Cassini view of Dione's leading hemisphere. The large craters on or near the terminator are (from bottom to top) Evander, Erulus, Lagus and Sagaris. The Palatine Chasmata fractures stretch across the lower right limb, and the trough Aufidus Catena extends along the bottom near the south pole.
|Discovered by||Giovanni Cassini|
|Discovery date||March 21, 1684|
|Alternative names||Saturn IV|
|Semi-major axis||377 396 km|
|Orbital period||2.736 915 d|
|Inclination||0.019° (to Saturn's equator)|
|Dimensions||1128.8 × 1122.6 × 1119.2 km|
|Mean radius||561.4 ± 0.4 km (348.96 mi)|
|Surface area||3,964,776.51 km2|
|Mass||(1.095 452 ± 0.000 168)×1021 kg (3.28×10−4 Earths)|
|Mean density||1.478 ± 0.003 g/cm³|
|Equatorial surface gravity||0.233 m/s2|
|Escape velocity||0.51 km/s|
|Rotation period||2.736 915 d
|Albedo||0.998 ± 0.004 (geometric)|
|Temperature||87 K (−186°C)|
|Apparent magnitude||10.4 |
Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("the stars of Louis") to honor king Louis XIV. Cassini found Dione using a large aerial telescope he set up on the grounds of the Paris Observatory. The satellites of Saturn were not named until 1847, when William Herschel's son, John Herschel published Results of Astronomical Observations made at the Cape of Good Hope, and suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
Physical characteristics 
At 1122 km in diameter, Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. It is composed primarily of water ice, but as the third densest of Saturn's moons (after Enceladus and Titan, whose density is increased by gravitational compression) it must have a considerable fraction (~ 46%) of denser material like silicate rock in its interior.
Though somewhat smaller and denser, Dione is otherwise very similar to Rhea. They both have similar albedo features and varied terrain, and both have dissimilar leading and trailing hemispheres. Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, meanwhile, contains an unusual and distinctive surface feature: a network of bright ice cliffs.
Scientists recognise Dionean geological features of the following types:
- Chasmata (chasms; long, deep, steep-sided depressions)
- Dorsa (ridges)
- Fossae (long narrow depressions)
- Catenae (crater chains)
The ice cliffs (formerly 'wispy terrain') 
When the Voyager space probe photographed Dione in 1980, it showed what appeared to be wispy features covering its trailing hemisphere. The origin of these features was mysterious, as all that was known was that the material has a high albedo and is thin enough that it does not obscure the surface features underneath. One hypothesis was that shortly after its formation Dione was geologically active, and some process such as ice volcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.
This theory was proven wrong by the Cassini probe flyby of December 13, 2004, which produced close-up images. These revealed that the 'wisps' were in fact not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata); Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.
The Cassini orbiter performed a closer flyby of Dione at 500 km (310 mi) on October 11, 2005, and captured oblique images of the cliffs, showing that some of them are several hundred metres high.
Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and areas of tectonic fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres (62 mi) in diameter. The plains areas tend to have craters less than 30 kilometres (19 mi) in diameter. Some of the plains are more heavily cratered than others. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what some scientists expected; Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with the highest cratering rates on the leading hemisphere and the lowest on the trailing hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation. Because Dione is relatively small, an impact causing a 35 kilometer crater could have spun the satellite. Since there are many craters larger than 35 kilometres (22 mi), Dione could have been repeatedly spun during its early heavy bombardment. The pattern of cratering since then and the bright albedo of the leading side suggests that Dione has remained in its current orientation for several billion years.
On April 7, 2010, instruments on board the unmanned Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions (O+
2) around the moon, so thin that scientists prefer to call it an "exosphere" rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 cm-3.
The Cassini probe instruments were unable to directly detect water from the exosphere due to high background levels, but it seems that highly charged particles from the planets' powerful radiation belts could split the water in the ice into hydrogen and oxygen.
Dione was first imaged by the Voyager space probes. It has also been probed three times from closer distances by the Cassini orbiter. There was one close targeted fly-by, at a distance of 500 km on 2005 October 11; another similarly close fly-by was performed on 2010 April 7. A third fly-by was performed on 2011 December 12 at an altitude of 99 km.
See also 
- Dione in fiction
- The moon Helene, which orbits in Dione's leading Lagrangian point, L4.
- The moon Polydeuces orbits in Dione's trailing Lagrangian point, L5.
Dione as seen by Voyager 1; the craters prominent at upper and lower left are Dido and Aeneas; to the latter's right are the troughs Latium and Larissa chasmata.
Wispy terrain on Dione's trailing hemisphere. The Eurotas (top) and Palatine Chasmata run from upper right to lower left; the Padua Chasmata are near vertical at right, and the Carthage Fossae horizontal at left. The crater Cassandra and its ray system are at lower right.
Differently sized and oriented fractures within 60-km crater Amastrus (central peak at lower right). Larger arcuate fractures running from lower left to upper right are the Padua Chasmata, while smaller more parallel fractures from lower right to upper left may be related to the Aurunca Chasmata.
South polar impact basin Evander, 350 km in diameter, is by far the largest crater on Dione. The deep crater to its upper left is Sabinus.
|Wikimedia Commons has media related to: Dione|
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- Dione Profile at NASA's Solar System Exploration site
- The Planetary Society: Dione
- NASA probe video of approach to Dione
- Cassini images of Dione
- Images of Dione at JPL's Planetary Photojournal
- Dione global and polar basemaps (December 2011) from Cassini images
- Dione atlas (Sept. 2011) from Cassini images
- Dione nomenclature and Dione map with feature names from the USGS planetary nomenclature page