This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Unlike Earth, the moon no longer has a core-generated magnetic field. 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 bicycle dynamo, 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 Ph.D. student in the Department of Earth and Planetary Sciences at ETH Zurich. This is because analyses of rock samples brought back to Earth by the Apollo astronauts are 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 magnetized lunar rock: Impacts from massive meteorites or asteroids could have triggered magnetization 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 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. Their study is published in Science Advances.
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," Mittelholz says. This means that this region contains rock that is more strongly magnetized—and denser—than elsewhere.
"That is one of the reasons why this region is a potential window into the moon's internal structure," Yang says. 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 explains.
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