|Ganymede as seen from Galileo.|
|Diameter|| 5,268 km|
|Distance from primary|| 1070,000 km|
Ganymede is the largest satellite in the solar system and larger than the planet Mercury. Ganymede and Callisto are similar in size and density. Yet compared to "boring" Callisto, Ganymede has a very complex geologic history. The different geologic histories of Ganymede and Callisto have been a continuing puzzle. Impact craters, volcanic deposits, and tectonic structures are all common on the surface of Ganymede. Its craters record the flux of impacting projectiles, and its volcanic and tectonic features record part of the thermal and compositonal evolution of the interior. Recent theoretical work suggests that the eccentricity of Ganymede's orbit was tidally pumped up in ancient times, probably accounting for the long complex geologic history of Ganymede. Unlike Europa and Io, Ganymede is not undergoing tidal heating today.