The surface of Ganymede is halfway between that of Callisto and that of Europa. Portions of the crust are of ancient age, while other portions are relatively new.
The little white dots shown in this image are the many imact craters on Ganymede. Portions of Ganymede that are heavily cratered have not been resurfaced, indicating that they are of ancient age.
Besides the craters, the surface of Ganymede is characterized by grooved terrain, light terrain, dark terrain.
The many different kinds of terrain exhibited by Ganymede are an indication that it has experienced processes of alteration in the interior during it’s evolution.
source :windows to the universe