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

Leleu, Adrien Sebastien; Jutzi, Martin; Rubin, Martin (2018). The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets. Nature astronomy, 2(7), pp. 555-561. Nature Publishing Group 10.1038/s41550-018-0471-7

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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
size4–6. 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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