Scientists may have cracked a lunar mystery: how some moon rocks became intensely magnetic despite the Moon having no magnetism today. The answer lies in a rare cosmic chain reaction triggered by an ancient impact.
Key Points at a Glance
- Some lunar rocks show strong magnetism, though the Moon has no magnetic field today
- MIT scientists suggest an asteroid impact briefly amplified the Moon’s weak magnetic field
- Plasma from the impact focused on the far side, creating a short-lived magnetic surge
- Shockwaves helped lunar rocks record the amplified magnetic field
- This hybrid theory combines both dynamo activity and impact events
Lunar rocks collected from the Moon’s surface have long presented a riddle. Many exhibit intense magnetic signatures, yet the Moon today has no global magnetic field. How could inert surface rocks have locked in such strong magnetic imprints?
A new study from MIT may have finally pieced together the puzzle. The researchers propose a striking scenario in which a massive asteroid impact billions of years ago briefly boosted the Moon’s already-weak magnetic field, allowing certain rocks to “remember” that moment of cosmic intensity.
The team, led by MIT geophysics graduate student Isaac Narrett and published in Science Advances, used advanced simulations to reconstruct a hypothetical sequence of events. They assumed that early in its history, the Moon had a weak magnetic field generated by a small dynamo within its molten core—around 1 microtesla, fifty times weaker than Earth’s magnetic field today. On its own, this would not be enough to magnetize surface rocks to the levels observed.
But the game-changer was a large asteroid strike—specifically, one that could have created the Moon’s Imbrium Basin, a massive crater visible from Earth. According to the team’s simulations, such an impact would have vaporized surface material, generating a high-energy plasma cloud. This cloud, laden with charged particles, would have rapidly expanded, streaming around the Moon and concentrating on the opposite side from the impact—near the Moon’s far south pole.
It was here, on the far side, that the plasma compressed and momentarily amplified the Moon’s weak magnetic field, according to the researchers. The magnetic spike would have lasted just 40 minutes—but in the silent world of geology, that was enough.
Adding to the intrigue, the study posits that the impact also sent shockwaves rippling through the Moon’s body. These pressure waves converged on the opposite hemisphere, where the plasma had amplified the magnetic field. The collision of mechanical shock and magnetic surge briefly jostled the electrons in surrounding rocks, effectively “resetting” their orientation to align with the amplified field.
Benjamin Weiss, co-author and MIT professor, compares the process to throwing a deck of compass-needle-laced cards into the air. When they fall back, they realign to a new magnetic order—a record of a transient but powerful event.
The result? Rocks that were briefly and intensely magnetized, with signatures that have lasted for billions of years and are still detectable by orbiting spacecraft today. Notably, the region with the strongest magnetic readings—near the Moon’s far-side south pole—is exactly where the simulations predict this plasma-magnetic convergence would have occurred.
This theory elegantly bridges two long-standing ideas: that lunar magnetism came from a weak internal dynamo and that impacts played a key role. Earlier simulations suggested solar magnetic fields weren’t strong enough to explain the magnetized rocks, but this new model—focusing on the Moon’s own weak dynamo and a precisely timed impact—fits the data much better.
Moreover, it’s a testable hypothesis. Missions like NASA’s Artemis program plan to explore the Moon’s south polar region, providing an opportunity to directly sample the highly magnetized rocks and search for evidence of impact-induced shock.
The researchers emphasize that this momentary magnetism was a kind of cosmic accident—an unlikely confluence of internal dynamics and external catastrophe. Yet it left a lasting mark, a magnetic fossil hidden in plain sight on the Moon’s rugged terrain.
It’s a reminder that planetary history is often written in flashes—in this case, a 40-minute storm of invisible force etched into stone and carried through time.
Source: MIT News
