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The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets


The Cassini spacecraft revealed the spectacular, highly irregular shapes of the small inner moons of Saturn1, ranging from the unique 'ravioli-like' forms of Pan and Atlas2,3 to the highly elongated structure of Prometheus. Closest to Saturn, these bodies provide important clues regarding the formation process of small moons in close orbits around their host planet4, but their range of irregular shapes has not been explained yet. Here, we show that the spectrum of shapes among Saturn’s small moons is a natural outcome of merging collisions among similar-sized moonlets possessing physical properties and orbits that are consistent with those of the current moons. A significant fraction of such merging collisions take place either at the first encounter or after 1–2 hit-and-run events, with impact velocities in the range of 1–5 times the mutual escape velocity. Close to head-on mergers result in flattened objects with large equatorial ridges, as observed on Atlas and Pan. With slightly more oblique impact angles, collisions lead to elongated, Prometheus-like shapes. These results suggest that the current forms of the small moons provide direct evidence of the processes at the final stages of their formation, involving pairwise encounters of moonlets of comparable size4,5,6. Finally, we show that this mechanism may also explain the formation of Iapetus’ equatorial ridge7, as well as its oblate shape8.

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The authors acknowledge support from the Swiss NCCR PlanetS and the Swiss National Science Foundation.

Author information

A.L. performed the dynamical modelling and analysed the results. M.J. performed the collision modelling and analysed the results. M.R. contributed initial ideas for the study. All authors contributed to interpretation of the results and the preparation of the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to A. Leleu.

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Fig. 1: Shapes of small moons of Saturn compared to model outcomes.
Fig. 2: Impact probability distribution and collision regimes.
Fig. 3: Shapes resulting from merging collisions.
Fig. 4: Ridge formation on Iapetus.