The far side of the moon provides clues to a past magnetic field

The far side of the moon has revealed evidence of a past magnetic field, according to a study published in *Science Advances*. The researchers, including ETH Zurich scientists Xi Yang and Anna Mittelholz, analyzed data from lunar orbit probes, such as NASA's *GRAIL* and *Lunar Prospector*, to detect magnetic field anomalies. They found that the Dewar region, on the far side, has a strong magnetic field anomaly and a gravity anomaly, suggesting a dense, magnetized rock body beneath the surface. The study concludes that 4.2 billion years ago, the moon had an internally generated magnetic field. The researchers did not rely on rock samples but on orbital data, and they created an accurate model of the subsurface using gravity and magnetic field measurements. The region contains a 60 km wide, 9 km deep rock body that is denser and more magnetized than its surroundings. The magnetic field strength is estimated to be at least 10 microtesla, much stronger than Earth's current field of 50 microtesla. The study rules out the possibility that a single impact caused the magnetic field, as the Dewar region is outside the areas considered likely candidates. The findings provide insights into the moon's internal structure and the mechanisms that could have generated a strong magnetic field. The study also suggests that the magnetic field could have played a role in protecting the surface from solar wind, as indicated by the presence of lunar swirls. The results may help future lunar missions and provide information about planetary dynamos.

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