Callisto

October 13, 2010

Image courtesy of NASA Images Europa Callisto and Jupiter 249x300 CallistoCallisto is the third-largest moon in the Solar System, with a diameter of over 4,800 km (2,985 miles),  and the second largest in the Jovian system, after Ganymede. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1,880,000 km. It does not form part of the orbital resonance that affects three inner Galilean satellites—Io, Europa and Ganymede—and thus does not experience appreciable tidal heating. Callisto’s surface is also 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, and it has the lowest density of the Galilean satellites (about 1.83 g/cm3). 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.

The surface of Callisto is most heavily cratered objects in the solar system and also the oldest with a surface age of about 4 billion years. It is also the darkest of the Galileans, but it is twice as bright as our own Moon. It is thought to be a long dead world, with hardly any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface.

CALLISTO 300x225 Callisto

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 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 the moon. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.

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