Jupiter's water world.

The Voyager spacecrafts which traveled past Jupiter in 1979 revealed the cratered countenance of Callisto, the valleys and ridges of Ganymede, the cracked face of Europa, and the spewing volcanoes of Io. But it would take a spacecraft named for Italian scientist Galileo, who discovered the moons in 1610, to reveal the true complexity of these worlds and to begin to divulge their interior secrets. Viewed from afar, Callisto appears heavily cratered, with few signs of internal activity. Up close, Callisto's dark surface appears as a smooth fog that cloaks the moon's contours. The process of sublimation, which occurs when solid material turns directly into gas, best explains the origin of this rubbly surface, A mixture of ice and rock delivered by colliding comets and ice-rich planetesimals can sublimate slowly over time, leaving dusty debris behind. Of the Galilean satellites, Callisto's ancient, cratered surface should hold the most pristine and primitive material, the stuff from which the moons were built. The gravity studies suggest that Callisto is a mix of rock and ice deep into its interior and that its constituents are only partially segregated. This implies that the moon's innards never melted throughout, somehow staying relatively cool as Callisto accreted in the ancient jovian nebula. The magnetic field around Callisto provides another peek beneath the moon's surface and points to an astonishing conclusion: Callisto probably hides a subsurface ocean today. Although Callisto does not generate its own magnetic field, Galileo found that the powerful magnetic field of Jupiter induces one in Callisto. Although the decay of radioactive elements left over from Callisto's formation does not generate enough heat to melt pure ice today, salts or just a bit of ammonia could play the role of antifreeze. As pressure increases within Callisto, common ice like that found in our freezers is transformed into more compact forms.