Impactor Populations and Terrain Age Estimates from Craters on Pluto and Charon

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. We also discuss what limits can be put on the ages of Pluto’s middle-aged and younger terrains. 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 ~10-15 km in diameter have a shallower SFD power-law slope than those larger than this break diameter. The SFD slope for smaller craters is q ~ -1.7 ± 0.2 and this slope is fairly consistent down to the smallest sizes measured of ~1-km-diameter craters. For craters larger than the SFD break diameter, average is q ~ -3, although there is variation over the size range available on Pluto and Charon. This distinct break to a shallower slope on Pluto and Charon is different than what is observed for craters on the Earth’s Moon, and suggests the Kuiper belt impactor SFD has a different shape than that of the asteroid belt. We translate the crater slope into an impactor slope using Schmidt-Holsapple-Housen impactor scaling laws (also see Singer et al., 2019 and supplement) and use this new SFD shape revealed by the New Horizons data for the impactors in the terrain age estimates presented here. For impactors smaller than ~1-2 km in diameter, the translated impactor SFD slope is ~ -1.5 or -1.6 for a non-porous or porous target surface, respectively.

The results of this work can be found in the forthcoming book chapter: Singer, K. N., Greenstreet, S., Schenk, P. M., Robbins, S. J., and Bray, V. J. (2021). Pluto and Charon craters and terrain age estimates, in The Pluto system. edited by S. A. Stern, L. A. Young, J. M. Moore, W. M. Grundy, and R. P. Binzel, University of Arizona Press, Tucson.