With a radius of about 1,500 miles (2,400 km), Callisto is about the size of Mercury. It is the third largest moon in the solar system, after Jupiter's Ganymede and Saturn's Titan. Callisto orbits just outside Jupiter's main radiation belt from its magnetic field and is therefore relieved of the magnetic storms and ionic stripping that Io experiences. Callisto, like the other Galilean satellites, does produce its own magnetic field of about 4 x 10-9Tesla, probably from circulating salty water.

Callisto is named after a woman from Greek myth who was either the daughter of Lycaon, king of Arcadia, or a river nymph. Like the women other moons of Jupiter are named after, Callisto was pursued by Zeus. Zeus fell in love with her astonishing beauty and disguised himself as Apollo to win her over. Hera, Zeus's wife, took revenge on Callisto by turning her into a bear, and in the end, Zeus placed her and her son in the heavens as the constellations Ursa Major and Ursa Minor.

Callisto has an icy, cratered, ancient surface, with no recent resurfacing by volcanic activity. Its surface is thought to be among the oldest in the solar system, dating back to 4.56 billion years before present. Craters are the only surface features on this moon:

It has no mountains, grooves, or scarps. Since its surface is icy, like Ganymede's, its craters also tend to flatten and become palimpsests over time. Among its surface features are two immense impact craters, Valhalla and Asgard (the hall of Odin and the home of the gods, both from Norse mythology). The main depression of Valhalla crater is 190 miles (300 km) in diameter, and its outer rings (formed by the initial shock and collapse of the crater) stretch to 1,900 miles (3,000 km) in diameter. The second basin, Asgard, has outer rings 1,000 miles (1,600 km) in diameter. To the south of Asgard basin lies a region of exceptionally strange landforms. Shown in the figure below, this terrain consists of many sharp ice and dust spires and knobs. As the ice slumps or sublimes, the dark material apparently slides down and collects in low-lying areas. Over time, as the surface continues to erode, the icy knobs will likely disappear, producing a scene similar to the bottom inset. The number of impact craters in the bottom image indicates that erosion has essentially ceased in the dark plains shown in that image, allowing impact craters to persist.

The knobs are about 260 to 330 feet (80 to 100 m) tall, and they may consist of material thrown outward from a major impact billions

Spires of ice on Callisto are thought to be made by erosion and sublimation of ice from the surface. (NASA/JPL/Galileo)
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