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. . . .