Dione is probably composed of a rocky core making up one-third of the moon's mass, with the rest water-ice. Its ice coverage is less than that of Tethys and Rhea. Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and wispy material. The heavily cratered terrain has numerous craters greater than 100 kilometers in diameter. The plains area tends to have craters less than 30 kilometers in diameter. Some of the plains are heavily cratered while others are not. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This is opposite from what some scientists expected. Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with the highest cratering rates on the leading hemisphere and the lowest on the trailing hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation.
(Source: NASA/JPL/Space Science Institute/ Solar View) |
