My present theory is that Ganymede formed partly from the oceans of Maldek (Asteroid Belt) when it was nuked out of existence

 I can imagine the oceans leaving the planet as it flew apart and in space with no gravity water would tend to join together while circling the gravity of all the pieces of maldek (the asteroid belt) after a nuclear war blew it up basically 65 million years ago. Then a piece of this blown up planet came to earth and killed all the larger dinosaurs like T-Rex. (which seems sort of convenient for the survivors of Maldek to come settle here on earth?