What happened to the moon’s magnetism? Scientists may finally have the answer

For decades, scientists have been trying to explain why some lunar rocks appear to carry signs of strong magnetism, even though the moon has no magnetic field today.

The mystery began when orbiting spacecraft detected patches of highly magnetized rock, especially on the far side of the moon. How could that be, if the moon has no global magnetic field now?

A team of scientists may finally have the answer. The study suggests that the moon’s magnetic rocks could be the result of a short-lived magnetic burst caused by an ancient asteroid impact.

This event, combined with a weak magnetic field that once existed on the moon, could have created the conditions for temporary magnetism.

To test this idea, a research team led by the Massachusetts Institute of Technology conducted detailed simulations.

A sudden surge of magnetism

The study revealed that a large asteroid impact could have created a cloud of plasma – charged, ionized particles – around the moon.

This cloud, according to the simulations, would have streamed around the moon and concentrated on the side opposite the impact. There, it would have interacted with and briefly boosted the moon’s weak magnetic field.

If any rocks were in that region at the time, they could have recorded this spike in magnetism before the field quickly faded.

Temporary magnetism on the moon

Interestingly, one of the moon’s biggest impact sites – the Imbrium basin – is located on the near side. Its exact opposite point, on the far side near the south pole, is where scientists have detected strongly magnetized rocks.

The study suggests the Imbrium impact could be the very event that created the conditions for this temporary magnetism.

Isaac Narrett is a graduate student in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) and the study’s lead author.

“There are large parts of lunar magnetism that are still unexplained,” said Narrett. “But the majority of the strong magnetic fields that are measured by orbiting spacecraft can be explained by this process – especially on the far side of the moon.”

Rethinking the source of lunar magnetism

For a long time, scientists believed the moon may have once generated its own global magnetic field through a “dynamo” – a process involving a molten, churning core, like the one Earth uses to maintain its magnetic field.

But the moon’s core is small, and any such dynamo would have been weak. It didn’t seem strong enough to account for the highly magnetized rocks seen on the far side.

Other scientists had explored the idea of a plasma-generating impact. A 2020 study simulated whether a solar magnetic field, boosted by an asteroid impact, could have produced the observed magnetism. That idea didn’t hold up – the sun’s magnetic field at the moon’s distance was too weak to matter.

Testing a new hypothesis

This time, the researchers changed the starting point. Instead of focusing on the sun, they assumed the moon had a dynamo-generated magnetic field of its own – about one microtesla in strength, or roughly 50 times weaker than Earth’s field today.

The team simulated a massive impact like the one that formed the Imbrium basin. This impact would have vaporized a large amount of surface material, creating a plasma cloud.

Collaborators at the University of Michigan developed a code to simulate how that cloud would move and interact with the moon’s magnetic field.

The power of impact and shockwaves

The simulations revealed that the plasma would partly escape into space, but some of it would wrap around the moon and converge on the far side. There, it would briefly intensify the moon’s weak magnetic field. This enhancement would last for only about 40 minutes.

Would that be enough time for the rocks to remember the magnetism? Apparently, yes – thanks to another side effect of the impact. The researchers found that a shockwave from the collision would travel through the moon like a seismic wave and reach the far side.

This pressure wave would disturb the rocks’ electrons – which align with magnetic fields – just as the field peaked. As the rocks settled, their electrons would lock into this stronger magnetic alignment.

“It’s as if you throw a 52-card deck in the air, in a magnetic field, and each card has a compass needle,” said study co-author Benjamin Weiss. “When the cards settle back to the ground, they do so in a new orientation. That’s essentially the magnetization process.”

Studying the moon’s magnetized rocks

The team believes this chain of events – a weak lunar dynamo, a large impact, plasma generation, and a shockwave – could explain many of the magnetized rocks on the moon, particularly those on the far side.

One way to confirm the idea is by collecting rock samples from that region and checking for signs of both shock and strong magnetism. This might happen soon. The rocks are near the lunar south pole, where NASA’s Artemis program is planning future missions.

“For several decades, there’s been sort of a conundrum over the moon’s magnetism – is it from impacts or is it from a dynamo? And here we’re saying, it’s a little bit of both,” noted study co-author Rona Oran. “And it’s a testable hypothesis, which is nice.”

The full study was published in the journal Science Advances.

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