The Moon today is almost devoid of a magnetic field. The current magnetic field measured on the surface is 1 000 times weaker than that on the Earth's surface. However, the lunar rock samples brought back to Earth by the Apollo missions (1969 and 1972) show a high level of magnetisation, suggesting that they were formed in the presence of a very strong magnetic field, similar in strength to the Earth's magnetic field.
But a body the size of the Moon should not be able to generate a field as strong as that of the Earth.
Intermittent magnetic field
Scientists at Stanford University's Department of Geophysics speculated that the lunar magnetic field might not be constant. The Moon would then have produced an intense field only intermittently. They therefore set out to investigate the mechanisms that might underlie this hypothesis.
Their idea is that lunar rocks could have sunk into the mantle, right down to the core, each time causing convective movements. Scientists assume that at the beginning of its life, the Moon was covered with molten rock. As this magma began to solidify, the denser minerals naturally sank to the bottom. The remaining liquid magma, rich in titanium and heat-producing elements (thorium, uranium, potassium), took longer to solidify. As these titanium blocks are relatively cold compared to the core, the temperature difference would have been sufficient to boost convection movements, giving rise to a strong magnetic field at the surface for about a century.
The hypothesis is quite credible, as it explains not only the magnetic signature detected in the lunar samples brought back to Earth, but also the fact that it differs from one sample to another.
Study published in Nature Astronomy