Photo credit: Mark A. Wieczorek
Some Moon rocks brought by Apollo's teams were magnetic in nature, that means that the moon had it's own magnetic field (once upon a time) not like now, deprived of magnetic field.
A Magnetic field is generated by the what’s called a dynamo, which is caused by the fluid motion of a conducting material, such as liquid iron. In the case of the Earth’s magnetic field, this motion occurs in the planet’s outer core, and is caused by the convection of heat.
But the moon isn’t large enough for convection to take place.
the moon’s solid-rock middle layer, called its mantle, stirs up its liquid iron core. The researchers think this happens because the moon’s core and its mantle rotate around slightly different axes, and the boundary between them is not quite spherical, so their relative motion causes the fluid to mix around.
The strength of this stirring is determined by the angle between the core and the mantle, and the distance between the Earth and the moon, because the tidal gravitational tug from the Earth causes the moon’s mantle to rotate differently than the coreThis model would explain why the moon used to have a magnetic field, but no longer does. That’s because the angle between the mantle and the core has narrowed over time, while the distance between the moon and the Earth has widened, causing the tidal forces to steadily decrease. While these forces used to be enough to generate a dynamo inside the moon, they aren’t anymore.
Based on their calculations, the researchers estimate the lunar magnetic field might have lasted for about a billion years, somewhere between around 2.7 billion and 4.2 billion years ago.
“The further out the moon moves, the slower the stirring, and at a certain point the lunar dynamo shuts off,”
