Have you ever pondered the mystery of the moon’s magnetism? Scientists have been intrigued by this question for decades, particularly since orbiting spacecraft detected signs of a strong magnetic field within lunar rocks, even though the moon today shows no inherent magnetism.
MIT scientists may finally have unraveled this enigma. Their research suggests that a blend of an ancient, weak magnetic field and a colossal impact event that generated plasma might have temporarily forged a strong magnetic field, concentrated primarily on the moon’s far side.
In a recent study published in the journal Science Advances, researchers conducted intricate simulations revealing that a large asteroid impact could have created an ionized particle cloud, which briefly enveloped the moon. This cloud then streamed around the moon and intensified the existing weak magnetic field on the opposite side. Consequently, rocks in that area could have recorded traces of this heightened magnetism before it faded swiftly away.
This phenomenon may clarify the presence of magnetic rocks found near the south pole on the far side of the moon. Interestingly, one of the moon’s most significant impact basins—the Imbrium basin—is located precisely where this initial impact could have occurred, suggesting a direct connection.
“While parts of lunar magnetism remain a puzzle,” explains Isaac Narrett, lead author and graduate student in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS), “most of the strong magnetic fields detected by orbiting spacecraft could be attributed to this process, especially on the moon’s far side.”
Narrett’s co-authors include Rona Oran and Benjamin Weiss from MIT, alongside collaborators from Curtin University and the University of Michigan at Ann Arbor.
Exploring Beyond the Sun
Researchers have recognized the moon’s remnants of a robust magnetic field for many years. Samples collected during NASA’s Apollo missions in the 1960s and 70s, along with global measurements from orbiting spacecraft, reveal significant remnant magnetism in surface rocks, particularly on the moon’s far side.
Typically, scientists attribute surface magnetism to a global magnetic field, generated by an internal “dynamo” composed of molten material. Earth produces its magnetic field via such a dynamo process, and it was once theorized that the moon did as well, albeit with a much weaker magnetic field. This weaker field may not adequately explain the highly magnetic rocks observed, particularly on the far side of the moon.
At times, scientists proposed alternative hypotheses, including substantial impacts that generated plasma, potentially amplifying any weak magnetic field present. In 2020, Oran and Weiss examined this possibility through simulations but found this scenario insufficient to elucidate the moon’s missing magnetism.
A Temporary Magnetic Surge
In their latest study, the researchers pivoted their approach. Instead of factoring in the sun’s magnetic fields, they postulated that the moon once harbored a dynamo, albeit a weak one. They estimated the magnetic field it would generate at around 1 microtesla—about 50 times weaker than what Earth has today.
The team then modeled a significant impact on the moon’s surface, much like the event that created the Imbrium basin on the near side. Utilizing impact simulations, they demonstrated how such an impact would create a plasma cloud as the surface material vaporized due to its force. An advanced simulation developed in collaboration with the University of Michigan illustrated how this plasma would interact with the moon’s weak magnetic field.
The results revealed that a portion of the plasma cloud would expand into space, while the remainder would stream around the moon and concentrate on the far side. Here, the resulting compression would cause a brief amplification of the moon’s weak magnetic field. Remarkably, this entire sequence transpired in an incredibly short window, approximately 40 minutes, according to Narrett.
So, could this fleeting opportunity have been sufficient for surrounding rocks to capture the fleeting magnetic spike? The researchers affirm this—with the assistance of another impact-related effect.
They discovered that an Imbrium-scale impact would generate a seismic shock, sending pressure waves through the moon. These waves converged on the other side, causing a “jitter” in the surrounding rocks, temporarily disturbing the electrons, which usually align with external magnetic fields. The researchers believe that as the impact’s plasma amplified the moon’s magnetic field, the rocks’ electrons settled into a new orientation, mirroring the momentary high magnetic field.
“It’s akin to scattering a deck of cards into the air in a magnetic field, with each card housing a compass needle,” Weiss elaborates. “When the cards land, they orient in a fresh alignment. That’s essentially how the magnetization occurs.”
According to the researchers, this interplay of a dynamo and a colossal impact, coupled with the impact’s shockwave, sufficiently explains the moon’s highly magnetized surface rocks, especially on its far side. A direct sampling of these rocks in the future—especially near the lunar south pole, which missions like NASA’s Artemis program plan to explore—could confirm this hypothesis.
“For years, the moon’s magnetism has posed a dilemma—whether it stems from impacts or from a dynamo?” Oran remarks. “We propose that it’s a blend of both, providing a testable hypothesis—an exciting outcome.”
The simulations were executed using MIT SuperCloud, and the research received partial support from NASA.
Photo credit & article inspired by: Massachusetts Institute of Technology
