Dione (moon)

Dione is a moon of Saturn discovered by Giovanni Cassini in 1684. It is composed primarily of water ice, but as the densest of Saturn's moons (aside from Titan, whose density is increased by gravitational compression) it must have a considerable fraction of denser material like silicate rock in its interior.

Though somewhat smaller, Dione is otherwise very similar to Rhea. They both have similar compositions, albedo features and varied terrain. Both have dissimilar leading and trailing hemispheres. On the trailing hemisphere of Dione there is a network of bright streaks on a dark background and few visible craters. The streaks overlay the craters, indicating that they are newer. The leading hemisphere is heavily cratered and is uniformly bright. The origin of the bright wispy material is somewhat obscure. Apparently, it is material with a high albedo and is thin enough that it doesn't obscure the surface feature underneath. It might have formed from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash.

Like Callisto, the craters lack the high relief features seen on the Moon and Mercury; this is probably due to slumping of the weak icy crust over geologic time.

It is thought that shortly after its formation Dione was geologically active, with some process such as ice volcanism resurfaced much of Dione and leaving the pattern of streaks over its whole surface. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and streaks of 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. Because Dione is relatively small, an impact causing a 35 kilometer crater could have spun the satellite. Since there are many craters larger than 35 kilometers, Dione could have been repeatedly spun during its early heavy bombardment. The pattern of cratering since then and the bright albedo of the leading side suggests that Dione has remained in its current orientation for several billion years.

The moon Helene orbits in Dione's leading Lagrangian point, L₄.