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.