Callisto is the second-largest moon of Jupiter, right after Ganymede, and the third largest in the entire Solar System. It is roughly the size of Mercury, at a third of its mass, and is essentially a dead space, due to a lack of tectonic activity. Its frozen surface is virtually entirely defined by aeons of impacts and bombardment. However, the existence of a subterranean sea suggests that it might actually harbor life, despite the hostile surface and a thin atmosphere of carbon dioxide. It is commonly considered to be a candidate for colonization, thanks to low radiation levels and likely existence of water.
Colonization[]
The moon is colonized as of 2320, by the United Jupiter Company that operates a maximum security prison on its surface. The Black Iron Prison houses criminals in conditions comparable to a concentration camp, with security and compliance enforced both by automated robots and a crushing sense of despair and isolation in the far-flung reaches of the solar system. The moon is heavily excavated by the corporation and the prisoners are, apparently, subjected to experiments and exposure to whatever the corporation dredges up.