It leaves no DF at all unless structures or fleet are on it during the attack.

How do you get a moon anyway?

You get a moon by chance. Look at the end of a Combat Report and it will say "Probability of a moon forming: X%" If it forms it will form directly after the battle, the length that you keep the DF around has nothing t do with it. However, the larger the DF formed from the attack (Basically, the more epic the fight) the higher chance you have of a moon forming, with a max of 20%.