Larger in size than Luna, but darkly colored and not as heavily cratered, Ganymede and Callisto are very similar worlds. Neither is as dense (nor has as much gravity), as their mantles consist of more ice than iron rock. Both possess abundant volatiles and water (albeit frozen), making them ideal candidates for habitation. Ganymede, with its differentiated surface of rocky and icy terrain, has an iron core and thus a faint magnetic eld. Callisto, the smaller of the two, is composed mostly of icy silicate clays. As on Luna, most cities on Ganymede and Callisto are built below ground to shield them from meteor impacts (and, on Ganymede, from Jupiter’s radioactive bombardment).
While within the “protection” of the Jovian Republic, both moons are a patchwork of city-states. Some are full members of the Jovian polity, while others are only tolerated. Ganymede tends to swing more heavily toward the Junta, as its citizens still see the Junta-maintained infrastructure—accurately or not—as necessary in such a hostile environment. Callisto, outside the worst radioactive effects of the Jovian magnetosphere, is an easier place for techno-progressivism to gain a foothold.