INNER SATELLITES
Mimas, Enceladus, Tethys, Dione, and Rhea represent a body size not previously explored by spacecraft. They are larger than Jupiter’s Amalthea and Mars’ Phobos and Deimos, yet smaller than Mercury, our Moon, or Jupiter’s large satellites. Their diameters range from 390 kilometers (240 miles) for Mimas to 1530 kilometers (950 miles) for Rhea, and they are probably composed primarily of water ice.
With the exception of Enceladus, all of these moons have heavily cratered surfaces, looking much like the Moon and Mercury. Mimas displays an impact crater whose diameter is one-fourth that of the satellite—such an impact must have nearly shattered the icy satellite. Tethys has a valley 70 kilometers (40 miles) wide that stretches 800 kilometers (500 miles) across the satellite, an apparent crustal fracture resulting from seismic activity. Several sinuous valleys, some of which appear to branch, are visible on Dione’s surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces, perhaps caused by ice thrown out of craters by meteorite impacts.
Of the five inner moons, Enceladus appears the smoothest, but we will have to wait for Voyager 2 to photograph the satellite at greater resolution in 1981. Since the maximum intensity of the E-Ring occurs near Enceladus’ orbit, Enceladus may be a source of E-Ring particles.