Image of Callisto obtained in 2001 by NASA's Galileo spacecraft (Courtesy NASA/JPL-Caltech)
|Discovered by||G. Galilei
|Discovery time||January 7, 1610|
|Shortest distance from what it orbits around||1,869,000 km|
|Longest distance from what it orbits around||1,897,000 km|
|Avg. distance from the center of its orbital path||1,882,700 km|
|How egg-shaped its orbit is
|How long it takes to complete an orbit||16.6890184 d|
|Average speed||8.204 km/s|
|Angle above the reference plane
|0.192° (to Jupiter's equator)|
|What it orbits||Jupiter|
|Size and Other Qualities|
|Average distance from its center to its surface||2410.3 ± 1.5 km (0.378 Earths)|
|Area of its surface||7.30×107 km² (0.143 Earths)|
|Volume inside it||5.9×1010 km³ (0.0541 Earths)|
|Mass||1.075938 ± 0.000137×1023 kg (0.018 Earths)|
|Average density||1.8344 ± 0.0034 g/cm³|
|Gravity at its surface||1.235 m/s2 (0.126 g)|
|Slowest speed able to escape into space
|How long it takes to turn around one time||synchronous|
|Angle at which it turns
(in relation to its orbit)
|How much light it reflects||0.22 (geometric)|
|Make up||~4×108 cm-3 carbon dioxide
up to 2×1010 cm-3 molecular oxygen
Callisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the second largest moon orbiting Jupiter, after Ganymede, and the third largest moon in the Solar System. Callisto is almost the same size as the planet Mercury, being 99% the diameter of the planet, but is much less massive. Of the four Galilean moons of Jupiter, Callisto is the farthest, with an orbital radius of about 1,880,000 kilometres. It does not form part of the orbital resonance that affects three inner Galilean satellites, Io, Europa and Ganymede. Like the Earth's Moon, Callisto rotates synchronously with its orbital period. This means that one side is always turned toward Jupiter, while the other side never sees the face of the planet. Callisto's surface is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away.
Callisto is composed of approximately equal amounts of rock and ices, with an average density of about 1.83 g/cm³. Compounds on the surface include water ice, carbon dioxide, silicates, and organics. The Galileo spacecraft made investigations of the moon, and revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water more than 100 kilometers deep.
The surface of Callisto is heavily cratered and extremely old. The surface does not show any sign of processes occurring below the surface such as plate tectonics, earthquakes or volcanoes, and is thought to have evolved mainly under the influence of meteorite impacts. Notable surface features include multi-ring structures, impact craters in different shapes, and chains of craters (known as catenae) and associated scarps, ridges and deposits. On a smaller scale, the surface is diverse and is made up of small bright frost deposits at around hill tops, surrounded by a smooth blanket of dark material in valleys. This is thought to result from the degradation of small landforms, caused through sublimation, which is supported by the general lack of small impact craters and the presence of numerous small knobs, considered to be their remains. The absolute ages of the landforms are not known.
Callisto is surrounded by an extremely thin atmosphere made out of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. An ocean is thought to exist within Callisto. This may mean that life might exist. However, this is less likely than on nearby Europa. Various space probes from Pioneers 10–11 to Galileo and Cassini have studied the moon. Callisto has long been considered the most suitable place for humans to settle for future exploration of the Jovian system.
References[change | edit source]
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- Calculated on the basis of other parameters
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