Ganymede, a satellite of the planet Jupiter. With a diameter of 3,270 miles (5,260 km), Ganymede is the largest satellite in the solar system, and is larger than the planet Mercury. Its surface is composed of ice and has distinct dark and light areas. The dark areas are heavily cratered and the light areas are covered by long, parallel channels. Ganymede was discovered by Galileo Galilei in 1610.
Our knowledge of space changes all the time as new discoveries are made. But some ideas about the universe have never really held water – including the one that everything is made of frozen H2O.
A look at the night sky at any time of year will reveal a faint band of light stretching across the sky -- our solar system's home, the Milky Way. How much do we really know about it?