Callisto

**This page/section has been marked for fixing under Project/Paraphrase**
This page is to be fixed and paraphrased under the project's guidelines. Please do not remove this tag until the project has ended.**Incomplete**

This page is incomplete because, **it has some parts of its infobox missing**.Please do not remove this tag, unless the page has been completed. You can help the wiki by finishing it.
*Not what you're looking for? See: **Calypso, a moon of Saturn.***

**Callisto**, 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.

## Callisto

Callisto as imaged in true color by the Voyager 2 spacecraft on 8 July 1979. The image is composed from orange, green and blue color channels.

## Orbit Data

### Orbital Parent

Jupiter

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 independently by Simon Marius and Galileo Galilei in 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.