What was the shape of the Moon’s ancient magnetic field?

Even though the Moon currently lacks a global magnetic field, its rocks and crust are highly magnetized, as evidenced by samples returned during the Apollo missions. (For an explanation of how that can happen, see the article by David Dunlop, Physics Today, June 2012, page 31 .) Laboratory and spacecraft measurements imply that the magnetization is the result of a (now-extinct) dynamo created in the Moon’s core. The dynamo, which persisted for 2 billion years, would have produced a magnetic field with a surface strength as high as Earth’s. That strength was a surprising finding, considering that the Moon’s core is tiny—with a radius just one-tenth that of Earth’s.

Because the rocks returned during the Apollo missions had been so battered by meteor impacts, however, it’s been difficult to tell what direction the rocks were facing when they were originally magnetized. And that information is important for inferring the type of dynamo that could have generated the field in the first place. Most Apollo samples were taken from loose rocks scattered across the lunar surface.

That’s not the case with every Apollo sample, though. Oxford University geoscientist Claire Nichols , Apollo 17 astronaut Harrison Schmitt, and their colleagues have now gone back to Schmitt’s original Apollo photographs to look for features that reveal how some of those rock samples were oriented when they were part of the lunar bedrock.

The only geologist to have set foot on the Moon, Schmitt, along with Eugene Cernan (shown in the figure next to one of the parent boulders), hammered and photographed two samples—75035 and 75055—directly from the edge of Camelot Crater during the Apollo 17 mission in 1972. Visible on the parent boulders are planar features that Nichols and her colleagues used to discern the boulders’ original orientation and thus the angle of the magnetic field relative to the lunar surface.

The team’s analysis of the two basalt samples supports earlier evidence that 3.7 billion years ago the field was surprisingly strong (approximately 50 µT) and inclined at an angle of 34 ± 10°. (For comparison, Earth’s surface magnetic field near the North Pole is 60 µT.) It also shows that the lunar field’s geometry may have been dipolar—much like a bar magnet and Earth’s magnetic field today. That was completely unexpected. Given the high field strength on the ancient Moon, subsequent calculations show that such a geometry could not have been generated by the same mechanism that drives Earth’s magnetic field. Because the Moon rotates slowly compared with Earth, dynamo models predicted that the field should have been multipolar.

No one knows just how the Moon created such a strong magnetic field. According to one model, the Moon’s rocky outer layer, its liquid outer core, and its solid inner core all rotated at different rates by gravitational stirring. The different rates likely generated friction between the layers, which may have been responsible for a lunar dynamo. Other recent work by the University of Rochester’s John Tarduno and colleagues argues that a lunar dynamo may not have existed at all. (C. I. O. Nichols et al., Nat. Astron., 2021, doi:10.1038/s41550-021-01469-y .)