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"Animation of Jupiter's moon Callisto passing in front of the planet. Callisto is the second largest of Jupiter's Galilean moons, and the third largest in the Solar System, with a diameter of some 4800 kilometres. It is the most distant of the Galilean moons from Jupiter, orbiting some 1.9 million kilometres from the centre of the planet, and is the only one of the four moons not locked in an orbital resonance with the others. As its orbit is not stabilised by such a resonance, the tidal heating seen in the inner three moons does not occur. Due to this lack of energy it is thought that it is largely undifferentiated, and its interior is a broadly homogenous mixture of ices and silicates. It has one of the oldest and most heavily-cratered surfaces known, and no evidence of geological activity. Despite this, there is evidence for a subsurface ocean, as is the case with Europe and Ganymede."