Scientists analyzing measurements from several lunar orbiters have identified a massive, highly magnetized rock formation hidden beneath the far side of the Moon. The discovery, reported in a September 23 article in Science Advances, provides fresh evidence that the Moon once possessed a magnetic field comparable to a fraction of Earth’s present‑day field.
Discovery of a buried magnetic anomaly
The feature, located in a region named Dewar that cannot be seen from Earth, spans roughly 60 kilometers (37 miles) in width and extends about 9 kilometers (5.6 miles) deep. Researchers combined magnetic data with gravity measurements from NASA’s Lunar Prospector (1998), Kaguya (2007) and GRAIL (2011) to determine both the density and magnetization of the structure. The resulting model indicates a rock body that is denser and more strongly magnetized than the surrounding material, consistent with an ancient volcanic complex that solidified while still under the influence of a global magnetic field.
According to co‑author Anna Mittelholz, a lecturer at ETH Zurich, the magnetic signature suggests the Moon’s field at the time of formation—about 4.2 billion years ago—was between one‑fifth and one‑third the strength of Earth’s current field. This estimate places the ancient lunar dynamo well before the field’s known decline around 3.2 billion years ago, when the Moon’s interior cooled and solar radiation began stripping away any residual atmosphere.
Implications for the lunar dynamo debate
The existence of a strong early magnetic field has been contentious since Apollo samples first revealed magnetization signatures in the 1970s. While many researchers have long believed a lunar dynamo operated roughly between 4.25 and 3.5 billion years ago, newer analyses of Apollo rocks have sometimes failed to detect a clear magnetic signal for parts of that interval, prompting some to question whether a dynamo ever existed.
By relying on orbital observations rather than surface samples, the new study sidesteps the complications of handling rocks that have been removed from their geological context for over half a century. Mittelholz notes that laboratory exposure can alter a sample’s magnetic record through heating, shock, or routine storage, potentially obscuring original signals.
Instead of looking solely at magnetic anomalies, the researchers integrated gravity data to pinpoint the physical structure responsible for the signal. This combined approach allowed them to identify a specific buried formation rather than an ambiguous magnetic imprint that could arise from many rock configurations.
Adrien Broquet of the German Aerospace Center emphasizes that understanding the Moon’s magnetic history is essential for reconstructing its early geological evolution. “The Moon was smaller, fully molten for a long time, and experienced intense early volcanism,” he said. “Magnetic data help us infer how the Moon looked and behaved in its formative era.”
Broader significance for planetary science
The findings may extend beyond lunar research. Mittelholz suggests that confirming a dynamo on a small body like the Moon helps define the minimum conditions required for any rocky or icy world to generate a magnetic field. Such benchmarks could inform models of magnetic activity on bodies such as Mercury, Mars, or exoplanets.
The study also touches on lunar swirls—bright, patchy surface features that often coincide with magnetic anomalies. Lead author Xi Yang, a doctoral student at ETH Zurich, proposes that these anomalies may shield the surface from solar wind, preserving the swirls’ brightness. If validated, the relationship could guide future mission planners in selecting sites that are naturally protected from radiation.
External experts, including Claire Nichols of the University of Oxford, view the work as reinforcing the notion that the Moon generated a substantial magnetic field more than four billion years ago, though the duration and variability of that dynamo remain open questions. Nichols urges continued observations from upcoming missions such as NASA’s Artemis program and China’s Chang’e series to fill remaining gaps.
Overall, the integration of magnetic and gravity data from orbiting probes has shifted the conversation from “did the Moon have a dynamo?” to “how did that dynamo operate?” and sets the stage for future comparative studies of planetary magnetism across the solar system.
Helene Elliott is the Lead Science & Space Reporter at News Raise. She reports on aerospace missions, astrophysics discoveries, quantum research, and environmental technology.
