Saturn's Moons


Dione (Saturn IV)
Dione ( "dy OH nee" ) is the twelfth of Saturn's known satellites.

orbit:377,400 km from Saturn
diameter:1120 km
mass:1.05e21 kg
Discovered by:Cassini in 1684.

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.

On the trailing hemisphere 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 uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

This is 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.

Helene orbits in Dione's leading Lagrange point.
Helene (Saturn XII)
Helene is the thirteenth of Saturn's known satellites.

orbit:377,400 km
diameter:33 km (36 x 32 x 30)
mass: ?
Discovered by :Laques and Lecacheux in 1980 from ground-based observations.

Helene is in Dione's leading Lagrange point and hence was sometimes referred to as "Dione B".
Rhea (Saturn V)
Rhea ("REE a") is the fourteenth of Saturn's known satellites and the second largest.

orbit:527,040 km from Saturn
diameter:1530 km
mass:2.49e21 kg
Discovered by:Cassini in 1672.

Though somewhat larger, Rhea is otherwise very similar to Dione. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.

Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass. The leading hemisphere is heavily cratered and uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. Rhea's history is probably very similar to Dione's.

  • Difficulties:-
    1-How did Helene come to be in Dione's Lagrange point? Is it a fragment of an earlier larger body?
    2-What was the cause of the resurfacing? Why did it leave only albedo features on Dione?
    3-Why are there so many fewer craters on the trailing hemisphere?
    4-Why does Dione lack anything like the deep valley on Tethys?
    5-The surface features of Rhea and Dione seem almost identical yet Rhea is twice as massive. Why are they so similar?

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    Last Modified : 5 Sep. 2001
    Created By#C0115361 Team