Which Galilean moon has the lowest density?

Among the four Galilean moons, Callisto has the lowest density. Its density is about 1.83 grams per cubic centimeter, lower than Ganymede, Europa, and Io because Callisto contains a comparatively large proportion of water ice mixed with rock.

Callisto is the outermost of the Galilean moons and the second-largest, after Ganymede. Galileo Galilei and Simon Marius independently observed the four moons in 1610, transforming the argument that every celestial body orbited Earth. Callisto's distance from Jupiter also places it outside the strong orbital resonance linking Io, Europa, and Ganymede.

That weaker tidal interaction means Callisto experiences far less tidal heating than the inner Galilean moons. Its ancient surface is heavily cratered, and the enormous Valhalla impact basin spans roughly 3,000 kilometers. Beneath the surface, magnetic observations have suggested the possibility of a salty subsurface ocean.

A common mistake is to equate lowest density with smallest moon. Callisto is actually larger than Io and Europa. The key comparison is composition: Io is rockier and densest, while Callisto's mixture of roughly equal rock and ice makes it the least dense member of the quartet.