Scientists have found evidence for a magnetic field around the Earth going back at least 3.5 billion years, 250 million years earlier than previously thought.
The results also reveal that, at 50-70% of present-day levels, the field would have been much weaker than it is now, meaning that the early Earth would have taken a harsh battering from the solar wind, stripping away water and light elements like hydrogen.
The discovery, which is published in Science, was made by University of Rochester researcher John Tarduno and his colleagues. He and his team scoured South Africa for ancient quartz rock samples dating back more than 3.5 billion years. These were tested using a device called a SQUID (superconducting quantum interface device) magnetometer, which can read the magnetic field imprinted into rocks when they first form and being to cool.
In this way the Earth's magnetic field can be reconstructed going back billions of years. Previously, similar approaches had yielded evidence for a magnetic field around the planet from about 3.2 billion years ago but the new results push this date back to 3.5 billion years, indicating that the "geodynamo" - the pool of circulating iron at the Earth's core - was active by this time.
Critically, the results also show that the magnetic field was only half as strong as it is today. This, coupled with the fact that the young Sun was ejecting a solar wind far fiercer than it is today, would have meant that the planet had a harder time fending off the onslaught, which would have robbed the Earth of large amounts of water and would probably have meant the northern lights would have been visible from London rather than northern Norway!