On the in situ formation of Pluto's small satellites

Abstract

The formation of Pluto's small satellites - Styx, Nix, Keberos and Hydra remains a mystery. Their orbits are nearly circular (eccentricity e = 0.0055 or less) and near resonances and coplanar with respect to Charon. One scenario suggests that they all formed close to their current locations from a disk of debris, which was ejected from the Charon-forming impact. We test the validity of this scenario by performing N-body simulations with Pluto-Charon evolving tidally from an initial orbit at a few Pluto radii. The small satellites are modeled as test particles with initial orbital distances within the range of the current small satellites and damped to their coldest orbits by collisional damping. It is found that if Charon is formed from a debris disk and has low initial eccentricity, all test particles survive to the end of the tidal evolution, but there is no preference for resonances and the test particles' final e is typically > 0.01. If Charon is formed in the preferred intact capture scenario and has initial orbital eccentricity ~ 0.2, the outcome depends on the relative rate of tidal dissipation in Charon and Pluto, A. If A is large and Charon's orbit circularizes quickly, a significant fraction of the test particles survives outside resonances with e >~ 0.01. Others are ejected by resonance or survive in resonance with very large e (> 0.1). If A is small and Charon's orbit remains eccentric throughout most of the tidal evolution, most of the test particles are ejected. The test particles that survive have e >~ 0.01, including some with e > 0.1. None of the above cases results in test particles with sufficiently low final e.This work is supported in part by Hong Kong RGC grant HKU 7030/11P. (c) 2015: American Astronomical Society