The many ages of Triton: New crater counts on the voyager high-resolution image sequence and implications for impactor provenance

Abstract

Triton is sparsely cratered overall, but because of the high concentration of its impact craters on the leading hemisphere of motion, a dominantly prograde planetocentric origin for the impactors has been hypothesized. With only a modest heliocentric contribution, the average age of Triton's datable terrains could be no more than ~10 Ma, which would make Triton the most globally active icy satellite of all. This is obviously important, but is it true? There are clear stratigraphic age differences between terrains on Triton's leading hemisphere, especially on the 10-frame high-resolution Voyager 2 mosaic, which complicates simple or facile interpretations of the crater spatial distribution. Moreover, crater counts on the high-resolution mosaic, which we present here, indicate a steep slope for the impactor size-frequency distribution (SFD) at small (sub-km) sizes, completely at odds with crater counts on Charon, nominally the best estimate of the time-integrated Kuiper belt flux (Charon's corresponding SFD slope being quite shallow in comparison). Most likely, this implies that the smaller craters we measure on Triton are indeed predominantly planetocentric, but the origin of such a geologically recent prograde impactor swarm is puzzling. Here we consider several possible sources or explanations in detail, and find debris from a collision or collisions between neptunian irregular satellites, to be the least objectionable, albeit unproven. Secondaries or sesquinaries from a large but unknown crater elsewhere on Triton are a distinct alternative.