An unusual rock buried on the far side of the moon is providing fresh clues for a longstanding scientific puzzle: the ancient lunar magnetic field.
Analyzing data from lunar probes, a team of researchers found a rock formation that was much denser and more strongly magnetized compared with the surrounding material. The subterranean feature could be an ancient volcanic complex that became magnetized when the magma rose from the moon’s interior and solidified before reaching the surface, according to a new study published September 23 in the journal Science Advances.
The characteristics of the large rock body, measuring about 60 kilometers (37 miles) wide and around 9 kilometers (5.6 miles) deep, suggest that the moon once had a magnetic field when the rock formed about 4.2 billion years ago. The field was likely between one-fifth and one-third the size of Earth’s own magnetic field currently active today, the study authors noted.
Life on Earth as we know it likely wouldn’t be possible without its magnetic field. It creates the magnetosphere, a vast region in space that surrounds our planet. This celestial shield keeps harmful solar wind, cosmic rays and radiation at bay, preventing the atmosphere from being stripped away.
Molten metal constantly churning in Earth’s outer core generates electrical currents that sustain the magnetic field, in a process called the dynamo effect. The mechanical principle is similar to a bicycle dynamo, which uses the bike’s rotating wheel to spin magnets inside the generator and produce an electrical current to power lights.
The moon may also have had a core-powered magnetic field in its past, but that activity has long ceased. About 3.2 billion years ago, as the moon’s interior cooled down, the field significantly weakened, according to NASA. Without strong protection, solar radiation ripped apart the moon’s atmosphere.
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The ancient magnetic field’s formation and existence, however, has long been the subject of discussion in astronomy. “The debate stretches back to the 1970s, when the Apollo missions first returned lunar rocks that showed signs of having recorded a magnetic field,” said study coauthor Anna Mittelholz, a lecturer at the Swiss Federal Institute of Technology Zurich, or ETH Zurich.
For a long time, Mittelholz added, experts broadly agreed that the moon’s past dynamo was active from roughly 4.25 billion to 3.5 billion years ago. “This is probably still what the majority of the community believes, but in the last decade, new analyses of Apollo samples have found no magnetic signal at all for parts of that same period,” she wrote in an email, “and some researchers have gone as far as questioning whether an early dynamo existed.”
The new study brings evidence for that early dynamo, but without using samples from the Apollo missions. Instead, researchers used data collected from orbit by lunar probes to analyze a region not visible from Earth called Dewar, where they detected the magnetic rock.
Pinning down the history of the moon’s past magnetic field is key to understanding the moon itself, according to study coauthor Adrien Broquet, a researcher at the German Aerospace Center in Berlin.
“Earth and the moon had drastically different geological histories. The moon was smaller and fully molten for a long time. It also had a lot of volcanism early on, and there are many questions we cannot exactly answer if we don’t know how the moon looked like back then,” Broquet said. “Analyzing the magnetic field of the moon is a good way to estimate how the moon looked like early on.”
The findings could potentially extend beyond the moon, helping scientists study other celestial bodies, Mittelholz added. If the moon “managed to run a dynamo for any length of time, that tells us something about the minimum conditions needed to sustain a dynamo,” she said. “Getting its history right helps calibrate what we think is possible for other small rocky and icy bodies too.”
A rocky debate over Apollo lunar samples
The researchers used data from NASA probes, including Lunar Prospector, launched in 1998; Kaguya, launched in 2007; and GRAIL, launched in 2011.
One advantage of using orbital data instead of moon rocks is that the samples were pulled out of their geological context more than 50 years ago, according to Mittelholz.
“Their magnetic record can be disturbed quite easily through heating, shock, or even routine handling and storage in a lab,” she said. “So two samples that look similar can end up giving different answers, not necessarily because the moon’s field wasn’t there, but because the evidence itself has had a lot of chances to get scrambled along the way.”
Researchers have used orbital data before to investigate the lunar magnetic field, but the new study takes a different approach.
“Other magnetic studies using satellite data sets generally look at the magnetic signal by itself and try to infer what kind of rock might have produced it,” Mittelholz said. “We instead combined magnetic data with gravity data in one model.”
Gravity measurements described the density of the unearthed rock, while magnetism informed how strongly it’s magnetized, she explained.
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“Together they let us pin down an actual buried structure rather than just a signal that could be produced by many magnetized rock configurations,” she said.
The researchers were unable to estimate how long the active dynamo could have existed and said that it’s unclear how the small lunar core could have generated a magnetic field potentially as strong as Earth, as previous studies have suggested. However, the authors of the new study said they believe that their findings shift the debate from whether the moon had a dynamo to how it worked.
Earlier studies have pointed to different hypotheses about the formation of the lunar magnetic field. A 2025 paper indicated that an impact from a large asteroid could have temporarily strengthened an existing, weaker magnetic field. A February study based on Apollo samples suggested instead that the melting of rocks rich in titanium deep inside the moon temporarily boosted the satellite’s magnetic field, making it flicker over time.
As part of the new study, the researchers also observed something peculiar about lunar swirls, patches of the moon’s surface that are brighter than their surroundings. The swirls are often paired with magnetic anomalies, or regions where the rock is magnetized, the study found.
“The leading hypothesis is that these magnetic anomalies deflect solar wind and protect the surface from getting weathered by it,” said lead study author Xi Yang, a doctoral student in the department of Earth and planetary sciences at ETH Zurich.
While this observation is only preliminary, Yang added, it could mean that in the future, the swirls may help identify areas of the moon’s surface that are more protected from harmful solar wind, which would be valuable for lunar missions.
Magnetic fields of the moon and beyond
The new study further supports the idea that the moon was able to generate an intense dynamo early in its history, according to Claire Nichols, an associate professor of geology of planetary processes at England’s University of Oxford. She did not participate in the new study.
However, a lot more work is needed to understand for how long the moon could generate a dynamo, and how much that dynamo’s activity varied, Nichols added in an email.
“Overall, I think this study provides further motivation to keep adding to our observations during upcoming missions such as Artemis and Chang’e — there are still a lot of open questions about the moon we are yet to figure out,” she said, referring to lunar exploration programs from NASA and the China National Space Administration, respectively.
The new research also adds to the growing body of evidence that the moon once possessed a stronger magnetic field more than 4 billion years ago, said Isaac Narrett, lead author of the unrelated 2025 study on the lunar magnetic field. Narrett, a doctoral student in the department of Earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology, was not involved in the study.
By using data collected by orbiting lunar probes, Narrett added, the research is leaving room for future work while “laying the groundwork for uncovering clues of planetary evolution with other gravity and magnetic datasets of the moon, Mercury and Mars.”
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