Callisto

**Callisto** (/kəˈlɪstoʊ/), or **Jupiter IV**, is the second-largest moon of Jupiter, after Ganymede. In the Solar System it is the third-largest moon after Ganymede and Saturn's largest moon Titan, and as large as the smallest planet Mercury, though only about a third as massive. Callisto is with a diameter of 4821 km roughly a third larger than the Moon and orbits Jupiter on average at a distance of 1883000 km, which is about six times further out than the Moon orbiting Earth. It is the outermost of the four large Galilean moons of Jupiter, which were discovered in 1610 with one of the first telescopes, being visible from Earth with common binoculars.

The surface of Callisto is the oldest and most heavily cratered object in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (*catenae*) and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known. Callisto is composed of approximately equal amounts of rock and ices, with a density of about 1.83 g/cm3, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. Investigation by the *Galileo* spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.

It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that it always faces the same direction, making Jupiter appear to hang directly overhead over its near-side. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from *Pioneers 10* and *11* to *Galileo* and *Cassini* have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable to base possible future crewed missions to study the Jovian system.

## History

### Discovery

Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—Ganymede, Io, and Europa.

### Name

Callisto is named after one of Zeus's many lovers or other sexual partners in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis. The name was suggested by Simon Marius soon after Callisto's discovery. Marius attributed the suggestion to Johannes Kepler.

> Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell... I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...
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However, the names of the Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Callisto is referred to by its Roman numeral designation, a system introduced by Galileo, as **Jupiter IV** or as "the fourth satellite of Jupiter".

There's no established English adjectival form of the name. The adjectival form of Greek Καλλιστῴ *Kallistōi* is Καλλιστῴος *Kallistōi-os*, from which one might expect Latin *Callistōius* and English \*Callistóian (with 5 syllables), parallel to Sapphóian (4 syllables) for *Sapphōi* and Letóian for *Lētōi*. However, the iota subscript is often omitted from such Greek names (cf. *Inóan* from *Īnōi* and *Argóan* from *Argōi*), and indeed the analogous form **Callistoan** is found. In Virgil, a second oblique stem appears in Latin: *Callistōn-,* but the corresponding **Callistonian** has rarely appeared in English. One also sees *ad hoc* forms, such as **Callistan**, **Callistian** and **Callistean**.