At the center of the Earth, a giant sphere of solid iron is slowly swelling. It’s the inner core, and scientists have recently uncovered intriguing evidence that suggests its birth half a billion years ago may have played a key role in the evolution of life on Earth.
At that time, our planet’s magnetic field was failing – and that would have had critical consequences, they claim. 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 dropped to a fraction of its current strength – before abruptly regaining its power. And as a result of this planetary reboot, Earth witnessed the sudden proliferation of complex multicellular life on its surface. It was the Cambrian Explosion, when most major animal groups first appeared in the fossil record. Now scientists have linked it to events at the very center of the Earth.
Our planet is made up of spheres. There is a layer of rock 5 to 70 km thick that covers the Earth like an eggshell. This is called the crust and underneath is the mantle, made up of a 3,000 km layer of silicates. Further down there is the outer core, made of molten iron, and inside there is another sphere – of solid iron. It measures more than 2,000 km in diameter and is growing by about one millimeter per year.
“Earth’s magnetic field is generated by swirling iron in the outer core,” said John Tarduno, professor of geophysics at the University of Rochester, New York. “Before the Cambrian explosion, the core was entirely molten and its ability to generate a magnetic field was collapsing.”
Analysis of crystals in rocks in Quebec by Tarduno’s team showed that the Earth’s magnetic field was less than 10% of its current strength and would have provided poor shielding against cosmic and solar radiation. The dynamo that powers the Earth’s magnetic field was probably losing power due to rapid heat loss from the core, it is claimed.
Then the core began to solidify in its center, which had profound consequences. Essentially, it turbocharged the movements in the outer core, restoring the strength of the planet’s magnetic field. “Our research indicates that the formation of the inner core began around 550 million years ago and it happened just before the Cambrian explosion,” Tarduno said.
Why and how the inner core was born was a mystery. From its tiny beginnings half a billion years ago, it has evolved into a solid iron sphere the size of a moon. It is the most metallic place on Earth and has had a major impact on surface conditions.
More importantly, it provided our world with a magnetic field. Observations of 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 continuous stream of protons and electrons pouring down from the sun’s surface – the Martian atmosphere was forced out into space, leaving its surface dead and waterless.
“Earth would not have evolved like Mars but it would certainly have lost more water than it has today if it hadn’t restarted its magnetic field,” Tarduno added. “It certainly would have been a much drier planet than the one we live on today.”
However, the geophysicist was hesitant to speculate exactly how the revival 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 unrelated. But we cannot yet say what the exact pattern of events was. This requires more studies.