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Callisto, name of the 4th satellite of Jupiter, discovered by Galileo January 7, 1610. Callisto is one of the four major moons of the planet Jupiter, the third-largest moon in the solar system, the second largest in the Jovian system, after Ganymede, and the largest object in the solar system to be properly differentiated.

Discovered by Galileo Galilei in 1610, Callisto has a diameter of 4,821 km, equivalent to 99% of the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon in order of distance from Jupiter, being approximately 1.88 million km from the planet.

Callisto does not participate in orbital resonance involving the other three Galilean satellites: Io, Europa and Ganymede, so it does not suffer the tidal heating, endogenous phenomena that originate on Io and Europa. Without internal magnetic field and just outside of the radiation of the gas giant band, not particularly interacts with the magnetosphere of Jupiter. Callisto is more or less equally, by rocks and ice, with an average density of about 1.83 g / cm³, the lowest among the Galilean Moons – Callisto, Europa, Ganymede, or Io.

On its surface it was detected spectroscopically the presence of water ice, carbon dioxide, silicates and organic compounds. Studies conducted by the Galileo spacecraft revealed that Callisto may have a small core of silicates and possibly a layer of liquid water below the surface, at depths of over 100 km. The surface of Callisto is the oldest and most heavily cratered in the Solar System.

No traces of subsurface processes such as plate tectonics or volcanism; there is no sign that geological activity has ever occurred in the past and the evolution of its surface is mostly manufactured for meteorite impacts. The main surface features include structures with multiple concentric rings, with embankments, ridges and deposits associated with them, impact craters of various shapes and crater chains. The ages of the different morphologies are not known. The likely presence of an underground ocean of Callisto leaves open the possibility that it could harbor life. However, the conditions seem to be less favorable than the nearby Europe. Several space probes Pioneer 10 and 11, the Galileo and Cassini have studied Callisto, which, because of its low levels of radiation, has long been considered the most suitable place for a human base for future exploration of the Jovian system .

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Callisto, name of the 4th satellite of Jupiter, discovered by Galileo January 7, 1610.