Moon-moon Scattering and the Origin of Irregular and Runaway Moons

Observations of the Solar system show that planetary satellites exist in various configurations; some have circular, co-planar orbits, and these are termed regular satellites. Other irregular satellites, have typically eccentric, inclined, and even retrograde orbits. Regular satellites are formed through core-accretion; similar to planet formation scenarios, but the origin of irregular satellites is still debated. Various formation scenarios have been suggested, involving capture of external unbound objects, either following a disruption of a binary minor planet, interaction of a single planetesimal with the planetary atmosphere of the planet, or through chaotic capture of planetesimals during rapid growth of the planetary embryos. However, it is difficult to reconcile the number of irregular moons with these hypotheses. Here we present a different hypothesis for the origin of irregular moons, through the in-situ formation of regular moons, which then scatter each other into irregular inclined and eccentric configurations. Such interaction could possibly lead to ejection from the system, producing “runaway moons”. We find instability regions where moons similar to the two biggest moons of Jupiter, Saturn and Uranus, could have become dynamically unstable due to mutual interactions. We show that moon-moon scattering in these regions could lead to ejection of moons from the system, and explore the eccentricity and inclination excitations of the moons' orbits as a function of distance from the host planet.