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Chaos-assisted capture of irregular moons

Abstract

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.

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Figure 1: The intersection of the Hill sphere in 3D with the surfaces of zero velocity, and histograms of inclination distributions for 108 test particles originating at the Hill sphere.
Figure 2: Poincaré surfaces of section showing regions of chaotic (‘shotgun’ pattern) and regular (nested curves) dynamics.
Figure 3: The normalized probability distribution of 3D orbits that survived in Monte Carlo simulations (without dissipation) for 20,000 years.
Figure 4: Comparison of the orbital properties of the known irregular satellites of Jupiter5,6 (to 20027) and Saturn8 with dissipative Monte Carlo simulations.

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Acknowledgements

This work was supported by the US National Science Foundation, the Royal Society (UK) and the US Office of Naval Research.

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Correspondence to Stephen Wiggins or David Farrelly.

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Astakhov, S., Burbanks, A., Wiggins, S. et al. Chaos-assisted capture of irregular moons. Nature 423, 264–267 (2003). https://doi.org/10.1038/nature01622

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