Ganymede

The largest and most densely settled of Jupiter's icy Galilean moons, Ganymede was colonized following its successful terraformation in the 2140s, and quickly became a preferred destination for immigrants from Earth due to its low cost of living. The moon's large size, uniform shape and dense metallic core made altering the surface gravity with warp fields easier and cheaper than it was even on Earth's own moon, and so people kept flocking to it, seeking escape from the destitution of Earth's lower levels. The city on Ganymede continued to spread out until it covered the entire surface of the moon, and then it built upwards in order to accommodate for the still-growing population. By the 2200s, Ganymede was already the second most populated world in Sol, right behind Earth and just in front of Titan. Though the planet does not have multiple layers like Earth, it does have towering skyscrapers the width of entire city blocks that stretch far into the sky and connect to one another with bridges and ramps. Ganymede is also the most important industrial hub in the Jupiter system, as it has the single largest geothermal power plant ever constructed, which was created using metallic material dredged up from the planets core. Ganymede has similar, but less extreme economic conditions to Earth and Luna. The very wealthy live in mansion-sized penthouses at the tops of skyscrapers, looking down on the poor below them, who huddle together at or near street level. Meanwhile, the majority of the population lives in between.