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.

A surprisingly strong magnetic field

“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?”.

Mysterious lunar swirls

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.