I think a lot of the maps in US2 are partly fictional (They look like what the actual planet does, but aren't exact) And yes, Jupiter, Earth, Mercury and Neptune are based on real data, at least for the most part, I'm not sure (I know for a fact Earth is 100% correct on landmass colors and shapes and the such) but others, like Saturn, are probably highly fictional since it changes and so it wouldn't be feasible to keep changing the textures (Though we'll be able to in the net few updates choose our own textures...yay!!!)
Although I do remember seeing Saturn's texture having the hexagon at some point.