A huge ball of solid iron slowly swells in the center of the earth. This is 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 weakening – and that would have had critical consequences, they argue. Normally, this field protects surface life by repelling cosmic rays and charged particles emitted by our sun.
But 550 million years ago, it had shrunk to a fraction of its current strength – before abruptly regaining its power. And in the wake of this planetary reset, Earth has witnessed the sudden proliferation of complex multicellular life on its surface. This was the Cambrian explosion when most large groups of animals 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 covering the earth like an eggshell. This is called the crust, and beneath it lies the mantle, which is made up of a 3,000 km long layer of silicate. Further down is the outer core of molten iron, and inside is another sphere – solid iron. It has a diameter of more than 2,000 km and is growing by about one millimeter a year.
“Earth’s magnetic field is created 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 its ability to create a magnetic field collapsed.”
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 protection against cosmic and solar radiation. The dynamo that powers Earth’s magnetic field has probably lost power because of rapid heat loss from the core, it is argued.
Then the nucleus began to solidify at its center, with profound consequences. Essentially, it sped up the motions in the outer core and restored the strength of the planet’s magnetic field. “Our research shows that the formation of the inner core began around 550 million years ago, and that this occurred 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 grown into a moon-sized sphere of solid iron. It is the most metallic place on earth and has a major impact on surface conditions.
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 Mars, which lost its magnetic field 4 billion years ago. Unprotected from the solar wind — the continuous stream of protons and electrons emanating from the Sun’s surface — the Martian atmosphere was blown into space, leaving its surface dead and waterless.
“Earth wouldn’t have evolved like Mars, but it would certainly 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 reluctant to speculate exactly how the rebirth of Earth’s magnetic field would have affected the evolution of life. “I don’t think the return of Earth’s magnetic field and the subsequent explosion of life on Earth can be unrelated. But we can’t yet tell what the exact pattern of events was. That needs to be studied more.”