Ganymede is the largest moon of Jupiter and also the largest moon in the solar system. It is about the size of Mercury, having a diameter of 5,262 km (3,280 miles) and viewable by a pair of good field glasses. In fact, Ganymede appeared as a star-like object around Jupiter to Simon Marius and Galileo Galilei (with a telescope) in 1610 when the moon was discovered.
Ganymede has several large surface features such as the Galileo Regio, which is a large dark region about 3,200 km in diameter. There is a bright north polar region as well as various bright spots, the remnants of relatively new meteorites. All across the surface of Ganymede are lots of ridges, valleys and mountains, the result of complex internal activity and tectonic plate movement. When the Voyager spacecraft took pictures of Ganymede, it confirmed the existence of large, dark circular patterns which told a story about the crust's ancient history.
The surface temperature of Ganymede is much colder than Earth, with daytime temperatures ranging from 90 to 160 Kelvin (or -297 to -171 degrees Fahrenheit). There is no known atmosphere on the surface but Hubble Space Telescope did detect a layer of ozone at the surface, the result of the interaction between trapped energy particles from Jupiter's magnetic field and the water particles on Ganymede.
The interior of Ganymede is much like Callisto, consisting of a rocky core with water and ice layers that surround this core. The density is 1.94 g/cm^3, meaning that 50% of the moon is made of this inner core.
Ganymede 
