GEOLOGY OF PLUTO AND CHARON
Charon, like Pluto, is differentiated with a water ice crust, but lacks abundant surface volatiles. A prominent ~250 km-wide belt of extensional faulting dominates the encounter hemisphere. The eastern border of the encounter hemisphere features prominent peaks extending about 3-5 km above the floors of surrounding moats, and the moats lie 1-2 km above surrounding plains. Charon also displays a variety of basins, extensional faults and probable cryovolcanic-resurfaced plains. Charon features a prominent reddish north polar cap that may be tholins sourced from atmospheric escape from Pluto and delivery to Charon.
Charon and Pluto probably have a collisional origin, and freeing of a subsurface water ocean has caused the dominantly extensional tectonism.