Formation and Evolution of Pluto’s Small Satellites

Abstract

Plutoʼs system of five known satellites is in a puzzling orbital configuration. Each of the four small satellites are on low-eccentricity and low-inclination orbits situated near a mean motion resonance with the largest satellite Charon. The Pluto-Charon binary likely formed as a result of a giant impact, and so the simplest explanation for the small satellites is that they accreted from the debris of that collision. The Pluto-Charon binary has evolved outward since its formation due to tidal forces, which drove them into their current doubly synchronous state. Meanwhile, leftover debris from the formation of Charon was not initially distant enough from Pluto-Charon to explain the orbits of the current small satellites. The outstanding problems of the system are the movement of debris outward and the small satellites' location near mean motion resonances with Charon. This work explores the dynamical behavior of the collisionally interacting debris orbiting the Pluto-Charon system. While this work specifically tests initial disk and ring configurations designed to mimic the aftermath of the disruption of satellites by heliocentric impactors, we generally find that collisional interactions can help move material outward and keep otherwise unstable material dynamically bound to the Pluto-Charon system. These processes can produce rings of debris whose orbits evolve rapidly due to collisional processes, with increasing pericenters and decreasing semimajor axes. While these rings and disks of debris eventually build satellites that are significantly farther out than the initial locations of a disrupted satellite, they do not show a strong preference for building satellites in or near mean motion resonances with Charon under a wide array of tested conditions.

The Fate of Debris in the Pluto-Charon System

“…The disc has 27060 test particles that range in barycentric distance from 0-65000 km with eccentricities randomly drawn from a uniform distribution from 0 to 0.01 and inclinations randomly drawn from a uniform distribution from 0 to 0.5 • . Walsh & Levison (2015) find that a stable ring around the Pluto-Charon binary should collisionally circularize in about a decade, meaning that any free eccentricity in the debris' orbit set from the progenitor collision should damp within a few decades. While the disc will not be perfectly circular due to a forced eccentricity from the binary, in the regions under consideration, the forced eccentricity will be less generally much less than 0.3 using the formulation from Leung & Lee (2013).…”

“…Despite a compelling explanation for the formation of Charon, a theory for the emplacement of the four small circumbinary moons remains elusive. Many works, such as Ward & Canup (2006), Lithwick & Wu (2008a,b), Canup (2011), Cheng et al (2014b), Kenyon & Bromley (2014), and Walsh & Levison (2015), have tried to explain the location of the small moons. Dynamical stability studies by Youdin et al (2012) predicted low masses and high albedos for the moons, which were confirmed by Brozović et al (2015), and New Horizons (Stern et al 2015).…”

“…The destabilizing influence of the binary almost certainly rules out in-situ formation if Charon undergoes eccentric tidal evolution. As noted by Kenyon & Bromley (2014), Walsh & Levison (2015), and Bromley & Kenyon (2015), the Holman & Wiegert (1999) binary instability boundary provides strict limitations on the stable locations of particles around the Pluto-Charon binary. The location of this empirical boundary, acrit, is a function of both binary eccentricity and mass ratio (µ = MC/(MP +MC)), as shown in equation 1.…”

A Pluto–Charon Sonata: Dynamical Limits on the Masses of the Small Satellites

“…Using numerical simulations to test the plausibility of this picture requires accurate masses for the four small satellites. The total mass of the satellite system derived here, 10 20 g, provides a new target for theoretical models that attempt to predict the growth of satellites within circumbinary debris from the Pluto-Charon collision (Kenyon & Bromley 2014;Walsh & Levison 2015) or the long-term stability of satellites as the binary circularizes and expands following the collision (Bromley & Kenyon 2015b;Cheng et al 2014b;Smullen & Kratter 2017;Woo & Lee 2018). The smaller masses for the satellites may also improve our understanding of their rotational evolution (Quillen et al 2017).…”

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

“…Lower mass planetesimals with r = 10-20 km have a factor of ten smaller escape velocities and therefore stir tracers to veloci-ties ∼ 10% of the local escape velocity. Small particles with these velocities might survive collisional evolution within circumbinary rings (Kenyon & Bromley 2014;Walsh & Levison 2015;Bromley & Kenyon 2015).…”