Could the Craters on the Mid-Sized Moons of Saturn Have Been Made by Satellite Debris?

Saturn's mid-sized moons have usually been assumed to be primordial. However, Charnoz et al. (2011) and Crida and Charnoz (2012) showed that the steep trend of mass vs. distance of the moons out to Rhea is consistent with the spreading of an early massive ring (e.g., Canup 2010) beyond Saturn's Roche limit. In this model, these moons would be billions of years old, but with Mimas forming perhaps 1 Gyr after Rhea.Cuk et al. (2016) investigated the dynamical evolution of the, mid-sized saturnian moons due to tides. They infer that the moons have migrated little. Tethys and Dione probably did not cross their 3:2 resonance, but the system likely did cross a Dione-Rhea 5:3 resonance and a Tethys-Dione secular resonance. These crossings would have happened recently; for Q = 1500 (Lainey et al. 2012), within the past 100 Myr. Cuk et al. suggested that a previous generation of moons underwent an orbital instability, perhaps due to a solar evection resonance, leading to catastrophic collisions between them (Movshovitz et al. 2016). Today's moons would have reaccreted from the debris. This model implies that most craters on the moons were formed by this debris, with impacts taking place at much lower speeds than applies for impacts by comets.Many crater properties, such as the depth-to-diameter ratio (Bray and Schenk 2015) and the amount of melting and vaporization (Kraus et al. 2011), depend on the impact velocity. We will discuss how measurements of craters in Cassini images of saturnian moons can be used to distinguish between the Cuk et al. scenario and the view in which the largest craters are made by comets and planetocentric debris makes only smaller craters (Alvarellos et al. 2005).We thank the Cassini Data Analysis Program for support and Amy Barr Mlinar for discussions.Alvarellos, J.L., Zahnle, K.J., Dobrovolskis, A.R., Hamill,P. (2005). Icarus 178, 104Bray, V.J., Schenk, P.M. (2015). Icarus 246, 156Canup, R.M. (2010). Nature 468, 943Charnoz, S., et al. (2011). Icarus 216, 535Crida, A., Charnoz, S. (2012). Science 338, 1196Cuk, M., Dones, L., Nesvorny, D. (2016). Astrophys. J. 820:97Kraus, R.G., Senft, L.E., Stewart, S.T. (2011). Icarus 214, 724Lainey, V., et al. (2012). Astrophys. J. 752:14Movshovitz, N., et al. (2016). Icarus 275, 85