Dione is one of the densest of Saturn’s 47 moons. While most of Saturn’s moons have a mostly icy composition, Dione likely has a rocky core which accounts for one third of the moon’s mass, with the rest being ice. The surface of Dione is cratered, but varying in intensity, with heavily cratered plains, moderated cratered plains, lightly cratered plains and streaks that appear lighter. The leading hemisphere and the trailing hemisphere are different, with most of the heavily cratered plains on the leading hemisphere, and the lightly cratered plains along with the wispy material on the trailing hemisphere.
Until the Cassini probe flew by in December of 2004, the origin of the bright streaks on Dione was unknown. The original hypothesis was proved wrong when the latest images from Cassini were made available. It appears from the latest photographs that the bright streaks are ice cliffs created by tectonic fractures. Due to Cassini’s closer fly by in October of 2005, it is now known that some of the cliffs are almost 1000 feet high.