Callisto (moon)

From Wikipedia, the free encyclopedia

Callisto

Click image for description
Discovery
Discovered by:	G. Galilei S. Marius
Discovery date:	January 7, 1610
Orbital characteristics
Periapsis:	1,869,000 km (0.0125 AU)
Apoapsis:	1,897,000 km (0.0127 AU)
Mean radius of orbit:	1.8827×106 km (0.012585 AU)
Orbital circumference:	11,829,000 km (0.079 AU)
Eccentricity:	0.0074
Orbital period:	16.6890184 d (0.04569 a)
Avg. orbital speed:	8.204 km/s
Max. orbital speed:	8.265 km/s
Min. orbital speed:	8.143 km/s
Inclination:	2.02° (to the ecliptic) 0.21° (to Jupiter's equator)
Satellite of:	Jupiter
Physical characteristics
Mean radius:	2410.3 km (0.378 Earths)
Surface area:	7.30×107 km² (0.143 earths) [1]
Volume:	1 E19 m3
Mass:	1.0759×1023 kg (0.018 Earths)
Mean density:	1.834 g/cm³
Equatorial surface gravity:	1.235 m/s2 (0.126 g)
Escape velocity:	2.440 km/s
Rotation period:	synchronous
Axial tilt:	zero
Albedo:	0.17
Temperature:	~120 K
Apparent magnitude:	5.7
Atmosphere
Surface pressure:	trace
Composition:	100% Carbon dioxide

Callisto (kə-lis'-toe, IPA: [kəˈlɪstoʊ]; 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 99% the size of the planet Mercury but much less massive.

Contents 1 Name 2 Physical characteristics 2.1 Internal Structure 2.2 Surface Features 2.3 Atmosphere 2.4 Possibility of extraterrestrial life 3 See also 4 In fiction 5 Notes and references

[edit] 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.

[edit] Physical characteristics

[edit] Internal Structure

Callisto's battered surface lies on top of an icy layer that is about 150 kilometers thick. A salty ocean in excess of 10 kilometers thick may lay beneath the crust. Its presence is indicated by 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.

[edit] Surface Features

Callisto is one of the most heavily cratered moons 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.

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.

[edit] Atmosphere

Callisto has a very tenuous atmosphere composed of carbon dioxide^[2]. Because such a thin atmosphere can easily be lost in a variety of ways (see atmospheric escape), scientists believe that it is constantly replenished, possibly by slow sublimation of carbon dioxide ice from the satellite's icy crust.

[edit] Possibility of extraterrestrial life

Like with Europa and Ganymede, the idea has been brought up that extraterrestrial microbial life may exist on Callisto due to the salty ocean under its surface. However, scientist Torrence Johnson said the following about comparing the odds of life on Callisto with the odds on other Galilean moons^[3]:

"The basic ingredients for life -- what we call 'pre-biotic chemistry' -- are abundant in many solar system objects, such as comets, asteroids and icy moons. Biologists believe liquid water and energy are then needed to actually support life, so it's exciting to find another place where we might have liquid water. But, energy is another matter, and currently, Callisto's ocean is only being heated by radioactive elements, whereas Europa has tidal energy as well, from its greater proximity to Jupiter."

Based on Johnson's statement above, and on other scientific observations, it has been found that of all of Jupiter's Galilean moons, Europa has the greatest chance of supporting microbial life.

[edit] See also

• List of craters on Callisto
• List of geological features on Callisto
• The asteroid 204 Kallisto
• Colonization of Callisto

[edit] In fiction

See Callisto in fiction.

[edit] Notes and references