Chaos-assisted capture of irregular moons

It has been thought1,2,3 that the capture of irregular moons—with non-circular orbits—by giant planets occurs by a process in which they are first temporarily trapped by gravity inside the planet's Hill sphere (the region where planetary gravity dominates over solar tides4). The capture of the moons is then made permanent by dissipative energy loss (for example, gas drag3) or planetary growth2. But the observed distributions of orbital inclinations, which now include numerous newly discovered moons5,6,7,8, cannot be explained using current models. Here we show that irregular satellites are captured in a thin spatial region where orbits are chaotic9, and that the resulting orbit is either prograde or retrograde depending on the initial energy. Dissipation then switches these long-lived chaotic orbits10 into nearby regular (non-chaotic) zones from which escape is impossible. The chaotic layer therefore dictates the final inclinations of the captured moons. We confirm this with three-dimensional Monte Carlo simulations that include nebular drag3,4,11, and find good agreement with the observed inclination distributions of irregular moons at Jupiter7 and Saturn8. In particular, Saturn has more prograde irregular moons than Jupiter, which we can explain as a result of the chaotic prograde progenitors being more efficiently swept away from Jupiter by its galilean moons.