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Callisto is a huge Jovian moon, the third largest satellite in the solar system, after Ganymede and Titan. At 4820 km in diameter, Callisto is about 40% greater in size than the Moon and only a third the size of the Earth. Callisto has a surface area of 7.30×107 km², greater than that of Asia.
Callisto is one of the Galilean satellites, discovered by Galileo Galilei in 1610 with one of the earliest telscopes. Callisto's diameter is 99% that of Mercury, but it has far less mass, due to the tremendous amount of ice it contains. Callisto is about half rock and half ice.
Callisto's surface is extremely cratered and old. The cratering on its surface has almost reached saturation — that is, every new crater must erase an old one. Its surface looks like mud being pelted by raindrops, but frozen in stone and ice.
As one of the most heavily cratered bodies in the solar system alongside Mercury, Callisto does not have any major geographic features aside from those associated with impacts. There are no major Callistoan mountains, ridges, or linea. Some of its craters, particularly the large Valhalla, form cracked cocentric ring structures.
Callisto has a tenuous atmosphere of carbon dioxide and molecular oxygen, and a thin subsurface ocean at 100-150 km below. Speculation about on extraterrestrial life in the oceans of the Jovian moons has largely focused on Europa, however. Callisto is widely regarded as the most likely site of a future base, due to its location in a relatively radiation-free region around Jupiter and its stable surface geology. By contrast, Io is located in one of the most extreme radiation-saturated areas around Jupiter, its plains are covered with sulfur, while volcanoes are constantly erupting all around.
Callisto orbits Jupiter at a distance of 1,880,000 km, making it the fourth Galilean moon in terms of distance. By comparison, the Moon orbits the Earth at a distance of about 400,000 km, significantly closer.