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.
In brief
- Rock samples from the Moon paint a contradictory picture: some point to a strong, ancient magnetic field, while others do not.
- Researchers at ETH Zurich used gravity and magnetic field measurements from a specific region on the far side of the Moon to determine the structure and magnetisation of the lunar crust beneath the surface there.
- Based on these findings, the researchers conclude that 4.2 billion years ago the Moon had a magnetic field stronger than ten microtesla, which corresponds to between one-fifth and one-third of the current strength of the Earth's magnetic field.
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.
A window into the internal structure
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.
