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

Abstract

The Cassini spacecraft revealed the spectacular, highly irregular shapes of the small inner moons of Saturn 1 , ranging from the unique "ravioli-like" forms of Pan and Atlas 2,3 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 planet 4 , 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 4,6,7 .Finally, we show that this mechanism may also explain the formation of Iapetus' equatorial ridge 8 , as well as its oblate shape 9 .The small inner moons Atlas, Prometheus, Pandora, Janus, and Epimetheus are repelled by the rings of Saturn at a rate that is proportional to their mass and decreases with their distance 11,4 . It has thus been proposed that these moons were formed in a pyramidal regime (i.e. by a series of mergers of similar sized bodies) as they migrated away from the rings 4,7 . This scenario is supported by the observations of the small inner Saturnian satellites, as bodies with similar semi-major axis have comparable masses: mPrometheus/mPandora~1.16 and mJanus/mEpimetheus ~3.6 (JPL SSD, not accounting for error bars), and the mass of these bodies increase with their distance to the rings. The pyramidal regime provides an alternative to the

Fate of the Runner in Hit-and-run Collisions

“…Lastly, there is a mismatch for the set of dynamical evolution where all the collision were modelled as occurring in the orbital plane. If all collisions happen in the same plane, then a different distribution of impact angles, shifted towards steeper values, is expected (Emsenhuber 2017; Leleu et al 2018). In the latter case, more head-on collisions are favored, and the median is located at 30 • , compared to 45 • in the general case following Shoemaker (1962).…”

Interior properties of the inner saturnian moons from space astrometry data

“…Such a scenario is supported by the monotonic distribution of the moons' masses as a function of distance to their primary, as well as their small densities of 0.4-0.7 g/cm 3 (Thomas et al 2013). Since collisions between moonlets are expected during such a formation process (Charnoz et al (2010), Leleu et al (2018)), a possible inhomogeneity may arise within the interior of these small icy moons. Here, we try to assess the interior distribution of these small moons by determining their physical librations.…”

High-resolution simulations of catastrophic disruptions: Resultant shape distributions

“…2b). Nearly head-on collisions between the remnants are reasonably probable because their orbital planes are similar (Leleu et al 2018). The existence of flat asteroids belonging to asteroid families may suggest the importance of the evolution of shapes through collisions between member asteroids.…”