Callisto

Callisto is one of Jupiter’s moons. It was discovered in 1610 by the Italian astronomer Galileo Galilei.

Callisto is the second largest and outermost of Jupiter’s moons. It is one of the four Jupiter moons – the others are Europa, Io and Ganymede – that Galileo was able to see from Earth. Jupiter has 67 moons, 49 of which have been named by the International Astronomical Union (IAU). 16 carry numbers and 2 are uncertain. 13 of the moons were discovered by early observations from Earth. Voyager spacecraft missions in the 1970s discovered 3 and observations from Earth using sensitive devices found 41 moons, of which two remain unconfirmed.

The names of most Jupiter’s moons derive from characters from Greek mythology who were the lovers and descendants of Jupiter (Zeus), the king of the gods. Jupiter’s as-yet-unnamed moons have numbers. Callisto was the daughter of the King of Arcadia (Lycaon). A companion to Diana (Artemis), the goddess of hunting, she vowed to remain chaste, but Jupiter (Zeus in Greek) disguised himself as Diana and seduced her.

German astronomers Simon Marius and Johannes Keppler first named Callisto. Galileo called the moon Jupiter IV. The mythological name was revived in the mid-20th century and was formally adopted by the IAU over the period 1955-2004.

The composition of Callisto is equal parts of water-ice and rock with some volatile ammonia ice. Unlike the other three large Jovian moons that have a differentiated interior of crust, mantle and core, Callisto consists of a thin crust of between 50 and 200 km underlain by a salty ocean of up to 300 km thickness. Flyby spacecraft registered a weak magnetic field on Callisto. The remaining interior seems fairly uniform. There is a thin atmosphere made up of mostly carbon dioxide, through some condensed oxygen may be present on the moon’s surface.

Callisto has the most heavily cratered surface in the Solar System. There are multi-ringed craters, the largest of which is called Valhalla and measures 360 km in diameter. Crater chains (catenae) extend east to west. The Gomul catena runs to the north of Valhalla. There are no tectonic features such as mountains and volcanoes. The cratered surface is dotted with ice features up to 100 m in height that are slowly eroding.

Callisto orbits Jupiter in its equatorial plane and with an orbital period of 16.7 Earth days. It orbits synchronously, meaning that it rotates on its own axis in the same direction as its orbit around Jupiter. As a result, the same hemisphere of Callisto always faces towards Jupiter and Callisto’s period of rotation, its day, is the same as its orbital period. Along with Jupiter and its other satellites, Callisto takes 11.86 Earth years to orbit the sun.

Spacecraft flyby missions Pioneer 10 in 1973 and Pioneer 11 in 1974 passed Callisto at distances of 1.4 million and 786,500 km respectively. The first photographs were obtained by the 1979 Voyager 1 and 2 missions that passed at distances of 8 million and 1.4 million km respectively. The Galileo missions to Jupiter between 1995 and 2003 photographed Callisto’s surface from a distance of less than 138 km.