Unlike the earth, the Moon no longer has a core-generated magnetic field today. On our planet, the movement of liquid iron in the outer core generates a global magnetic field. This so-called geodynamo works on a similar principle to a dynamo on a bicycle, which converts mechanical motion into electrical energy. “Today, there is an ongoing heated debate as to whether the Moon also operated a dynamo in the past,” says Xi Yang, a PhD student in the Department of Earth and Planetary Sciences at ETH Zurich. This is because the analysis of rock samples brought back to Earth by the Apollo astronauts is contradictory.
“Some researchers assume there was a strong magnetic field that existed over a long period between 4.25 and 3.5 billion years ago, while others, however, find no evidence of this,” says geophysicist Anna Mittelholz, who is a lecturer in the same department. In addition to the dynamo theory, there is a second possible explanation for the magnetised lunar rock: impacts from massive meteorites or asteroids could have triggered magnetisation processes on the Moon.
A study by the two ETH researchers, in collaboration with colleagues at the Institute of Space Research, DLR, and the Technical University of Berlin now supports the dynamo theory. It comes to the conclusion that 4.2 billion years ago – some several hundred million years after its formation – the Moon did indeed possess an internally generated magnetic field. The researchers did not base their findings on rock samples, but on data collected by probes in lunar orbit, such as gravity measurements from NASA’s ‘GRAIL’ probes and magnetic field models drawing on orbital measurements from the Lunar Prospector and Kaguya missions.
The focus is on a specific region called Dewar situated on the far side of the Moon, which we never see from Earth. “The Dewar region is a genuine stroke of luck: one of the strongest magnetic field anomalies on the far side of the Moon and a distinct gravity anomaly coincide spatially there,” as Mittelholz relates. This means that this region contains rock that is more strongly magnetised – while at the same time - denser than elsewhere. “That is one of the reasons why this region is a potential window into the Moon’s internal structure,” as Yang stated.
In most cases, the origin of magnetic field anomalies measured from lunar orbit is unknown. “The gravity data, however, give us insight into the density and thus into the material beneath the surface,” explains Mittelholz. “Where the magnetic field and gravity signals coincide, it is possible to combine the two and attribute the anomaly to a specific geological structure. This is precisely the opportunity that Dewar offered.” And the researchers made the most of it: for the first time, they created an accurate model of the subsurface by jointly processing gravity and magnetic field data.
The result: in the Dewar region, beneath the lunar surface, lies a rock body approximately 60 kilometres wide, extending to a depth of around 9 kilometres. It is much denser than the surrounding crust, while strongly magnetised at the same time. Combined with the surface geochemistry and an arched topography, the researchers conclude that this is solidified magma that has risen from the subsurface – a buried volcanic complex. The age of the structure – 4.2 billion years – can be determined from the various deposits of impact material on the lunar surface.
“Because we know how much iron is present in such a rock body, we can estimate the minimum strength the magnetic field must have had as the magma cooled slowly. For me, that is a very important finding,” as Mittelholz states. And Yang adds: “We have found that the magnetic field on the Moon at that time was very likely stronger than 10 microtesla. On Earth today, the magnetic field strength stands at around 50 microtesla.”
The researchers rule out the possibility that a violent impact could have caused this magnetic field. This is because the Dewar region lies outside the areas that are considered possible candidates. “We can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the core,” says Yang.
It remains unclear, however, how the small lunar core could have generated such a strong magnetic field. For this reason, the ETH researchers do not yet consider the question of the existence of an early lunar dynamo to be fully resolved. “But we are examining the question from an entirely new perspective,” says Mittelholz. And so the question has shifted from “Was there a dynamo?”, to “How did it work?”.
Moreover, the study provides insights into another, puzzling phenomenon, namely lunar swirls. These bright, curved or striped patterns on the Moon’s surface stand out clearly against their darker surroundings. Wherever such a swirl appears, researchers always find a magnetic anomaly. There is also a lunar swirl on the surface in the Dewar region above the observed anomaly. The origin of swirls is a matter of debate.
One possible explanation is that swirls only form where the magnetic field runs horizontally at the surface, as is the case with the Dewar Swirl. The horizontal field deflects the solar wind, thereby protecting the surface from weathering. As a result, this area remains brighter than its surroundings. “This is important information for future astronauts,” as Mittelholz outlines: “Magnetic field lines could offer protection from solar winds, and swirls indicate the locations of such constellations.”
The researchers’ findings are also intended to assist future lunar missions in selecting priority targets for on-site measurements, and they may help in the analysis of lunar samples to draw conclusions about the Moon’s magnetic evolutionary history. “The method could also be applied to other celestial bodies to obtain information about a planetary dynamo based solely on data from orbit,” says Yang. Mars, for example, would be an interesting case, although data of sufficiently high quality is currently lacking.
Science Advances
Magmatic origin of the Dewar magnetic anomaly: Implications for an early lunar dynamo
23-Sep-2026