In this image of the Valhalla region on Callisto we notice thae numerous rings surrounding the impact basin. Could these rings be the result of a gradual resolidification of the surface over a briny ocean below? Could the cause of the rings be similar to the ripples on a pond when you cast in a pebble?
Also there seem to be an extrordinary ammount of cratering with apparent central uplift zones. could these also be caused by the existance of a subsurface ocean?