ETH Zurich Study Points to Strong Ancient Magnetic Field on the Moon

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, a mechanism known as a geodynamo that works on a similar principle to a bicycle dynamo, converting mechanical motion into electrical energy. "Today, there is an ongoing heated debate as to whether the Moon also operated a dynamo in the past," said Xi Yang, a PhD student in the Department of Earth and Planetary Sciences at ETH Zurich. This is because analysis of rock samples brought back to Earth by the Apollo astronauts has proven 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 find no evidence of this," said geophysicist Anna Mittelholz, 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 concludes that 4.2 billion years ago, several hundred million years after its formation, the Moon did possess an internally generated magnetic field. The researchers based their findings not on rock samples but on data collected by probes in lunar orbit, including 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 is never visible 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," Mittelholz said. This means the region contains rock that is both more strongly magnetised and denser than elsewhere. "That is one of the reasons why this region is a potential window into the Moon's internal structure," Yang said.

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," Mittelholz explained. "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." For the first time, the researchers created an accurate model of the subsurface by jointly processing gravity and magnetic field data.

The result: beneath the lunar surface in the Dewar region 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 this is solidified magma that rose 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," Mittelholz said. Yang added: "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, since the Dewar region lies outside the areas considered possible candidates for such impacts. "We can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the core," Yang said.

It remains unclear how the small lunar core could have generated such a strong magnetic field, so the ETH researchers do not yet consider the question of an early lunar dynamo to be fully resolved. "But we are examining the question from an entirely new perspective," Mittelholz said, noting the question has shifted from whether a dynamo existed to how it worked.

The study also provides insight into lunar swirls, bright, curved or striped patterns on the Moon's surface that stand out against their darker surroundings. Wherever such a swirl appears, researchers find a magnetic anomaly, and a lunar swirl is present on the surface above the Dewar anomaly. One possible explanation is that swirls form where the magnetic field runs horizontally at the surface, deflecting the solar wind and protecting the surface from weathering, which keeps the area brighter than its surroundings. "This is important information for future astronauts," Mittelholz said. "Magnetic field lines could offer protection from solar winds, and swirls indicate the locations of such constellations."

The findings are also intended to help future lunar missions select priority targets for on-site measurements, and may assist 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," Yang said. Mars would be an interesting case, though data of sufficiently high quality is not yet available.

CONTACT: https://doi.org/10.1126/sciadv.aec0341