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