# Obliquity evolution of the minor satellites of Pluto and Charon

## TL;DR

This study investigates the obliquity evolution of Pluto's small satellites using simulations, revealing that mean motion resonances and past orbital migrations could explain their high obliquities, which are not solely due to tidal effects.

## Contribution

It introduces a novel simulation approach combining a damped mass-spring model with N-body dynamics to analyze obliquity evolution of circumbinary satellites.

## Key findings

- Tidal dissipation alone does not cause large obliquity variations.
- Mean motion resonance interactions can induce high obliquities.
- Past orbital migration likely contributed to current obliquity states.

## Abstract

New Horizons mission observations show that the small satellites Styx, Nix, Kerberos and Hydra, of the Pluto-Charon system, have not tidally spun-down to near synchronous spin states and have high obliquities with respect to their orbit about the Pluto-Charon binary (Weaver et al. 2016). We use a damped mass-spring model within an N-body simulation to study spin and obliquity evolution for single spinning non-round bodies in circumbinary orbit. Simulations with tidal dissipation alone do not show strong obliquity variations from tidally induced spin-orbit resonance crossing and this we attribute to the high satellite spin rates and low orbital eccentricities. However, a tidally evolving Styx exhibits intermittent obliquity variations and episodes of tumbling. During a previous epoch where Charon migrated away from Pluto, the minor satellites could have been trapped in orbital mean motion inclination resonances. An outward migrating Charon induces large variations in Nix and Styx's obliquities. The cause is a commensurability between the mean motion resonance frequency and the spin precession rate of the spinning body. As the minor satellites are near mean motion resonances, this mechanism could have lifted the obliquities of all four minor satellites. The high obliquities need not be primordial if the minor satellites were at one time captured into mean motion resonances.