Callisto is the eighth of Jupiter's known satellites and the second largest. It is the outermost¹ of the Galilean moons.

　　orbit: 1,883,000 km from Jupiter

　　diameter: 4800 km

　　mass: 1.08e23 kg

　　Callisto was a nymph, beloved of Zeus and hated by Hera. Hera changed her into a bear and Zeus then placed her in the sky as the constellation² Ursa Major.

　　Discovered by Galileo and Marius in 1610.

　　Callisto is only slightly smaller than Mercury but only a third of its mass.

　　Unlike Ganymede, Callisto seems to have little internal structure; however there are signs from recent Galileo data that the interior materials have settled partially³, with the percentage of rock increasing toward the center. Callisto is about 40% ice and 60% rock/iron. Titan and Triton are probably similar.

　　Callisto's surface is covered entirely⁴ with craters⁵. The surface is very old, like the highlands of the Moon and Mars. Callisto has the oldest, most cratered⁶ surface of any body yet observed in the solar system; having undergone little change other than the occasional impact for 4 billion years.

　　The largest craters are surrounded by a series of concentric rings which look like huge cracks but which have been smoothed out by eons of slow movement of the ice. The largest of these has been named Valhalla. Nearly 3000 km in diameter, Valhalla is a dramatic example of a multi-ring basin, the result of a massive impact. Other examples are Callisto's Asgard, Mare⁷ Orientale on the Moon and Caloris Basin on Mercury.

　　Like Ganymede, Callisto's ancient craters have collapsed⁸. They lack the high ring mountains, radial rays and central depressions common to craters on the Moon and Mercury. Detailed⁹ images from Galileo show that, in some areas at least, small craters have mostly been obliterated¹⁰. This suggests that some processes have been at work more recently, even if its just slumping¹¹.

　　Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight line. This was probably caused by an object that was tidally disrupted as it passed close to Jupiter (much like Comet SL 9) and then impacted on Callisto.

　　Callisto has a very tenuous¹² atmosphere composed of carbon dioxide.

　　Galileo has detected evidence of a weak magnetic field which may indicate some sort of salty fluid below the surface.

　　Unlike Ganymede, with its complex terrains¹³, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently¹⁴ has a much simpler geologic¹⁵ 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.