Which Galilean moon is best to live on?

Answer 1

For humans, I'd say Callisto. Callisto is the farthest of the Galilean moons from Jupiter, and orbits outside Jupiter's powerful radiation belts, so it would be the least dangerous to land on. It also has ample water ice, and possibly even an underground ocean (as do Europa and Ganymede), so it could be a useful resource for us.

For any kind of life though, I'd say Europa. Europa almost certainly has a thick underground ocean with more water than all of Earth's oceans combined - we're more certain of this than we are for Ganymede and Callisto, and their oceans are locked away under much more ice than Europa's, so they'd never get any sunlight. Europa's core is also thought to be hotter than Ganymede and Callisto's, as it is closer to Jupiter and gets flexed more by its gravity. Underwater volcanoes, hot springs and black smokers could provide havens for bacteria in Europa's warm oceans. Not a good place to set up a human colony, though! And I think we shouldn't do that anyway, just in case we contaminate Europa's ocean and then accidentally "discover" our own "alien life" down there...

Answer 2

Europa is often considered the most promising Galilean moon to potentially support life due to its subsurface ocean. This moon has a thin icy crust that may harbor microbial life in its subsurface ocean, shielded from harmful radiation and extreme temperatures on the surface.