On Ganymede one can easily see two different types of impacts and be able to discern their relative ages.
Because its an ice covered ocean, hard impacts go right through the ice surface and leave concentric ripple like rings for craters.
Occasionally an impactor is traveling slowly enough that it simple lodges in the ice without breaking all the way through, as can be seen by the much darker objects standing out from the more uniform background colour.
Recent impacts are more white due to the disturbance of the surface ice and older impacts simply fade to the surrounding uniform colour.
Snip taken from:
By NASA / Jet Propulsion Lab - http://photojournal.jpl.nasa.gov/catalog/PIA00081, Public Domain, Link
