Callisto (astronomy) - astronomy.

Callisto (astronomy) - astronomy. Callisto (astronomy), large satellite of the planet Jupiter. The moon orbits Jupiter at an average distance of 1,883,000 km (1,170,000 mi). Callisto completes an orbit and rotates in the same amount of time, once every 16.69 Earth days. Its nearly circular orbit parallels Jupiter's equator. Callisto is spherical and is the third-largest moon in the solar system. The moon has a radius of 2,403 km (1,493 mi), making it nearly the same size as the planet Mercury. Since Callisto consists mostly of low-density water ice, however, the moon is only one-third as massive as rocky, metallic Mercury. Callisto's interior is probably not differentiated into a rocky core surrounded by lighter icy material, like that of the other three large moons of Jupiter--Io, Europa, and Ganymede. Instead, scientists believe that the entire moon is a mixture of rock and ice, with the percentage of rock in the mixture increasing toward the moon's center. Callisto is the most heavily cratered world known. This cratering suggests that the surface has not changed in billions of years and that there is little internal activity affecting it. If Callisto's internal activity was strong enough, it would melt or disturb the ice near the moon's surface, and many of the craters would be erased. Instead, Callisto seems to be the most geologically quiet of the four large moons of Jupiter. This is probably because Callisto is the outermost of the large moons. No large outer satellite of Jupiter subjects it to the gravitational tug-of-war with Jupiter that can produce internal heating. The only sign of change on the surface is the slumping of older crater walls that occurs as the ice that forms them flows downward over extremely long periods of time. Despite Callisto's apparent stillness, however, in 1998 the Galileo spacecraft revealed signs that the moon may have a liquid or slushy ocean beneath its surface. Callisto may also have a tenuous atmosphere composed mostly of carbon dioxide. The largest crater on Callisto is Valhalla, a 300-km (190-mi) basin surrounded by a system of concentric rings 1,500 km (930 mi) wide. In addition, Callisto has 12 known crater chains. Astronomers believe that these chains were caused by comets or asteroids that broke up when they passed too near Jupiter and crashed into Callisto. The longest chain, Gipul Catena, is about 640 km (about 400 mi) long. Callisto was discovered independently by Italian astronomer Galileo and German astronomer Simon Marius. Callisto and the three other large moons of Jupiter--all of which were found in 1610--are collectively known as the Galilean moons. Marius named Callisto and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Callisto was a nymph who was changed into a bear by Zeus's jealous wife Hera. In Greek mythology, Callisto is the basis of the constellation Ursa Major. Craters on Callisto are generally named for heroes and heroines of northern mythologies. The United States Voyager space probes and the United States Galileo orbiter have provided a wealth of information about Callisto. When the Voyager probes passed Jupiter in 1979 they provided some detailed photographs. Galileo made its first flyby of Callisto in 1996 and examined the moon several times before its mission ended in 2003. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.