Lunar palaeomagnetism and its implications

The final great impacts, creating the multi-ring basins on the Moon, must have altered its principal axes of inertia and, assuming solid state creep in its interior, caused successive reorientation relative to its pole of rotation. Lunar palaeomagnetism has been explained by an early lunar magnetic field generated by a core dynamo. The palaeomagnetic directions of the lunar crust determined from the Apollo 15 and 16 subsatellite magnetometer surveys, by L.L. Hood and colleagues, challenges interpretation on this idea. The palaeoequators so determined for Imbrium, Nectarian and pre-Nectarian times place impacts of the same age in low latitude: there must have been small moons in the Earth-Moon system, which impacted the Moon in its retreat from the Earth. Sources of presumed Imbrium age are magnetized in agreement with the dipole formula: proving the existence of an early lunar core-dynamo field.