A dense magnetized body under the Moon's far side argues for an early core dynamo

An orbital magnetometer registers magnetized rock somewhere below the surface. It does not say what that rock is, how deep it goes, or when it cooled. Most lunar magnetic anomalies measured from orbit have sources nobody has identified ^[11]. Gravity supplies the other half of the identification. "The gravity data, however, give us insight into the density and thus into the material beneath the surface," said Anna Mittelholz, a geophysicist and lecturer at ETH Zurich ^[9][25]. "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," she said ^[10]. The ETH account calls this the first accurate model of the subsurface built by processing gravity and magnetic field data jointly ^[12].

The model puts the body at about 60 kilometres wide, reaching roughly 9 kilometres down ^[13], close to seven times wider than it is deep ^[22]. Its age of 4.2 billion years comes from the various deposits of impact material on the lunar surface ^[15].

Impact magnetization is the standing alternative: a strike by a massive meteorite or asteroid setting off magnetization processes in lunar rock ^[5]. At Dewar the authors exclude that on geography, because the region falls outside the areas considered possible candidates ^[18]. Both steps, the identification of the body and the exclusion of a shock origin, are inferences from the anomaly's geological setting ^[14][18]. "We can therefore be almost certain that the magnetic field must originate from a longer-lasting dynamo generated in the core," said Xi Yang, a doctoral student in the same department ^[19][25].

The field estimate runs through the iron in the rock. "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. For me, that is a very important finding," Mittelholz said ^[16]. Yang said the field was "very likely stronger than 10 microtesla", against around 50 microtesla for Earth's field today ^[17], a floor one fifth of the present terrestrial field ^[21].

The study pins one structure at one moment. Mittelholz said some researchers place a strong field across the interval from 4.25 to 3.5 billion years ago ^[3], a span of 750 million years ^[23], and Dewar's 4.2 billion years sits about 50 million years into it ^[24]. Yang and Mittelholz say it remains unclear how the small lunar core could have produced a field that strong ^[20].