The fourth-largest satellite of Saturn, 1123 km in diameter; also known as Saturn IV. Dione orbits 377 415 km from Saturn in 2.737 days, sharing its orbit with the much smaller Helene. Its axial rotation is the same as its orbital period. Dione was discovered in 1684 by G. D. Cassini. The two hemispheres of Dione show different crater densities, the trailing face being more heavily cratered. The trailing hemisphere also displays extensive bright complexes of scarps and fissures which give an overall appearance of wispy streaks many hundreds of kilometres in length. Dione’s density is 1.5 g/cm3, suggesting that it consists of ice with a rocky core.
http://photojournal.jpl.nasa.gov/catalog/PIA08256