So, Callisto has several advantages over the inner three moons, which is why she was picked for the Colony. First, because she is in an orbit almost two million kilometers out from Jupiter, the high energy particles generated in Jove’s magnetosphere that bathe the other three moons are much less intense. That is an important feature because even with our magnetic shielding and all, when we are on the surface, some of that stuff still gets through and can be bad for the DNA. Less is better when it comes to radiation. Second, Callisto is geologically pretty quiet when compared to the others. The tidal forces generated by Jupiter’s powerful gravity field are not present in sufficient strength to cause internal heating of the type found in varying degrees inside the other moons. Without the heat, there are no volcanoes, or tectonic activity to cause earthquakes. Callisto had about as much geological action as Earth’s moon, which is not much. Third, the old girl had water and lots of it. There was ice scattered around on the surface, permafrost below the surface and an underground ocean about 50 kilometers down that was like something out of Jules Verne. Of course water gave us what we needed for the fuel cells, allowed us to make propellants for the point to point rockets we used for our exploration of Callisto and when distilled, provided many valuable minerals and salts.
Excerpt from Chapter IV: Jupiter Space