Io has no recognizable impact craters, and is continually being resurfaced by volcanic eruptions that cover any craters that might form. Generally, the further away from Jupiter, the older the moon. Io has volcanism. Europa and Ganymede have tectonic processes that fracture or distort craters. Callisto's craters are mainly destroyed by subsequent impacts, but landslides have also modified the crater walls.
Smaller craters are more common than larger ones.
The patterns to the size distribution of craters on Ganymede and Callisto are similar but not identical to the simulated asteroid impact results. There are fewer large impacts in the size distribution for the Ganymede image. The craters on Ganymede and Callisto might have been created by comets or some other objects. Assumed the area in the image is large enough to include full range of crater sizes. Region might not be representative of the moon.