The paleoinclination of the ancient lunar magnetic field from an Apollo 17 basalt

Abstract Paleomagnetic studies of Apollo samples indicate that the Moon generated a core dynamo lasting for at least 2 billion years. However, the geometry of the lunar magnetic field is still largely unknown because the original orientations of essentially all Apollo samples have not been well-constrained. Determining the direction of the lunar magnetic field over time could elucidate the mechanism by which the lunar dynamo was powered and whether the Moon experienced true polar wander. Here we present measurements of the lunar magnetic field 3.7 billion years (Ga) ago as recorded by Apollo 17 mare basalts 75035 and 75055. These samples formed as part of basalt flows in the Taurus-Littrow valley that make up wall-rock within Camelot crater, now exposed at the rim of the crater. Using apparent layering in the parent boulder for 75055, we inferred its original paleohorizontal orientation on the lunar surface at the time of magnetization. We find that 75035 and 75055 record a mean paleointensity of ~50 µT. Furthermore, 75055 records a paleoinclination of 34 ± 11°. This inclination is consistent with, but does not require, a selenocentric axial dipole field geometry (i.e., a dipole in the center of the Moon and aligned along the spin axis). Additionally, although true polar wander is also not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies are consistent with our measured paleoinclination.