Two of Jupiter’s 63 moons are thought to be “twins.” The two moons were formed at the same time and are the same shape and size, but they are far from identical.
While these two moons are very similar, Callisto and Ganymede also have their differences. Ganymede has an ice layer on top of its rock layer. Callisto’s surface has rock and ice too, but instead of forming layers, the ice and rock spread across this spotted moon in a mixture.
Scientists also believe that Ganymede is hotter than Callisto is because four billion years ago, comets and asteroids entered Jupiter’s gravitational pull. Because Ganymede is closer to Jupiter, it sustained twice the impact from these comets as Callisto. Eventually, the heat of these comets changed Ganymede’s structure: its ice melted and its rocks fell to the moon’s core because of the impact. This movement of rocks from the surface to the core decreased the moon’s gravitational energy, which caused the ice to melt even more. The rocks continued to move until all of the rocks were surrounding Ganymede’s core, and the ice then refroze on the surface.
Since Ganymede took most of the heat from the comets, Callisto was not damaged as much as Ganymede. So it still stands as rock and ice. In this way, you might even say that Callisto was saved by its twin Ganymede.
[Sources: NASA; Science Illustrated]