It's more interesting to come up with details for each of the Moons of Jupiter. But I couldn't resist creating a d20 table to spur the imagination. Here it is.
Moon 1: Terrain - living growth. This moon is a forest of toadstools, mushrooms, and other fungi. It has one main antagonist type: an evil intelligence. A roll of 3 indicates beholders. Excellent: a fungus forest in which lurks a beholder hive mother and her many drones. They enslave anybody unfortunate enough to land, and are busy constructing a magical portal by which they can invade other worlds.
Moon 2: Terrain - Icy. A glacial ball hovering in the void. Main antagonist types are fomorians and a lammasu. There is a special condition of 'unstable'. Clearly, the lammasu was placed on this moon to protect an ancient artefact, temple, shrine, monument or captive. The fomorians dwell in caverns in the ice, perhaps plotting its conquest - or a way to get off this icy sphere and to somewhere where they can satisfy their cruel lusts.
Moon 3: Terrain - Icy. Another glacier world. Main antagonist type is salamanders. Curioser and curioser. The salamanders are clearly exiles from the elemental plane of fire. Having committed some obscure crime, they have been sentenced to punishment by their efreeti overlords to live in suffering for 1,001 years in severe and unrelenting cold. They are desperate to escape, or - even better - return their home plane to wreak horrible vengeance on their betrayers.
I love D&D.