Satellite view of the Jack Hills in Australia (Image: NASA/GSFC/METI/Japan Space Systems, and U.S./Japan ASTER Science Team)

Earth’s magnetic field powered up early. The finding, gleaned from field lines frozen into zircon crystals up to 4.2 billion years old, offers a glimpse at our planet’s infancy – and may hint at how the Earth was able to hold on to its life-sustaining atmosphere.

Looking back so far in time is not easy. In its 4.5 billion year history, our planet’s surface has changed constantly, and the early pages of Earth’s story remain frustratingly blank. “It’s like you have chapters 19 and 20 of a 20-chapter book,” says James Head of Brown University in Providence, Rhode Island, who was not involved in the study.


To find out more about the origin of Earth’s magnetism, a team led by John Tarduno of the University of Rochester in New York retrieved rock samples from the Jack Hills in Western Australia – home to some of the oldest rocks on Earth.

The team spent months picking out zircons – tiny silicate crystals a tenth of a millimetre in size – from the surrounding rock. Inside these crystals, even tinier deposits of magnetic minerals contained traces of fossil magnetism locked in from the moment they first cooled. With a custom magnetometer, the team was able to measure the strength of the ancient magnetic fields, which they matched with estimates of the crystals’ ages based on the isotopes found in them.

They found that the fields were surprisingly ancient – the youngest crystal was 3.3 billion years old, and the oldest 4.2 billion years old.

Interconnected era

That age helps to settle a debate about how the infant Earth cooled down. The planet’s magnetic field is driven by a rotating liquid-metal core called a geodynamo. Models of how heat flowed in the early Earth suggested that in order to begin spinning, the geodynamo would have needed convective currents of heat swirling between the core and the mantle. Unless the Earth started out much hotter than we think it did, those currents should not have been available until about 3.5 billion years ago.

But an early magnetic field would have protected the oceans and atmosphere from being swept away in the fierce solar winds of the early solar system – and the fact that Earth’s atmosphere and oceans have survived suggests that this is just what happened. The new measurement offers support for this early magnetic shield.

“This is more compatible with the atmosphere and the ocean as we know it today,” Tarduno says.

This could mean that the genesis of the geodynamo took place in a mysteriously interconnected era, says Head. Four billion years ago, the same kind of convection that is necessary to power magnetic fields may have also had something to do with the first stirrings of plate tectonics. Plate tectonics drove volcanoes to release the gases that became Earth’s atmosphere – which in turn may have been guarded by magnetic fields.

“It puts more cards on the table in terms of what might have happened at that point,” Head says.

Journal reference: Science, DOI: 10.1126/science.aaa9114