Ganymede, the third of the Galilean moons, is the largest moon of Jupiter and the largest moon in the Solar System, almost twice as big as our moon.
Natural color view of Ganymede from the Galileo mission's first encounter with the moon. The dark areas are the older, more heavily cratered regions and the light areas are younger, tectonically deformed regions. The brownish-gray color is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta.
Voyager took this picture from a distance of about 3.4 million kilometers (nine times the distance from the Earth to the Moon) revealing detailed surface features for the very first time. The large dark area near the center and the bright patch in the southern hemisphere are crossed by irregular light streaks somewhat similar to rays seen on the Moon.
Pioneer 10 took this photo as it passed by from a distance a little greater than that from the Earth to the Moon.
Italian scientist Galileo Galilei has discovered what he calls the Medicean stars (in honor of the Medici family) Jupiter I, II, III and IV orbiting the planet Jupiter when he pointed his telescope to the heavens.
Of course photography did not exist in the 1600's so the picture above is a modern day view through a small telescope that closely represents what Galileo would have seen. Of course Jupiter III we now know as Ganymede. This discovery led to the demise of the long held theory that the Earth was at the center of the universe and a new understanding that the planets (including the Earth) orbit the Sun.