[Paper Review] Formation of irregular and runaway moons/exomoons through moon-moon scattering

Hagai B. Perets, Matthew J. Payne|arXiv (Cornell University)|Jul 9, 2014

Astro and Planetary Science3 citations

TL;DR

This paper proposes that moon-moon scattering in post-disk satellite systems can dynamically excite regular moons into high-eccentricity, high-inclination orbits, producing prograde irregular moons and runaway moons. Through N-body simulations, it shows that massive moons are commonly ejected, leaving 1–4 survivors with irregular orbits, while a significant fraction of small moons are temporarily recaptured into retrograde orbits before colliding with the planet.

ABSTRACT

Gas giant planets in the Solar system host large satellite systems with multiple regular and irregular moons. Regular moons revolve around their host planet in circular, low inclination short period orbits, and are thought to form in-situ through coagulation processes. In contrast, irregular moons have highly inclined (and even retrograde), typically eccentric and long period orbits around their host planet. Irregular moons are therefore often thought to be unbound objects in helio-centric orbits that were later captured to their current orbits around the planet. Here we study the a different possibility in which regular moons form in-situ, and dynamically evolve through mutual moon-moon scattering. We find that such evolution can excite the satellites into high eccentricities and inclinations. We find that moons are either ejected from the host planet to become runaway moons, or stay bound and become prograde orbiting irregular moons with inclined and eccentric orbits around their host planet. Ejected moons, unbound to the planet, can later be temporarily re-captured by the host planet even at retrograde orbits. Such moons are eventually re-ejected from the system or collide with the planet, at least in the absence of dissipative processes (e.g. collisions with existing bound moons, a debris disk or through tidal interactions with the host planet), not currently modeled. Uncaptured runaway moons may eventually be ejected from the Solar system, or be captured into stable helio-centric orbits and contribute to the populations of asteroidal or trans-Neptunian objects. Such scenarios are potentially relevant both for the gas-giant satellites in the Solar system and for the dynamical evolution of exomoons.

Motivation & Objective

• To investigate whether moon-moon gravitational scattering can explain the origin of irregular moons in the Solar System and exomoons.
• To explore the dynamical evolution of multi-moon systems after the dissipation of the gaseous circum-planetary disk.
• To determine whether such scattering can produce prograde and retrograde irregular moons, including temporarily captured runaway moons.
• To assess the potential for dissipative processes (e.g., tidal interactions) to stabilize retrograde moons, which are otherwise unstable in the simulations.
• To examine the implications of moon ejection for the populations of minor bodies in the Solar System, such as asteroids and trans-Neptunian objects.

Proposed method

• Conduct N-body simulations of multi-moon systems with varying initial masses and orbital configurations following gaseous disk dissipation.
• Model moon-moon gravitational scattering using direct numerical integration with high-precision orbital integrators.
• Include massless particles to represent low-mass moonlets and track their orbital evolution and ejection.
• Analyze orbital parameters (eccentricity, inclination, semi-major axis) of surviving and ejected moons to classify them as regular, irregular, or runaway.
• Simulate temporary recaptures of ejected moons by the host planet and assess retrograde capture rates.
• Compare simulation outcomes with observed properties of irregular moons (e.g., inclinations, eccentricities) and minor body populations.

Experimental results

Research questions

• RQ1 Can moon-moon scattering alone produce the high-inclination and high-eccentricity orbits observed in prograde irregular moons?
• RQ2 What fraction of small moons are temporarily recaptured into retrograde orbits after ejection, and under what conditions?
• RQ3 How do the final architectures of moon systems (number of survivors, orbital parameters) depend on initial conditions such as moon mass and spacing?
• RQ4 Why are no massive retrograde irregular moons observed in the Solar System, and could dissipative processes explain this?
• RQ5 Can moons ejected from their host planet become part of the background populations of asteroids or trans-Neptunian objects?

Key findings

• More than 50% of small moons are ejected from the system, with approximately 5–10% of these being temporarily recaptured into retrograde orbits before colliding with the planet.
• The surviving massive moons typically have high eccentricities and inclinations, matching the orbital characteristics of observed prograde irregular moons.
• The number of surviving moons is typically 1–4, regardless of the initial number, mirroring the outcome of planet-planet scattering in multi-planet systems.
• No stable retrograde orbits are achieved for massive moons in the absence of dissipative processes, suggesting that such processes may be necessary for long-term stability.
• Ejected moons can become transiently bound to the host planet or be captured into heliocentric orbits, potentially contributing to the populations of minor bodies like asteroids and trans-Neptunian objects.
• The mechanism provides a plausible alternative to traditional capture models for irregular moons, avoiding fine-tuned initial conditions.