Callisto
Summary of Facts
With a diameter of over 4,800 km (2,985 miles), Callisto is the third largest satellite in the solar system (only Ganymede and Titan are bigger), and is almost the size of Mercury. Callisto is the outermost of the Galilean satellites, and orbits beyonds Jupiter's main radiation belts.
Callisto has the lowest density of the Galilean satellites (1.86 grams/cubic centimeter). Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle. Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.
Callisto is the most heavily cratered object in the solar system. It is thought to be a long dead world, with a nearly complete absence of any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
There are no large mountains on Callisto, which is probably due to the icy nature of the satellite's surface. The surface features are dominated by impact craters and rings, and the craters are quite shallow. There are two large "bullseye" structures on Callisto, thought to be the result of a massive impact. The largest structure, Valhalla, has a bright patch 600 km across with rings extending out to almost 3000 km. The other ring structure, Asgard, is about 1600 km in diameter.
Seven impact crater chains have been mapped on Callisto. These chains probably formed when fragments of a comet were split apart by Jupiter's gravity and impacted on Callisto. In a similar scenario, Comet Shoemaker-Levy 9 split into 21 fragments and impacted Jupiter in 1994.
The Galileo spacecraft will make three close passes by Callisto during its 2 year orbital tour around Jupiter. The first close encounter occurred November 4, 1996.