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The far side of the Moon provides clues to a previous magnetic field
The question to whether the Moon ever possessed an internal magnetic field is a matter of debate. Now, researchers at ETH Zurich have opted for a new method to show that a mechanism once existed on the Moon that is still active on Earth today.
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.
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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 est