The moon's somewhat bizarre magnetic field may have been shaped by a massive asteroid impact, which left some of its material behind.
Huge amounts of highly magnetic material have been found in some parts of the moon's crust, but have so far not been satisfactorily explained.
But researchers from Harvard, MIT and the Institut de Physique du Globe de Parisnow suggest that an asteroid slammed into the moon around four billion years ago, leaving behind an enormous crater and iron-rich, highly magnetic rock.
While there is evidence that the moon once generated its own magnetic field, there's little to suggest it was strong enough to account for what's seen today.
"The conundrum has always been that the magnetism we see on the moon is not correlated with any surface geology," says professor Sarah Stewart-Mukhopadhyay.
"The theory that has been most commonly cited to explain it is an 'impact-induced field,' in which an impact concentrates and amplifies the moon's magnetic field. But it was difficult to test – people have tried to model it, but it is right at the edge of what could work."
The researchers had a simpler idea.
"Because the fields in this area are stronger than those found in any normal lunar rocks, our hypothesis is that it isn't lunar material," says Stewart-Mukhopadhyay.
"We know the magnetic properties of asteroidal material are much higher than that of the moon. It is possible that metallic iron from an asteroid could have been magnetized by the impact, and deposited on the moon."
When the team examined existing surveys alongside newer, more accurate ones, it quickly became clear that most of the magnetic anomalies are scattered around the rim of an enormous, 2,400 kilometer-diameter crater known as South Pole-Aitken.
The oldest definitive structure on the moon, the crater's between 3.9 and 4.5 billion years old, and is slightly elongated, suggesting it was formed by an object that struck the moon at an oblique angle.
The team modeled various types of impact, at different speeds and angles. And, says Stewart-Mukhopadhyay, every one produced similar results to the magnetic distribution seen on the moon today.
"Magnetism is one of the clues that let us construct a geologic history of the surface of a planet," she says.
"If we now have to consider that it may have come from a collision like this one, that's something we need to be aware of."