Dione

Saturn IV

Dione Facts

• Dione is the twelfth of Saturn's known satellites:
  • distance from Saturn: 377,400 km
  • diameter: 1120 km
  • mass: 1.05e21 kg

• Pronounced "dy OH nee"

• In Greek mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter).

• 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 (picture 2). The streaks overlay the craters, indicating that they are newer.

• The leading hemisphere is heavily cratered and uniformly bright (picture 3). 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 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.

Pictures

1. (above) higher res view of the streaks 142k gif

2. Dione trailing hemisphere 118k gif; 20k gif; 62k jpg

3. Dione leading hemisphere 180k gif; 46k gif; 88k jpg

4. Dione, moon of Saturn, showing surface pattern 147k gif
5. Dione, moon of Saturn, full view 162k gif
6. Dione, moon of Saturn, close shot 190k gif

More about Dione

• from LANL
• from JPL

Helene

• Helene (Saturn XII) is the thirteenth of Saturn's known satellites:
  • distance from Saturn: 377,400 km
  • diameter: 33 km (36 x 32 x 30)
  a.k.a. 1980 S 6, Dione B

• Discovered by Laques and Lecacheux in 1980 from ground-based observations.

• Helene is in Dione's leading Lagrange point.

Pictures

1. (above) Helene 8k gif

More about Helene

• from LANL

Open Issues

• How did Helene come to be in Dione's Lagrange point? Is it a fragment of an earlier larger body?
• What was the cause of the resurfacing? Why did it leave only albedo features on Dione?
• Why are there so many fewer craters on the trailing hemisphere?
• Why does Dione lack anything like the deep valley on Tethys?