planetary reboot, Earth witnessed the sudden spread of complex multicellular life on its surface. This was the Cambrian explosion, when most major animal groups first appeared in the fossil record. Now scientists have linked it to events at the center of the Earth.
Our planet consists 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 beneath it lies the mantle, which consists of a 3000 km long layer of silicates. Further down there is the outer core, made of molten iron, and inside there is another sphere – made of solid iron. It has a diameter of more than 2,000 km and grows at about a millimeter per year.
“Earth’s magnetic field is generated by swirling iron in the outer core,” said John Tarduno, a professor of geophysics at the University of Rochester, New York. “Before the Cambrian explosion, the core had completely melted and the ability to generate a magnetic field collapsed.”
Analysis of crystals in rocks in Quebec by Tarduno’s team showed that Earth’s magnetic field was less than 10% of its current strength and would have provided poor protection against cosmic and solar radiation. The dynamo that powers Earth’s magnetic field likely lost power due to rapid heat loss from the core, it’s claimed.
Then the core began to solidify in the center, with profound consequences. Essentially, it caused a turbocharged motion in the outer core, restoring the strength of the planet’s magnetic field. “Our research indicates that the formation of the inner core started about 550 million years ago and it happened just before the Cambrian explosion,” Tarduno said.
Why and how the inner core was born had been a mystery. From its tiny beginnings half a billion years ago, it has grown into a massive iron sphere the size of a moon. It is the most metallic place on Earth and has had a major impact on surface conditions.
Most importantly, it has 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 of this is Mars, which lost its magnetic field 4 billion years ago. Without protection from the solar wind — the constant stream of protons and electrons pouring out of the sun’s surface — Mars’ atmosphere was propelled into space, leaving the surface dead and waterless.
“Earth wouldn’t have evolved like Mars, but it certainly would have lost more water than it does today if it hadn’t restarted its magnetic field,” Tarduno added. “It would certainly have been a much drier planet than the one we live on today.”
However, the geophysicist was hesitant to speculate exactly how the rebirth of Earth’s magnetic field might have affected 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 independent of each other. But we cannot yet say what the exact pattern of events was. That needs more study.”