The Midsized Moons of Saturn

The midsized, icy moons of Saturn—Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus—are each remarkable worlds and together form a fascinating system. The innermost of these bodies orbit Saturn within nine Saturnian radii (RS): Mimas (3.08 RS), Enceladus (3.95 RS), Tethys (4.89 RS), Dione (6.26 RS), and Rhea (8.75 RS). The six satellites also come in diameter-sized pairs: ~500 km for Mimas and Enceladus, ~1,000 km for Dione and Tethys, and ~1,500 km for Rhea and Iapetus. Bulk densities all center around the value for water ice, implying a dominant icy bulk composition with varying amounts of rock. Enceladus, with its remarkable heavily tectonized surface and south polar plume discovered by the Cassini mission, hosts a possibly habitable subsurface ocean and is the source of Saturn’s E-ring, which extends from 3 to 8 RS. It is unclear whether the other inner midsized moons are also ocean worlds; their surfaces are largely dominated by impact craters. The surfaces of Mimas, Dione, Tethys, and Rhea all exhibit surface tectonic features indicating some level of endogenic activity, but evidence is lacking on whether oceans are required. Dione and Tethys also have regions of smoother terrain that may possibly be cryovolcanic. The formation and surface ages of the midsized moons are still heavily debated due to lack of knowledge of tidal dissipation levels in Saturn and the satellites, the satellites’ orbital evolutions, and the relevant impactor fluxes. The surface compositions of Saturn’s moons are largely controlled by their environments. At the inner midsize moons, which are embedded in the E-ring, gradients in water ice abundance, as well as hemispheric albedo dichotomies, with distance from Enceladus demonstrate the key role of E-ring particles in driving surface composition and albedo. A variation with distance from Enceladus is also seen in a reddish visible color in the system, the origin of which is still unclear. In the outer part of the system, the leading hemisphere of Iapetus (59.09 RS) is colored by dark, reddish organic-rich dust originating from Phoebe.