1. Department of Chemistry & Biochemistry, Utah State University, Logan, Utah 84322-0300, USA
2. School of Mathematics, University of Bristol, Bristol BS8 1TW, UK

Correspondence to: Stephen Wiggins²David Farrelly¹ Correspondence and requests for materials should be addressed to S.W. (Email: s.wiggins@bristol.ac.uk) or D.F. (Email: david@habanero.chem.usu.edu).

Abstract

It has been thought^{1, 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 moons^{5, 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 drag^{3, 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.