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Cassini image of trailing hemisphere of Dione showing the moon's complex of bright cliffs. At lower right is the feature called Cassandra, exhibiting linear rays extending in multiple directions. North is up. The image was taken in polarized green light with Cassini's narrow-angle camera at a distance of approximately 263,000 km (163,000 miles). Image scale is 2 km (1 mile) per pixel. Credit: NASA/JPL/Space Science Institute
The fourth-largest moon of Saturn and the third densest after Titan and Enceladus. It was discovered on March 21, 1684, by Giovanni Cassini and is also known as Saturn IV.

Dione's relatively high density suggests that it has a rocky core, making up about one third of its total mass, surrounded by water ice. In composition, albedo, and terrain, it is similar to Rhea. Both moons have markedly different leading and trailing hemispheres. The surface includes heavily-cratered terrain, moderately cratered plains, lightly cratered plains, and bright, wispy material. The heavily-cratered regions feature craters of all diameters up to and exceeding 100 km (60 miles), whereas the plains areas tend to have craters no larger than about 30 km across.

Contrary to what might be expected of a satellite in gravitational lock, it is the trailing hemisphere of Dione on which the majority of intense cratering exists. This suggests that, during the early period of heavy bombardment in the solar system, Dione was tidally locked to Saturn in the opposite direction to which it now spins. Subsequent major collisions with any objects large enough to cause craters more than about 35 km (21 miles) in diameter, of which there are many on Dione, would have been capable of altering the rate and direction of the moon's rotation. The likelihood is that Dione has been tidally locked in its current state for several billion years judging by the average albedo of the leading and trailing hemispheres. The albedo decreases from the leading to the trailing hemisphere since the former has suffered a higher rate of micrometeorite impacts. Streaks of bright material which interlace a dark portion of the surface are of uncertain origin but may be fractures associated with faults through which water has upwelled.

discovery 1684, by Giovanni Cassini
semimajor axis 377,396 km (234,553 miles)
diameter 1,123 km (698 miles)
mean density 1.5 g/cm3
escape velocity 0.51 km/s (1836 km/h, 1141 mph)
orbital period 2.74 days (2 days 17 hours 46 minutes)
orbital eccentricity 0.002
orbital inclination 0.02°
axial period synchronous
visual albedo 0.55

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   • Saturn, moons

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