Depths of Pluto's and Charon's craters, and their simple-to-complex transition

Impact craters form as point-like explosions on solar system bodies, excavating a cavity within the surface. Most information about the impactor itself is lost, save its energy, such that the target itself controls much of the final crater size and shape given the energy input. For this reason, impact craters are a useful probe of surface differences across a single body and between bodies. Two properties that are commonly used to compare craters from one body to another are the ratio between crater depth and diameter, and the diameter at which craters transition between simple, bowl-shaped morphologies to more complex morphologies that include flat floors, central peaks, wall terraces, and scalloped rims. Both metrics are important as input into theoretical models and for probing the strength of the surface material. In this work, we have measured these properties on Pluto and Charon and report on the results. However, the analysis is not entirely straightforward; a secondary goal of this work is to explore how to measure and quantify the two properties accurately (depth vs diameter and simple-to-complex transitions). We arrive at different results based on the method, and we report on that variation as a way to reasonably estimate the uncertainty in each quantity. For those reasons, it is difficult to place a single, exact value on each, but we do conclude that the morphology-based simple-to-complex transition occurs at approximately 11–12½ km on Pluto, and 13½–16 km on Charon. We also conclude that the existing, public topography data for at least Charon is likely too low in resolution to measure the simple crater depth vs diameter function accurately, such that a morphometry-based simple-to-complex transition diameter is not quantifiable at this time.