Ganymede

Ganymede is a moon in the Sol System, it is the third moon of the planet Jupiter and the largest moon in the entire system. Under large domes the surface ice is melted to form cool lakes filled with fish and water plants. The domes are opaque, lit on the inside by massive lighting arrays. All ten million people live along the edge of the domes, in fishing towns.

The moon's economy is based off of supplying Jovian gas harvesters with food and storing gas. also most major gas harvesting companies in the UNCA have their headquarters on Ganymede.

Colonization
Orbital colonies around Jupiter were the first human structures in the Jovian system, hauled in from Mars mostly. They were useful in the initial mapping out of Jupiter's storm patterns and as docks for gas harvesters. unfortunately they were dependent upon food from Earth. The EU, primary operators of the gas harvesting operations and pioneers of the fusion technology that the gas fueled decided more permanent installations were needed to ensure the uninterrupted flow of fuel gas. Ganymede was chosen as a site for colonization as it was not believed to have an ecosystem like Europa but it still had ample reserves of frozen water. Also the moon's magnetic field offered some protection from Jupiter's punishing radioactivity. When the first dome was completed the space colonies were emptied and their cylinders were converted into orbital gas tanks.