  We have performed topographic and geological mapping of 19 large impact
features on Ganymede and Callisto in order to gather morphometric and crater
age data that allow us to quantify how the diverse morphologies of these
features transition with size and age. The impact features are divided into two
main morphological groups, craters and penepalimpsests/palimpsests. The
morphologies of pit and dome craters are independent of age or geologic
context, indicating that the impacts that formed them were small enough to only
ever penetrate into a cold, rigid ice layer, with evolution of a pocket of
impact melt contributing to the development of their central pits and
surrounding raised annuli, while the domes formed mainly via viscous relaxation
of the pit topography. The subdued rims of anomalous dome craters indicate the
increasing effect of a warm subsurface ice layer on impact feature morphology
with increasing size. The low topographic relief of penepalimpsests and
palimpsests indicates that these impacts penetrated the ice shell to liberate
large volumes of underlying, pre-existing liquid. Penepalimpsests show a higher
frequency of concentric ridges within their interiors, indicating a more robust
subsurface state that could support the rotation and uplift of solid material
during impact. The size and age overlap of anomalous dome craters,
penepalimpsests, and palimpsests, with a few impact features appearing to be
transitional between anomalous dome craters and penepalimpsests, indicates that
impactor size, time of impact, and variation in temperature gradient across the
satellite are all factors in determining which of these morphologies emerges.
