Ganymede is the largest Galilean moon and the largest moon in the entire Sol System.
Like the rest of the Galilean moons, Ganymede formed around four billion years ago in Jupiter's orbital plane. The moon originally was a rocky body which was eventually covered in a thick ice sheet, the lower layer of which melted and became a salty underice sea due to the influence not only of Jupiter's powerful magnetic field but also due to the hydrothermal vents located on the lunarwide ocean floor.
Unlike Europa, however, life was unable to evolve in Ganymede's own oceans despite the fact that it is the only moon with its own natural magnetic field.
The moon was first thoroughly explored by the ESA's Jupiter Icy Moons Explorer in 2032 where the probe confirmed the existence of a salty ocean was confirmed by the probe.
Humans first set foot on Ganymede during the Great Solar System Tour.
When the Sol System inhabitants clamored for the terraforming of Europa, many also encouraged the terraforming of Ganymede and Callisto. In 4890 Terragen began terraforming Ganymede and Callisto, causing an explosion of jobs in the Sol System. Using lessons learned during Europa's initial terraforming stages, the moon was terraformed in a nearly identical way but with less regard for any life as none existed on the moon. Terraforming was complete in 5140.
Most Ganymedians have undergone genetic manipulation to become similar to mythical merfolk and have insisted that non-aquatic humans refer to them as such. They live in massive underground cities (the largest of which is New Atlantis), living off of the ocean lifeforms that have grown to call this moon their home.
The rest live on floating cities that spend their days fishing out of their artificial freshwater lakes and out of the ocean.
Like most oceanic worlds, its climate is very warm and predictable as there are no landmasses to block air currents.