Geologists at MIT and Oxford University have found ancient rocks in Greenland that bear the oldest remnants of Earth’s early magnetic field. 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 results of the study published in the Journal of Geophysical Research, represent some of the earliest evidence of a magnetic field surrounding the Earth. 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.

“If the Earth’s magnetic field was around a few hundred million years earlier, it could have played a critical role in making the planet habitable,” Benjamin Weiss from the Department of Earth, Atmospheric and Planetary Sciences at MIT and one of the authors told MIT News.

Scientists suspect that, early in its evolution, the Earth was able to foster life, in part due to an early magnetic field that was strong enough to retain a life-sustaining atmosphere and simultaneously shield the planet from damaging solar radiation. 

Exactly how early and robust this magnetic shield was was not known, though there has been evidence dating its existence to about 3.5 billion years ago. The objective of the researchers was to find rocks that still held signatures of the Earth’s magnetic field when the rocks first formed. To get to the rocks that were hopefully preserved and unaltered since their original deposition, the team sampled from rock formations in the Isua Supracrustal Belt in southwestern Greenland.

The team returned to MIT with whole rock samples of banded iron formations — a rock type that appears as stripes of iron-rich and silica-rich rock. Given their composition, the researchers suspect the rocks were originally formed in primordial oceans prior to the rise in atmospheric oxygen around 2.5 billion years ago. They used uranium to lead ratio and found that some of the magnetized minerals were likely about 3.7 billion years old. Through this careful process of remagnetisation, the team concluded that the rocks likely harbored an ancient, 3.7-billion-year-old magnetic field, with a magnitude of at least 15 microtesla. Today, Earth’s magnetic field measures around 30 microtesla.