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The moon may have kept our planet’s atmosphere safe from a more active sun 4 billion years ago, with a magnetic field that has long since disappeared.

While the moon has no magnetic field of note today, recent evidence from rock samples brought back by the Apollo missions show that between 4.2 and 3.4 billion years ago, when the moon was more than twice as close to Earth as it is now, it did have a magnetic field that was at least as strong as Earth’s present magnetic field.

James Green at NASA, Washington DC, and his colleagues used this information to model the interaction of the early moon’s magnetic field with Earth. They found that the magnetic fields of the moon and Earth should have combined to create a protective magnetosphere.

“The tidal forces from Earth interacting with the moon probably helped keep the current going and the magnetosphere active for several hundred million years,” says Green. Ultimately, the moon drifted away from Earth and its core cooled. “Its field died,” says Green.

The combined field would solve a key problem with the young Earth. Ů��С��Ƶs believe the sun was more active in its early life, ejecting up to 100 times more solar particles than now. This should have stripped Earth of its atmosphere, making prospects for life bleak. But instead life flourished. “We now know it had help, and that help came from the moon,” says Green.

Sampling the moon at its poles could reveal if the model is correct. At these locations, particles from Earth’s atmosphere such as nitrogen should have passed along the moon’s magnetic field lines and hit the ground, where they might still be detectable today.

Confirming the model could have implications in the hunt for life beyond our solar system. “Let’s look for terrestrial exoplanets that have moons,” says Green. “If those moons are large, they may have produced the same kind of protective effect.”