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

TL;DR

This paper demonstrates that the irregular shapes of Saturn's small moons can be explained by merging collisions of similar-sized moonlets, revealing insights into their formation processes.

Contribution

It introduces a collision-based formation model that accounts for the diverse shapes of Saturn's small moons, linking their morphology to specific merger scenarios.

Findings

Merging collisions produce shapes similar to observed moons.

Impact velocity range of 1-5 times escape velocity is critical.

Formation of Iapetus' ridge can also be explained by this mechanism.

Abstract

The Cassini spacecraft revealed the spectacular, highly irregular shapes of the small inner moons of Saturn, ranging from the unique "ravioli-like" forms of Pan and Atlas 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 planet, 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 size. Finally, we show that this mechanism may also explain the formation of Iapetus' equatorial ridge, as well as its oblate shape.