The difference between craters on an ice moon and a rocky moon.
On Ganymede the craters have no central uplift.
Instead the central uplift collapses back down again to form another ring.
So essentially we end up with a couple of ripples frozen in time
Snip taken from:
By NASA / Jet Propulsion Lab - http://photojournal.jpl.nasa.gov/catalog/PIA00081, Public Domain, Link