[uncertainquark]

That's a good question! The sources of lunar water are poorly understood.

Some of it is formed as a result of hydrogen ions in the solar radiation hitting the oxygen present in the surface and forming water, and then being transported to the permanently shadowed regions. Another source, and likely a major one, is asteroids and comets, which were impacting vigorously during the early solar system.

But to truly differentiate between the sources of water, sample are needed. This hasn't been done yet but will open a huge box of mysteries and also give insights into how the Moon formed.

[ShamelessC]

Wasn't the moon originally created due to a collision with the earth? Was there not any water for Earth to share with the moon when that happened?