Callisto is the 8th of jupiter's known satellites and the outermost satellite of the galilean moons. Callisto was discovered by Galileo and Marius in 1610.

Callisto is only slightly smaller then mercury, but less then 1/3 of it's mass.
Unlike Ganymede, Callisto seems to have little internal structure; However there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron.

Callisto is the oldest, most cratered body of all known body's in the solar system.
Callisto has a very tenuous atmosphere composed of carbon dioxide.

Unlike Ganymede, with its complex terrains, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists; (it may be related to the orbital and tidal evolution of Ganymede). "Simple" Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.