Dione (pronounced Die-oh-nee) is almost 700 miles wide. It is the third densest of Saturn's moons (Enceladus having recently been discovered to be denser than previously thought), and must consist of about 1/3 denser rocky materials and 2/3 lighter icy materials.
Like most of the moons in the Solar System Dione rotates synchronously, which means that it always keeps one face toward Saturn (the near side), and the opposite face away from Saturn (the far side). This also means, as shown in the diagram of synchronous rotation, below, that there is a "leading" side, which is always in front as Dione moves around Saturn, and a "trailing" side, which is always in back. The leading side of Dione is uniformly bright and covered by craters of various sizes, while the trailing side has a large number of lighter wispy streaks running through somewhat darker terrain. Until recently it was thought that the wisps were caused by thin layers of material somehow overlaid on the older craters. Close-up images by the Cassini spacecraft, however, show that the wisps are not "material" at all, but bright reflections from cliffs created by subsidence events. Why those events should take place primarily on the trailing side is not clear. Many of the moons of Saturn have differences between their leading and trailing sides, some even more dramatic than those on Dione, and an explanation of the differences between the two sides seems even more difficult now than before.
Diagram of Synchronous Rotation
Dione has some interesting gravitational interactions. As in the case of Tethys and its companion moons, Dione has trapped Helene
at the Lagrangian point 60 degrees ahead of Dione. In addition, it has locked Enceladus into an orbit which has exactly half the period that Dione has. This is similar to the interaction between Jupiter's moons Io and Europa, which are also locked into orbits with a period ratio of exactly 1 to 2. This type of gravitational interaction between objects is very common in the Solar System. It is called commensurability, which means that there is a common measure between two orbital periods, in that one orbital period, when divided into the other one, is exactly equal to the ratio of two small whole numbers. A similar example is the way in which Neptune controls the orbit of Pluto by forcing it into an orbital period which is 3/2 of the orbital period of Neptune.
The heavily cratered leading side of Dione as observed by Voyager 2
(NASA/JPL/Space Science Institute, Planetary Photojournal)
Wispy streaks on trailing side of Dione, also from Voyager 2
(NASA, JPL, Voyager 2, Calvin Hamilton, SolarViews)
Cratered leading side of Dione on left wispy trailing side on right. The wispy structures cross through older craters, indicating that they are younger than the period of bombardment which created the craters. (NASA, Voyager 1, Calvin J. Hamilton, apod951009)
An image taken by the Cassini spacecraft in July 2006 reveals that the wispy structures are reflection features created by sunlight shining off icy cliffs hundreds of yards in height. Apparently subsidence has fractured the trailing surface of Dione at some time in the relatively recent past (at least, more recent than the episode of bombardment which presumably ended the best part of 4 billion years ago). What caused the subsidence and why it seems to have occurred only on the trailing side of the moon remains a mystery. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod060905)
A closeup of Dione shows linear cliffs intersecting older craters. (NASA/JPL/Space Science Institute, Planetary Photojournal)
One of many images taken by the Cassini spacecraft as it neared closest approach to Dione in December, 2004. Craters cover the surface of the moon in innumerable numbers, while cracks and cliffs formed by subsidence crisscross the ancient landscape. On the right a star-shaped feature is probably a more recent impact, and rays created by secondary impacts of ejecta from the initial impact. (NASA/JPL/Space Science Institute, Planetary Photojournal)
Above, a portion of the highest resolution image of Dione currently available, taken by the Cassini spacecraft at its closest approach to the moon in December, 2004. The intensely cratered surface is crisscrossed by a complex network of fault scarps, which produce the "wispy" structure visible at a distance by reflecting sunlight from their icy surfaces.
Below, a closer view of the image above gives a closer approximation to the detail visible on the original images. Note the straight line of craters near top center, probably due to collision with a fragmenting comet. (NASA/JPL/Space Science Institute, Planetary Photojournal)
A portion of a look back at Dione after closest encounter shows a thin crescent thrown sharply into relief by the low angle of the Sun's rays. (NASA/JPL/Space Science Institute, Planetary Photojournal)
|Data for Dione|
Discovered by Giovanni Cassini in 1684
Named after one of the daughters of Uranus and Gaea
Orbital size 377,400 km (approximately 235,000 miles)
Orbital eccentricity 2%
Orbital inclination 0 degrees
Orbital period 2.73691 days
Rotation period 2.73691 days (synchronous rotation, one side permanently facing Saturn)
Diameter 1120 km (about 700 miles)
Mass 1/5500 of Earth, 1/70 of Earth's Moon
Surface gravity 2% of Earth, 1/7 of Earth's Moon
Density 1.4 times density of water (Composition probably 2/3 ice, 1/3 rock)
Albedo (reflectivity) 70%
Surface temperature 305 degrees below zero Fahrenheit
Traps Helene at its leading Lagrangian point