Geologists at MIT and Oxford University have uncovered ancient rocks in Greenland that bear the oldest remnants of Earth’s early magnetic field.

The rocks appear to be exceptionally pristine, having preserved their properties for billions of years. The researchers determined that the rocks are about 3.7 billion years old and retain signatures of a magnetic field with a strength of at least 15 microtesla. The ancient field is similar in magnitude to the Earth’s magnetic field today.

The open-access findings, appearing today in the Journal of Geophysical Research, represent some of the earliest evidence of a magnetic field surrounding the Earth. The results potentially extend the age of the Earth’s magnetic field by hundreds of millions of years, and may shed light on the planet’s early conditions that helped life take hold.

“The magnetic field is, in theory, one of the reasons we think Earth is really unique as a habitable planet,” says Claire Nichols, a former MIT postdoc who is now an associate professor of the geology of planetary processes at Oxford University. “It’s thought our magnetic field protects us from harmful radiation from space, and also helps us to have oceans and atmospheres that can be stable for long periods of time.”

Previous studies have shown evidence for a magnetic field on Earth that is at least 3.5 billion years old. The new study is extending the magnetic field’s lifetime by another 200 million years.