For decades, scientists have been puzzled by a strange feature of the Moon: while the Moon today has no magnetic field, some of the rocks on its surface are strongly magnetized—especially on the far side near the south pole.
Where did this magnetism come from, and how could it have left such a strong mark?
A new study by researchers at MIT may finally have an answer.
The team used computer simulations to show that a combination of two events—a weak ancient magnetic field and a massive asteroid impact—could have created a brief, powerful magnetic field on the Moon.
This sudden spike in magnetism may have lasted just 40 minutes but was long enough for nearby rocks to record it permanently.
Scientists have long debated whether the Moon once had a magnetic field created by a “dynamo”—a swirling, molten metal core like Earth’s that generates a magnetic field.
The Moon is much smaller than Earth, and its core likely created only a weak magnetic field, estimated at just 1 microtesla, or 50 times weaker than Earth’s current field.
In the new study, researchers started with this assumption of a weak lunar dynamo and then simulated a massive asteroid impact like the one that formed the Imbrium Basin, a giant crater on the Moon’s near side. When an impact of that scale hits, it can vaporize rock and create a huge cloud of ionized gas, or plasma. The researchers used models to show how this cloud would behave in the Moon’s environment.
They found that some of the plasma would have escaped into space, but much of it would have flowed around the Moon and gathered on the opposite side. There, it would have compressed the Moon’s weak magnetic field and temporarily made it much stronger. This intense but short-lived magnetic spike could explain the magnetized rocks seen on the far side of the Moon, directly opposite the Imbrium Basin.
But there’s more to the story. The same impact that created the plasma would also have sent shockwaves through the Moon’s surface—similar to a lunar earthquake. These waves would have traveled around the Moon and converged on the far side, “jittering” the rocks just as the magnetic field was at its strongest.
The researchers believe this jitter caused electrons in the rocks to shift their orientation and align with the temporary magnetic field. When the rocks settled, they locked in that magnetic signature, even after the field quickly faded.
It’s like tossing a deck of cards with tiny compass needles into the air during a magnetic storm. When they land, the needles all point in a new direction—and stay that way.
This theory combines two previously competing ideas about lunar magnetism: that it came either from a global magnetic field or from powerful impacts. The researchers now suggest it was both. A weak dynamo field, briefly amplified by a massive impact and shockwave, could explain the strong magnetism in many lunar surface rocks.
To confirm the theory, scientists may one day collect rock samples from the Moon’s far side—perhaps through NASA’s Artemis missions—which are planning to explore that very region. If those rocks show signs of both magnetic alignment and impact-related damage, it would provide strong support for this new explanation.
The findings were published in Science Advances and highlight how short-lived but powerful events in the Moon’s history may have left a lasting imprint that we can still detect today.
