The colors in several of these images represent views in spectral regions that are not visible to the eye. They show either differences in surface chemical composition or changes in the way the surface reflects sunlight. For example in the left middle image, bright red depicts material newly ejected from an active volcano on Io and the surrounding yellow materials are older sulphur deposits. The picture to its right shows enormous cracks in the ice shell that forms the surface of Europa. Blue represents ice and reddish areas probably represent a thin coating of darker material ejected by ice volcanoes that occur along the cracks.
North is to the top of the pictures.
The middle row shows regional color views, each covering an area about 1000 km by 750 km and scaled to about 1.8 km per pixel. The bottom row shows black and white views covering areas about 100 km by 75 km and scaled to about 180 meters per pixel. The Callisto global, Ganymede regional, and Io high resolution views were acquired in 1979 by NASA's Voyager spacecraft. Io Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter.