Accretion of ornamental equatorial ridges on Pan, Atlas and Daphnis

Abstract

We explore scenarios for the accretion of ornamental ridges on Saturn’s moons Pan, Atlas, and Daphnis from material in Saturn’s rings. Accretion of complex shaped ridges from ring material should be possible when the torque from accreted material does not exceed the tidal torque from Saturn that ordinarily maintains tidal lock. This gives a limit on the maximum accretion rate and the minimum duration for equatorial ridge growth. We explore the longitude distribution of ridges accreted from ring material, initially in circular orbits, onto a moon that is on a circular, inclined or eccentric orbit. Sloped and lobed ridges can be accreted if the moon tidally realigns during accretion due to its change in shape or because the disk edge surface density profile allows ring material originating at different initial semi-major axes to impact the moon at different locations on its equatorial ridge. We find that accretion from an asymmetric gap might account for a depression on Atlas’s equatorial ridge. Accretion from an asymmetric gap at orbital eccentricity similar to the Hill eccentricity, might allow accretion of multiple lobes, as seen on Pan. Two possibly connected scenarios are promising for growth of ornamental equatorial ridges. The moon migrates through the ring, narrowing its gap and facilitating accretion. The moon’s orbital eccentricity could increase due to orbital resonance with another moon, pushing it into its gap edges and facilitating accretion.