A Pluto–Charon Sonata. III. Growth of Charon from a Circum-Pluto Ring of Debris

Abstract Current theory considers two options for the formation of the Pluto–Charon binary. In the hit-and-run model, a lower mass projectile barely hits the more massive Pluto, kicks up some debris, and remains bound to Pluto. In a graze-and-merge scenario, the projectile ejects substantial debris as it merges with Pluto. To investigate the graze-and-merge idea in more detail, we consider the growth of Charon-mass objects within a circum-Pluto ring of solids. Numerical calculations demonstrate that Charon analogs form rapidly within a swarm of planetesimals with initial radii r 0 ≈ 145–230 km. On timescales of ∼30–100 days, newly formed Charon analogs have semimajor axes, a ≈ 5–6 r P , and orbital eccentricities, e ≈ 0.1–0.3, similar to Charon analogs that remain bound after hit-and-run collisions with Pluto. Although the early growth of Charon analogs generates rings of small particles at a ≈ 50–275 r P , ejection of several 145–230 km leftovers by the central Pluto–Charon binary removes these small solids in 10–100 yr. Simple estimates suggest that small particles might survive the passage of 10–20 km objects ejected by the central binary. Our results indicate that the Pluto–Charon circumbinary satellite system was not formed by a graze-and-merge impact when the formation of Charon within a circum-Pluto disk leads to the ejection of several 100–200 km particles through the orbital plane of the Pluto–Charon binary. If a growing Charon ejects only much smaller particles, however, graze-and-merge impacts are a plausible formation channel for the Pluto–Charon binary and an ensemble of small, circumbinary satellites.