In the depths of our planet, vortices of liquid iron produce our magnetic field which is an invisible shield that protects all life from the incredible solar winds. Knowing more about the magnetic field can provide clues to understanding the future evolution of the Earth, as well as that of other planets in the solar system.
If the magnetic field did not exist, the flow of high-energy particles from the Sun would destroy the ozone layer and with it the possibility of life on Earth. If this happened our landscape would look very much like the desert Mars, where the old magnetic field collapsed when the planet cooled down and the core stopped spinning.
How did they discover that the early magnetic field was so strong?
Thanks to some small crystals found in Australia rocks, which are helping scientists to unravel the ancient history of our planet's first magnetic field.These crystals show that this shield, which originated some 350 million years after the Earth was formed, was much more powerful than previously thought.
A mystery that could help answer the question of how life came to be here. For this magnetic field would have protected the Earth, its atmosphere, from the sun's high-energy particles, and perhaps helped life to take hold.
We're talking about a period known to geologists as the Hadean eon, which extended from 4.55 billion years ago to 4 billion years ago - of which there are fragmentary tracks in rocks in the Jack Hills area of Western Australia.
These rocks contain tiny crystals of a mineral called zircon, the oldest known earth material. Using new paleomagnetic and geochemical data, along with electron microscopes, experts dated and analyzed these zircon crystals, which are about two-tenths of a millimetre in size, and whose magnetic particles were imprinted with the Earth's magnetism at the time the mineral was formed.
"This research tells us something very important about the formation of a habitable planet," says John Tarduno, a geophysicist at the University of Rochester, New York (USA) and leader of the work published in the journal PNAS. "One of the questions we want to answer is why the Earth evolved the way it did and this work offers us even more evidence that the Earth's magnetic shielding was recorded very early on the planet".
What triggered the magnetic field?
The Earth’s core formed only 565 million years ago. So, the primitive magnetic field must have been driven by a different mechanism. What was it?
"We think that mechanism is the chemical precipitation of magnesium oxide within the Earth," said Tarduno.
The magnesium oxide, which had dissolved into the totally liquid core during the same impact that created our Moon, was slowly moving from the core into the mantle. That movement also generated a circulation in the liquid core that would eventually create the Earth's primitive magnetic field. By the time the "fuel", magnesium oxide, ran out (probably because of the extreme heat associated with the giant impact that formed the Earth's moon), the magnetic field had almost collapsed, scientists believe. But because the solid inner core formed at about the same time, it came at the perfect time to save life on Earth. Just in time, as the formation of the inner core provided a new energy source to power the geodynamo and the planetary magnetic shield that Earth possesses today.
As such, this study will help scientists draw conclusions about the sustainability of Earth's magnetic shield and whether there are other planets within the solar system with the right conditions to support life.
Reference: John A. Tarduno on al., "Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo," PNAS (2020). www.pnas.org/cgi/doi/10.1073/pnas.1916553117