Known Space -- Sol -- Saturn -- | Dione |
Dione, the twelfth of Saturn's moons, is an icy body similar to Tethys and Rhea. Its density is 1.43 gm/cm3, which makes it the densest moon of Saturn other than Titan. Dione is 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 whispy 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 suggests that during a 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.
Dione has probably been tidally locked in its current position for the past several billion years. This is reflected in the average surface albedo of the leading and trailing hemispheres. The surface albedo decreases from the leading to the trailing hemispheres due to a higher micrometeor dusting on the leading hemisphere.
The origin of the bright whispy 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.
Helene orbits in Dione's leading Lagrange point.
Orbital Distance: 377,400 km (average) Eccentricity: 0.002 Local Year: 2.737 days (2d 17h 41m 9s) Orbital Velocity: 10.028 km/s Inclination: 0.02° Local Day: 2.737 days (2d 17h 41m 9s) Axial Tilt: n/a Diameter: 1,120 km Composition: Rock and ice Density: 1.43 g/cm3 Mass: 1.05 x 1018 tonnes (0.0002 earths) Gravity: 0.023 G Escape Velocity: 0.500 km/s Area: 3.9 million km2 Surface Material: Mostly H2O ice Atmosphere: None Pressure: None Albedo: 0.70 Temperature: -186.56°C |