Callisto (moon)

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**Callisto**
Discovery
Discovered by	G. Galilei S. Marius
Discovered on	January 7, 1610
Orbital characteristics
Mean radius	1.8827 km (0.012585 AU)
Eccentricity	0.0074
Periapsis	1,869,000 km (0.0125 AU)
Apoapsis	1,897,000 km (0.0127 AU)
Revolution period	16.6890184 d (0.04569 a)
Orbital circumference	11,829,000 km (0.079 AU)
Orbital velocity	max: 8.265 km/s mean: 8.204 km/s min: 8.143 km/s
Inclination	2.02° (to the ecliptic) 0.21° (to Jupiter's equator)
Is a satellite of	Jupiter
Physical characteristics
Mean diameter	4820.6 km (0.378 Earths)
Surface area	7.3 km² (0.143 earths)
Volume	5.9 km³ (0.0541 Earths)
Mass	1.0759 kg (0.018 Earths)
Mean density	1.834 g/cm³
Surface gravity	1.235 m/s² (0.126 g)
Escape velocity	2.440 km/s
Rotation period	synchronous
Axial tilt	zero
Albedo	0.17
Apparent Magnitude	5.7
Surface temp.

min	mean	max
K	~120 K	K

Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (kə-lis'-toe, ; Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the third-largest moon in the solar system, about the same size as the planet Mercury.

Name

Callisto is named after Callisto, one of Zeus's many love interests in Greek mythology.

Although the name "Callisto" was suggested by Simon Marius soon after the moon's discovery, this name and the names of the other Galilean satellites fell into disfavour 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 simply referred to by its Roman numeral designation, Jupiter IV or as "the fourth satellite of Jupiter".

In scientific writing, the adjectival form of the name is usually Callistoan.

Physical characteristics

Callisto is one of the most heavily cratered satellites in the solar system. In fact, impact craters and associated concentric rings are about the only features to be found; there are no large callistoan mountains. This is probably due to the icy nature of its surface, with the largest craters and mountains being erased by the flow of the icy crust over geological time. Two enormous concentric ring impact basins are found on Callisto; Valhalla is the largest with a bright central region that is 600 kilometers in diameter and rings extending to 3000 kilometers in diameter, and the second-largest impact basin is Asgard measuring about 1600 kilometers in diameter. Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight line across Callisto's surface. This was probably caused by an object that was tidally disrupted as it passed close to Jupiter (much like Comet Shoemaker-Levy 9) before it impacted. Callisto's crust is thought to be approximately 4 billion years old, dating back almost to the formation of the solar system.

Callisto's battered surface lies on top of an icy layer that is about 150 kilometers thick. Beneath the crust lies a salty ocean in excess of 10 kilometers thick. The ocean was discovered from studies of the magnetic fields around Jupiter and its moons. It was found that the Callistoan magnetic field varies (flows in various directions at different times) in response to the background magnetic field generated by Jupiter; this suggests a layer of highly conductive fluid within the moon.

Beneath the ocean, Callisto seems to have a strange interior that is not entirely uniform but does not vary dramatically. Galileo orbiter data suggest that the interior is composed of compressed rock and ice, with the amount of rock increasing with depth due to partial settling of its constituents. Callisto has the lowest density of the four Galilean moons of Jupiter, only 1.86 g/cm³, and is about 40% ice and 60% rock/iron.

Callisto has a very tenuous atmosphere composed of carbon dioxide. Its source may be the slow sublimation of carbon dioxide ice from the satellite's icy crust.

Unlike neighbouring Ganymede with its tectonic grooved terrain, there is little evidence of tectonic activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geological history. The different geologic histories of the two has been an important problem for planetary scientists. "Simple" Callisto is a good reference for comparison with other more complex worlds.

Callisto in fiction

In Isaac Asimov's story "The Callistan Menace" (1940), Callisto has an atmosphere of carbon dioxide and is inhabited by large slugs that use magnetic fields to stun their prey.
Lin Carter's eight-novel Callisto series (1972-1978) is set on an inhabitable Callisto.
In Piers Anthony's sci-fi series "Bio of a Space Tyrant" (1983-), Callisto is the home planet of Hope Hubris, the Tyrant of Jupiter.
Jupiter Moon (1990) was a short-lived British attempt at making a space soap opera. It was set on a space university that orbited Callisto.
Kim Stanley Robinson's novel Blue Mars (1996) contains a description of a flourishing colony on Callisto.
Cowboy Bebop (1998) features a snowy Callisto, filled with fugitives and populated only by men.
In the game The 2nd Runner (2003), the protagonist, Dingo Egret, finds the Orbital Frame Jehuty buried in ice under Callisto's surface.
In the video game series G-police, your character is a pilot for a police force on Callisto.

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Name

Physical characteristics

See also

Callisto in fiction