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Where has the moon’s magnetism gone? Researchers have contemplated this inquiry for years, ever since orbiting spacecraft detected indications of a substantial magnetic field in lunar surface minerals. Today, the moon possesses no intrinsic magnetism.

Now, scientists at MIT might have unraveled the enigma. They suggest that a blend of an ancient, feeble magnetic field along with a significant, plasma-producing impact may have momentarily generated a robust magnetic field, particularly on the moon’s far side.

In a study published today in the journal Science Advances, the researchers demonstrate via intricate simulations that an impact, such as from a large asteroid, could have produced a cloud of ionized particles that briefly surrounded the moon. This plasma would have flowed around the moon and concentrated on the opposite side from the original impact. At that location, the plasma would have engaged with and momentarily amplified the moon’s weak magnetic field. Any rocks in that vicinity could have registered signs of the enhanced magnetism before the field swiftly dissipated.

This sequence of events might clarify the existence of notably magnetic rocks identified in a region close to the lunar south pole, on the moon’s far side. Coincidentally, one of the largest impact basins — the Imbrium basin — is situated precisely opposite on the near side of the moon. The researchers suspect that whatever caused that impact likely released the plasma cloud that initiated the theory in their simulations.

“Significant aspects of lunar magnetism remain unexplained,” remarks lead author Isaac Narrett, a graduate student in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS). “However, most of the strong magnetic fields recorded by orbiting spacecraft can be accounted for by this mechanism — particularly on the moon’s far side.”

Narrett’s co-authors comprise Rona Oran and Benjamin Weiss at MIT, along with Katarina Miljkovic at Curtin University, Yuxi Chen and Gábor Tóth at the University of Michigan at Ann Arbor, and Elias Mansbach PhD ’24 at Cambridge University. Nuno Loureiro, a professor of nuclear science and engineering at MIT, also provided insights and guidance.

Beyond the sun

Researchers have recognized for years that the moon retains remnants of a strong magnetic field. Samples from the lunar surface, returned by astronauts during NASA’s Apollo missions in the 1960s and 70s, as well as global assessments conducted remotely by orbiting spacecraft, indicate traces of remnant magnetism within surface materials, particularly on the moon’s far side.

The prevailing explanation for surface magnetism is a global magnetic field, produced by an internal “dynamo,” or a core of molten, swirling material. The Earth currently produces a magnetic field through a dynamo process, and it is believed that the moon may have once done the same, albeit its significantly smaller core would have generated a much weaker magnetic field that might not account for the exceptionally magnetized rocks observed, especially on the moon’s far side.

An alternative theory that scientists have periodically examined involves a massive impact that created plasma, which subsequently amplified any weak magnetic field. In 2020, Oran and Weiss investigated this theory with simulations of a significant impact on the moon, alongside the solar-generated magnetic field, which diminishes as it stretches out to the Earth and moon.

In the simulations, they examined whether an impact on the moon could enhance such a solar field sufficiently to explain the highly magnetic measurements of surface rocks. Ultimately, it turned out that it wasn’t, and their findings seemed to dismiss plasma-induced impacts as a factor in the moon’s missing magnetism.

A spike and a jitter

However, in their new investigation, the researchers adopted a different approach. Instead of factoring in the sun’s magnetic field, they presumed that the moon once possessed a dynamo that generated its magnetic field, albeit a weak one. Considering its core size, they estimated that such a field would have been around 1 microtesla, or about 50 times weaker than the Earth’s current field.

From this foundation, the researchers simulated a large impact on the moon’s surface, similar to what would have formed the Imbrium basin on the moon’s near side. Utilizing impact simulations from Katarina Miljkovic, the team then modeled the plasma cloud that such an impact would have created as the force of the impact vaporized surface material. They adapted a secondary code, developed by collaborators at the University of Michigan, to simulate how the resulting plasma would flow and interact with the moon’s weak magnetic field.

These simulations revealed that as a plasma cloud emerged from the impact, some would have dispersed into space, while the remainder would maneuver around the moon and concentrate on the opposite side. There, the plasma would have compressed and momentarily enhanced the moon’s feeble magnetic field. This entire sequence, from the instant the magnetic field was amplified to the moment it reverted to baseline, would have occurred incredibly quickly — roughly within 40 minutes, according to Narrett.

Would this fleeting opportunity have been sufficient for surrounding rocks to capture the transient magnetic spike? The researchers affirm, yes, aided by another impact-related effect.

They discovered that an Imbrium-scale impact would have generated a pressure wave throughout the moon, akin to a seismic shock. These waves would converge on the opposite side, where the shock would have “jittered” the neighboring rocks, momentarily disturbing the rocks’ electrons — the subatomic particles that inherently align their spins with any external magnetic field. The researchers speculate that the rocks were shaken just as the impact’s plasma enhanced the moon’s magnetic field. As the rocks’ electrons settled back, they assumed a new alignment, consistent with the fleeting high magnetic field.

“It’s like tossing a 52-card deck into the air within a magnetic field, where each card has a compass needle,” Weiss explains. “When the cards land back on the ground, they do so in a new orientation. That’s fundamentally the magnetization process.”

The researchers assert that this combination of a dynamo and a significant impact, along with the impact’s shockwave, is sufficient to account for the moon’s highly magnetized surface rocks — especially on the far side. One way to confirm this is to directly sample the rocks for indicators of shock and elevated magnetism. This may be feasible, as the rocks are located on the far side, near the lunar south pole, where missions like NASA’s Artemis program plan to investigate.

“For several decades, a puzzling issue surrounding the moon’s magnetism has persisted — is it a result of impacts or derived from a dynamo?” Oran remarks. “And here we suggest, it’s a combination of both. And it’s a testable hypothesis, which is encouraging.”

The team’s simulations were executed using the MIT SuperCloud. This research was partially supported by NASA.

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