Buried volcano beneath the Moon's Dewar swirl points to an ancient field of at least 11.4 microteslas

Each step in the Dewar inference, published in Science Advances ^[15], depends on the one before it. The team first needed a buried rock body that shows up in two separate signals. "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 said ^[3]. From the gravity signal they calculated the density of the material under the surface, and from the density they identified what the material was ^[4]. Rock type then sets an expected iron content. "Because we know how much iron is present in such a rock body, we can estimate the minimum strength the magnetic field must have had as the magma cooled slowly," Mittelholz said ^[6].

The 11.4-microtesla figure is that minimum, and the team says the real field could have been much higher ^[7]. The thing this doesn't tell you is how long the field lasted. Nichols told Discover the results could be consistent with work her group published in Nature Geoscience earlier this year, which found rare but intense magnetic events lasting less than 5,000 years ^[13]. On her reading, the strong field Dewar recorded as it cooled could belong to one of those short episodes.

The argument against impacts is local. Dewar is one of many lunar swirls ^[1], and the team's case against an impact origin is made for this site alone ^[8].

I think the paper makes a good case that this rock cooled in a strong field that an impact is unlikely to have produced ^[7][8]. Mittelholz draws a wider conclusion. "But we are examining the question from an entirely new perspective," she said ^[10]. In her view, the question has shifted from whether there was a dynamo to how it worked ^[11].