The Formation of Pluto's Low-Mass Satellites

Motivated by the New Horizons mission, we consider how Pluto's small satellites—currently Styx, Nix, Kerberos, and Hydra—grow in debris from the giant impact that forms the Pluto–Charon binary. After the impact, Pluto and Charon accrete some of the debris and eject the rest from the binary orbit. During the ejection, high-velocity collisions among debris particles produce a collisional cascade, leading to the ejection of some debris from the system and enabling the remaining debris particles to find stable orbits around the binary. Our numerical simulations of coagulation and migration show that collisional evolution within a ring or a disk of debris leads to a few small satellites orbiting Pluto–Charon. These simulations are the first to demonstrate migration-induced mergers within a particle disk. The final satellite masses correlate with the initial disk mass. More massive disks tend to produce fewer satellites. For the current properties of the satellites, our results strongly favor initial debris masses of 3–10 × 10 19 g and current satellite albedos A ≈ 0.4–1. We also predict an ensemble of smaller satellites, R ≲ 1–3 km, and very small particles, R ≈ 1–100 cm and optical depth τ ≲ 10 −10 . These objects should have semimajor axes outside the current orbit of Hydra.