Moons & Tides WELCOME TO THE FAMILY This diagram shows Saturn's rings (labeled in the order of their discovery), ring-shepherding moons, and icy midsize moons. (Technically, Iapetus is also a midsize moon, but its orbit lies outside Titan's, so it doesn't interact with the others and may have a different origin.) The orbits are shown to scale, but the moons' sizes are not. Moons smaller than a kilometer across are omitted. Ring shepherds Midsize moons Trojans/companions MIMAS This moon came to light as a dot in the telescope of William Herschel in 1789, long before the Voyagers and Cassini obtained closer views. Cassini's photo features the eponymous Herschel Crater that gives this geologically dead moon the appearance of the Death Star in Star Wars. D ring C ring B ring A ring F ring G ring SATURN Pan Daphnis Atlas Prometheus Pandora Janus Epimetheus Mimas Methone Anthe Pallene WOULD THEY FLOAT? The five midsize moons inside Titan's orbit exhibit a diversity of densities (and therefore compositions) that has long defied explanation. Percentage silicates Equatorial radius Floats* 1.5 1.0 2 Mimas *Density of water is 1 g/cm3 16 SEPTEMBER 2021 * SKY & TELESCOPE Enceladus 198 km 26% 252 km 57% Density (g/cm3)