Decline of the lunar core dynamo

•The Moon generated a core dynamo until at least 3.56 billion years ago (Ga).•The magnetic field declined sharply from ∼70 μT at 3.56 Ga to <∼4μT by 3.19 Ga.•This field decline suggests the rapid decline of at least one dynamo power source.

Recent analyses of Apollo samples have demonstrated that a core dynamo existed on the Moon between at least 4.25 and 3.56 billion years ago (Ga) with surface field intensities reaching ∼70 μT. However, it is unknown when the Moon's magnetic field declined. Determining the temporal evolution of the dynamo is important because it constrains secular changes in power at the lunar core-mantle boundary and, by implication, the Moon's thermal and orbital evolution and the field generation mechanism. Here we present paleomagnetic data from several younger mare basalts which demonstrate that the surface magnetic field had declined precipitously to <∼4μT by 3.19 Ga. It is currently unclear whether such a rapid decrease in field strength reflects either the cessation of the dynamo during this period or its persistence beyond 3.19 Ga in a drastically weakened state.