Callisto is the eighth and outermost of Jupiter's known moons and the second largest. It is also the third largest moon in the Solar System and only a little smaller than Mercury.
This moon is the most heavily cratered object in the solar system, and Its interior is probably quite similar to Ganymede, Jupiters’ largest moon, except the inner rocky core is smaller and is surrounded by a large icy mantle.
Callisto orbits beyond Jupiter's main radiation belts. is the outermost of the planets moons.
It was first observed by Galileo Galilee and Simon Marius in 1610.
The name Callisto comes from the beautiful daughter of Lycaon, who followed the chaste goddess of the hunt, Artemis. Unfortunately, Zeus seduced Callisto was seduced by Zeus and became pregnant she was banished by Artemis. In fear of her safety, Zeus turned Callisto into a bear to shield her from his wife Hera's jealousy. Later, Zeus placed Callisto and their son in the heavens, and then mother and son became known as Ursa Major and Ursa Minor, also known as Great Bear and Little Bear.
Callisto doesn’t seem to have very much internal structure; however scientists have discovered there are signs that for billions of years very little has changed except for the occasional impact.
The percentage of rock increases toward the center of the moon, which is about 40% ice and 60% rock/iron.
It has an ancient surface like that of the Mars and the Earth’s moon. The largest craters are surrounded by a series of rings that look like huge cracks but which have been smoothed by periods of slow progression of the ice. The largest of these craters is named Valhalla and is approximately 1864 miles in diameter.
Another interesting feature is Gipul Catena, a chain of long impact craters lined up in a straight line, and was probably caused by an object that passed close to Jupiter and impacted with Callisto.
Callisto has a very fragile atmosphere that is mainly composed of carbon dioxide.
Scientists detected evidence of a weak magnetic field, which may indicate some type of salty liquid underneath the surface.
There is practically no evidence of tectonic activity on Callisto. While Callisto and Ganymede are very similar, it apparently has a much simpler history.