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Orbit: 377,400 km from Saturn
Orbital period: 66 hours
Gravity: 0.232 m/s²
Circumference: 3,529 km
Diameter: 1120 km
Mass: 1.05e21 kg
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.
Dione is the densest of Saturn’s moons (aside from Titan, whose density is increased by  gravitational compression). It is composed primarily of water ice but must  have a considerable fraction of denser material like silicate rock.Though somewhat smaller, Dione is otherwise very similar to Rhea.  They both have similar compositions, albedo  features and varied terrain. Both  rotate synchronously  and have dissimilar leading  and trailing hemispheres.

Saturn's moon Dione

On the trailing hemisphere there is a network  of  bright streaks on a dark background and few visible craters (above). The  streaks  overlay the craters, indicating that they are newer.

Saturn's moon Dione

The leading hemisphere is heavily cratered  and uniformly bright  (above).   Like Callisto, the craters lack  the high relief features seen on the Moon and  Mercury.

This was interpreted as follows: shortly after its formation Dione was  active. Some processes (ice volcanism?) resurfaced much of Dione leaving  the pattern of streaks, probably on the whole surface. Later, after the internal activity and resurfacing ceased, a much less intense series of  impacts (which left craters too small to be seen in  Voyager’s  images) occurred.  This was concentrated on the leading hemisphere and  wiped out the streak patterns but left them intact on the trailing  hemisphere.

Saturn's moon Dione, lg

And as is often the case in  science and to the delight of all involved, newer data shows that the previous  hypothesis was wrong.  Higher resolution images from Cassini clearly show that the streaks are not ice flows but rather a complex network  tectonic fractures that are younger than most of the craters (click the image to  the left).

Helene orbits in Dione’s leading  Lagrange point. The tiny moon Polydeuces  (S/2004 S5), discovered by Cassini in 2004,  occupies the trailing Lagrange point.




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