Ganymede is the largest moon of Jupiter and is the largest in our solar system with a diameter of 5,262 km (3,280 miles). If Ganymede orbited the Sun instead of Jupiter it could be classified as a planet. Ganymede is most likely composed of a rocky core with a water/ice mantle and a crust of rock and ice. It has had a complex geological history. It has mountains, valleys, craters and lava flows. Ganymede is mottled by both light and dark regions. It is heavily cratered especially in the dark regions implying ancient origin. The bright regions show a different kind of terrain - one that is grooved with ridges and troughs. These features form complex patterns and have a vertical relief of a few hundred meters and run for thousands of kilometers. Jupiter fills the Ganymede sky with a deep beauty and foreboding.

HST, courtesy NASA