Discovered in 1684 by Giovanni Cassini, Dione is an icy and dense moon composed of a rocky core with large amounts of water and ice. It is tidally locked to Jupiter as reflected by data from albedo observations and has a mixture of cratered plains ranging from 30 kilometers to 100 kilometers in diameter. Most of the heavily cratered plains lie in the trailing hemisphere while the less cratered plains lie in the leading hemisphere. This fact was somewhat of a surprise to scientists since it was the exact opposite of what they had expected. If Dione had its heavily cratered plains in the trailing hemisphere and the less cratered plains in the leading hemisphere, that means it must have been tidally locked to Saturn in the opposite direction. Shoemaker and Wolfe had proposed this model and further theorized that this was caused by an impact that left a 35-kilometer (21-mile) crater to spin the moon.

Dione is second most dense moon of Saturn, next to Titan with a density of 1.43 g/cm^3. Most of the satellite is composed of water and ice (66%), with the rest rocky material (33%). The satellite also has bright misty material on its surface that is somewhat transparent. The exact origin of this material is unknown but it could be snow or ash from the moon's surface.

Physical Data Key Mass Mass Mass Mass compared to that of Earth. Radius Radius Radius Radius compared to that of Earth. Mean Density Average Density. Distance Distance to the planet's center. Magnitude The brightness of a star or object.

Orbital Data Key Rotational Period Number of days to make one complete rotation. Orbital Period Number of days to obital the planet. Mean Orbital Velocity Average obital speed. Orbital Eccentricity Eccentricity. Orbital Inclination The tilt of the moon or planet. Escape Escape Velocity. Albedo Visual geometric albedo.