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

Palaeomagnetic studies of Apollo samples indicate that the Moon generated a magnetic field for at least 2 billion years1,2. 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 ago as recorded by Apollo 17 mare basalts 75035 and 75055. We find that 75035 and 75055 record a mean palaeointensity of ~50 μT. Furthermore, we could infer from the magnetization direction of 75055 and the layering of its parent boulder that the inclination of the magnetic field at the time was 34 ± 10°. Our recovered inclination is consistent with, but does not require, a selenocentric axial dipole (SAD) field geometry: a dipole in the centre of the Moon and aligned along the spin axis. Additionally, although true polar wander is not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies4–6 are consistent with our measured paleoinclination. Basalt samples from the Moon gathered during the Apollo 17 mission hold information on the lunar magnetic field as it was 3.7 billion years ago. Its mean intensity was ~50 μT and its inclination 34 ± 10°. Such results suggest that the lunar dynamo was active at the time and was axially aligned and dipolar.