  Pluto's terrains display a diversity of crater retention ages ranging from
areas with no identifiable craters to heavily cratered terrains. This variation
in crater densities is consistent with geologic activity occurring throughout
Pluto's history and also a variety of resurfacing styles, including both
exogenic and endogenic processes. Using estimates of impact flux and cratering
rates over time, Pluto's heavily cratered terrains appear to be relatively
ancient, 4 Ga or older. Charon's smooth plains, informally named Vulcan
Planitia, did experience early resurfacing, but there is a relatively high
spatial density of craters on Vulcan Planitia and almost all overprint the
other types of volcanic or tectonic features. Both Vulcan Planitia and the
northern terrains on Charon are also estimated to be ancient, 4 Ga or older.
The craters on Pluto and Charon also show a distinct break in their
size-frequency distributions (SFDs), where craters smaller than approximately
10-15 km in diameter have a shallower SFD power-law slope than those larger
than this break diameter. This SFD shape on Pluto and Charon is different than
what is observed on the Earth's Moon, and gives the Kuiper belt impactor SFD a
different shape than that of the asteroid belt.
