Scientists have puzzled over this question for decades, ever since orbiting spacecraft picked up signs of a high magnetic field in lunar surface rocks. They propose that a combination of an ancient, weak magnetic field and a large, plasma-generating impact may have temporarily created a strong magnetic field, concentrated on the far side of the moon. There, the plasma would have interacted with and momentarily amplified the moon’s weak magnetic field. This combination of events could explain the presence of highly magnetic rocks detected in a region near the south pole, on the moon’s far side. Instead of accounting for the sun’s magnetic field, they assumed that the moon once hosted a dynamo that produced a magnetic field of its own, albeit a weak one.