The scientific controversy stems from decades of contradictory measurements on the rock samples that the Apollo astronauts carried home. On Earth, the churning of liquid iron in the outer core generates a global magnetic field through a process analogous to a bicycle dynamo, which converts mechanical motion into electrical energy. Some researchers who have studied lunar rocks contend that a strong magnetic field operated over a long stretch of time between about 4.25 and 3.5 billion years ago. Others examining similar samples find no evidence of any such field at all. Compounding the confusion, magnetised lunar rock does not automatically prove a dynamo. Massive meteorite or asteroid impacts could, in principle, have triggered magnetisation processes on the Moon, imprinting magnetic signatures in the crust without any internally generated field.

To sidestep the ambiguities of the sample record, geophysicist Anna Mittelholz and PhD student Xi Yang of ETH Zurich’s Department of Earth and Planetary Sciences, together with colleagues at the Institute of Space Research, the German Aerospace Center (DLR) and the Technical University of Berlin, turned to orbital data instead. Their analysis drew on gravity measurements from NASA’s twin GRAIL probes and on magnetic field models constructed from orbital measurements gathered by the Lunar Prospector and Kaguya missions. The target of their scrutiny was a region called Dewar, located on the far side of the Moon, the hemisphere that never faces Earth and one that remains poorly characterised compared with the near-side terrain studied since the Apollo era.

Dewar turned out to be a remarkable stroke of fortune. It hosts one of the strongest magnetic field anomalies on the lunar far side, and that magnetic signal coincides spatially with a distinct gravity anomaly, meaning the region contains rock that is simultaneously more strongly magnetised and denser than its surroundings. In most cases, the origin of magnetic anomalies measured from lunar orbit remains unknown, because a magnetic signal alone cannot reveal what lies beneath the surface. Gravity data, however, provide a window into subsurface density and therefore into the material hidden below. Where the two signals overlap, they can be combined and attributed to a specific geological structure, and Dewar offered precisely that opportunity. For the first time, the team created an accurate model of the subsurface by jointly processing the gravity and magnetic field data.

The model revealed something extraordinary hidden beneath the Dewar region: a buried rock body approximately 60 kilometres wide that extends to a depth of around 9 kilometres. It is much denser than the surrounding lunar crust and, at the same time, strongly magnetised. When the researchers combined this subsurface picture with surface geochemistry and a distinctive arched topography, they concluded that the body is solidified magma that once rose from the Moon’s interior, in other words, a buried volcanic complex. The age of the structure could be pinned down from the various deposits of impact material scattered across the lunar surface around it, yielding an estimate of about 4.2 billion years.

That age and composition allowed the team to perform a calculation with far-reaching implications. Because the iron content of such a rock body is known, the researchers could estimate the minimum strength the magnetic field must have had while the magma cooled slowly and locked in its magnetisation. Their conclusion is striking: the lunar magnetic field at that time was very likely stronger than 10 microtesla. For comparison, Earth’s magnetic field today stands at around 50 microtesla. A young Moon, barely a few hundred million years after its formation, may therefore have been generating a field approaching a fifth of the strength that shields our planet today.

The researchers also ruled out the leading alternative explanation. Impact-generated magnetisation requires specific conditions, and the Dewar region lies outside the areas considered plausible candidates for such processes. This exclusion matters enormously for the dynamo debate, because a magnetised volcanic complex of impact-independent origin, dated to 4.2 billion years ago, is exactly the kind of evidence that has been missing from the contradictory Apollo sample record. As Yang put it, the team can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the Moon’s core. The question, however, is not entirely settled. It remains unclear how the Moon’s small core could have generated such a strong magnetic field, and the ETH researchers do not yet consider the existence of an early lunar dynamo to be fully resolved. What has changed is the framing of the problem. The debate has shifted from asking whether a dynamo existed to asking how it worked, and the researchers are now examining the question from an entirely new perspective.

The study also sheds light on one of the Moon’s most photogenic mysteries: the lunar swirls. These bright, curved or striped patterns stand out sharply against the darker surface around them, and wherever a swirl appears, researchers invariably find a magnetic anomaly. A swirl also decorates the surface in the Dewar region, directly above the buried anomaly. One leading explanation holds that swirls form only where the magnetic field runs horizontally at the surface, as it does at the Dewar Swirl. Such a horizontal field deflects the solar wind, the constant stream of charged particles streaming from the Sun, thereby protecting the surface beneath from space weathering. That protected patch remains brighter than its surroundings, painting the swirl onto the landscape over billions of years.

Beyond their scientific elegance, the findings carry practical weight for the coming era of crewed lunar exploration. Because magnetic field lines could offer protection from solar wind radiation, swirls mark locations where such shielding constellations exist, information that Mittelholz describes as important for future astronauts planning extended stays on the surface. The team’s findings are also intended to help upcoming lunar missions select priority targets for on-site measurements, and they may guide the analysis of newly collected lunar samples in reconstructing the Moon’s magnetic evolutionary history. Perhaps most intriguingly, the method itself is portable. Yang notes that the same technique, combining gravity and magnetic data from orbit to infer the presence of a planetary dynamo, could be applied to other celestial bodies. Mars is an especially tempting case, although data of sufficiently high quality is currently lacking. For now, the buried magma of Dewar stands as a silent witness to a time when the young Moon, like Earth today, generated its own magnetic heart.

Subject of Research: Evidence for an early core-generated magnetic field on the Moon from the Dewar magnetic anomaly

Article Title: The far side of the Moon provides clues to a previous magnetic field

Article References: The far side of the Moon provides clues to a previous magnetic field. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Moon, lunar dynamo, magnetic field, Dewar region, far side of the Moon, GRAIL, Lunar Prospector, Kaguya, volcanic complex, lunar swirls, paleomagnetism, solar wind