![]() Moreover Pan and Atlas, and possibly Pandora and Daphnis as well, are quite similar in shape, making for an impressive coincidence of remarkable events. ![]() Additionally, constraints on the collision impact parameter are stringent, both so that the bodies aren’t torn apart by centrifugal forces and because any nonzero impact parameter would produce rotation perpendicular to the moons’ equators-opposite of their current orientations. ![]() The binary collision theory produces veridical equatorial ridges ( Leleu et al., 2018), but requires that the two bodies have nearly identical masses, and deform rather than fragment. This possibility does not appear to have been examined previously. We show here that troughs as well as ridges arise naturally if freely flowing grains follow equipotentials of a massive spheroid in a massive plane, as shown in Figures 1E–H). ![]() No existing theory predicts equatorial troughs. Theories for the formation of equatorial ridges include collisions between matched pairs of bodies ( Leleu et al., 2018), viscous deformation of ring material ( Charnoz et al., 2011), and accretion of material from a flat ring onto a central satellite ( Charnoz et al., 2007 Porco et al., 2007). The origins of these features are unknown. Some of these features persist even for a very low density and distant disk, which raises the prospect that nonlinear analysis of interactions from disks to moons and back again may lead to new insights. Moreover, analysis of these features reveals the possibility of previously unreported morphologies, for example, a stationary torus around a non-rotating satellite. Such ridges and other features likely record the history of the moons, and we find that if surface grains are freely flowing, then the combined gravity of Saturn itself and its tenuous ring generate similar circumferential features. ![]() We investigate here whether notable circumferential ridges seen on Saturn’s moons may be a byproduct of free flow of loosely packed regolith. Department of Physics and Astronomy, Rutgers University, Piscataway, NJ, United StatesĮxploratory missions have found that regolith on interplanetary bodies can be loosely packed and freely flowing-a state that strongly affects mission plans and that may also influence the large-scale shapes of these bodies. ![]()
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