We've been gazing at the Moon for a long time, yet it's still mysterious. We've sent numerous orbiters and landers to our satellite, and even brought some of it back to our labs. Those rocks only presented more mysteries, in some ways. Lunar rocks are magnetic, yet the Moon doesn't have a magnetosphere. How did this happen?
Lunar samples and data from lunar orbiters both show that some of the Moon's surface rocks are magnetic. This remnant magnetism is more prevalent on the lunar far side. Magnetism is usually explained by a planet's magnetosphere.
Earth's magnetosphere is generated by a rotating core of molten material, mostly iron and nickel. The rotation and convection create magnetic fields that shelter Earth from the Sun and lend magnetism to rocks. But the Moon doesn't have a molten core or a magnetosphere. So, how are its surface rocks magnetized?
New research in Science Advances might have the answer. It's titled "Impact plasma amplification of the ancient lunar dynamo," and the lead author is Isaac Narrett. Narrett is a graduate student in the MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS).
One of the first lunar samples is Sample 10003, collected by Apollo 11. It's about 3.9 billion years old, which helped establish the Moon's age. It also shows signs of magnetization.
This is a photograph of Apollo 11 lunar sample 10003, which is notable as one of humanity's first lunar samples. It helped confirm the Moon's age and revealed evidence of ancient lunar magnetization. Image Credit: NASA/LPI
Orbiters equipped with magnetometers have also detected surface magnetism. The Lunar Prospector, Kaguya, and others have detected remnant magnetism from the Moon's ancient past.
Scientists think the Moon likely had an interior dynamo like Earth's in its ancient past that has since cooled. It would've created a magnetic field that left an imprint on rocks. However, the Moon is much smaller than Earth, and there are questions around its ability to sustain a strong dynamo for a long enough time to explain the remnant magnetism in surface rocks.
This map of lunar magnetism is based on data from NASA's Lunar Prospector. Though weak compared to Earth, the magnetism is stronger on the far side. Image Credit: By Mark A. Wieczorek - Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1381296
The new research in Science Advances says lunar surface magnetism has a different cause: an ancient impact.
"Spacecraft magnetometry and paleomagnetic measurements of lunar samples provide evidence that the Moon had a magnetic field billions of years ago," the researchers write. "Because this field was likely stronger than that predicted by scaling laws for core convection dynamos, a longstanding hypothesis is that an ancient dynamo was amplified by plasma from basin-forming impacts."
The researchers ran dozens of impact and magnetohydrodynamic simulations of asteroid impacts on the Moon. They found that a powerful impact created a cloud of ionized particles that enshrouded the Moon. According to the authors, this cloud would've concentrated in a region opposite the impact site. It would've combined with and momentarily amplified the Moon's weak magnetic field, which was 50 times weaker than Earth's magnetic field, creating the magnetism detected in surface rocks.
This entire event was short-lived and lasted only about 40 minutes.
This figure shows how an ancient impact could've generated a cloud of plasma that concentrated opposite the impact site. The left column shows the plasma density, the center column shows magnetic field strength, and the right column shows the solar wind, the plasma, and the magnetic field. Image Credit: Narrett et al. 2025. Science Advances. https://creativecommons.org/licenses/by-nc/4.0/
A region of highly magnetized rocks was found on the Moon's far side near the south pole. This region is opposite one of the largest impact craters in the entire Solar System: Mare Imbrium. Mare Imbrium was most likely created when a protoplanet slammed into the Moon about 3.9 billion years ago.
"There are large parts of lunar magnetism that are still unexplained," said lead author Narrett in a press release. "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."
This process needed help from a pressure wave to create the magnetism. An Imbrium-sized impact would've created a pressure wave that quickly travelled to the Moon opposite the impact. "This process, coupled with impact-induced body pressure waves focusing at the antipode, could produce magnetization that can account for the crustal fields observed today," the authors write in their research article.
The shock would've been powerful enough to briefly disrupt the rocks' electrons. Since electrons orient their spins to external magnetic fields, they took on a new orientation to the momentary magnetic field, which is preserved as magnetism in the rocks today.
"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 co-author Benjamin Weiss from MIT's Department of Earth, Atmospheric, and Planetary Sciences. "When the cards settle back to the ground, they do so in a new orientation. That’s essentially the magnetization process."
These results could put an old debate about lunar rock magnetism to bed. Scientists have wondered if an impact caused it or if the Moon's ancient magnetic field is the cause. This research shows that it may have been an impact and a magnetic field working in tandem.
"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?" said co-author Rona Oran, also from MIT's Department of Earth, Atmospheric, and Planetary Sciences. "And here we’re saying, it’s a little bit of both. And it’s a testable hypothesis, which is nice."
"The short-lived impact-generated plasma amplification of a lunar dipole field provides a mechanism to explain some of the strongest and most spatially extensive crustal anomalies on the Moon," the authors write in their conclusion. "
These days, it seems like there's a steady stream of traffic to the Moon. As more landers visit it and bring home more samples, this theory will be tested.
"Current and future lunar sample return and magnetometer missions, like the Endurance rover and the Chang'e-6 lander, can explore and sample the Imbrium and Serenitatis basin antipodes (SPA region) and search for evidence of SRM (Shock Remnant Magnetism) from ancient lunar dipole antipodal amplification," the researchers conclude.
Press Release: Why are some rocks on the moon highly magnetic? MIT scientists may have an answer
Research: Impact plasma amplification of the ancient lunar dynamo