Callisto

Callisto is a natural satellite of the planet Jupiter. It is the outermost of the four Galilean moons and the third-largest moon in the Solar System. It was discovered by Galileo Galilei in 1610 and is named after Callisto, a nymph in Greek mythology who was turned into a bear.

Callisto orbits Jupiter at an average distance of about 1.88 million kilometers and completes an orbital period of 16.69 days. The moon is tidally locked to Jupiter, keeping the same face toward the planet. It measures about 4,821 kilometers in diameter and has a low density of around 1.8 g/cm3, indicating a composition that is rich in water ice with a rocky component.

The surface of Callisto is heavily cratered and among the most ancient in the Solar System, showing little sign of recent geological activity. Large impact basins, such as Valhalla, are prominent features. Surface temperatures range roughly from minus 143 °C at night to about minus 83 °C in sunlight. The terrain consists mainly of water ice with darker, non-ice material mixed in.

In terms of interior and geology, Callisto is thought to be only weakly differentiated, with a possible small rocky core and a substantial icy shell. Some models speculate a subsurface liquid water layer, but no confirmed ocean has been detected. Because of limited tidal heating, Callisto exhibits minimal current geologic activity.

Callisto has a very tenuous atmosphere, composed primarily of carbon dioxide, with negligible surface pressure. It was first imaged by Voyager 1 in 1979 and studied in detail by the Galileo spacecraft. Future missions may further explore its geology and potential subsurface environments.