Dione (moon)

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Not to be confused with the asteroid called 106 Dione.

**Dione**
Discovery
Image:Dione3 cassini big.jpg Cassini spacecraft view of Dione
Discovered by	G. D. Cassini
Discovery date	March 21, 1684
Orbital characteristics
Semi-major axis	377,396 km
Eccentricity	0.0022^[2]
Orbital period	2.736915 d^[3]
Inclination	0.019° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Dimensions	1127.6 × 1122 × 1120.6 km^[4]
Mean radius	561.70 ± 0.45 km^[5] (348.96 mi)
Mass	1.095452 ± 0.000168×10²¹ kg^[5] (3.28×10^-4 Earths)
Mean density	1.4757 ± 0.0036 g/cm³^[5]
Equatorial surface gravity	0.231 m/s²
Escape velocity	0.510 km/s
Rotation period	2.736915 d (synchronous)
Axial tilt	zero
Albedo	0.998 ± 0.004 (geometric)^[6]
Temperature	87 K (-186°C)
Apparent magnitude	10.4 ^[7]
Adjectives	Dionean

Dione (pronounced /daɪˈoʊni/ dye-OE-nee, or as Greek Διώνη) is a moon of Saturn discovered by Giovanni Cassini in 1684^[8]. It is named after the titan Dione of Greek mythology who (with Zeus) created Aphrodite. It is also designated Saturn IV.

1 Name
2 Physical characteristics
3 Exploration
4 See also
5 Gallery
6 References
7 External links

Name

Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII.

The names of all seven satellites of Saturn then known come from William Herschel's son, John Herschel (William Herschel discovered Mimas and Enceladus). In John Herschel's 1847 publication Results of Astronomical Observations made at the Cape of Good Hope,^[9] he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

Physical characteristics

$Cassini enhanced-color composite of Dione, showing the darker, fractured terrain on the trailing hemisphere$

Cassini enhanced-color composite of Dione, showing the darker, fractured terrain on the trailing hemisphere

Dione is composed primarily of water ice, but as the third densest of Saturn's moons (aside from 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, wispy streaks on a dark background that overlay the craters, indicating that they are newer. These are now known to be ice cliffs.

Scientists recognise the following types of Dionean geological feature:

Chasmata (chasms)
Lineae (wispy features)
Craters

Dione in a crescent taken by Cassini on October 11 2005

The ice cliffs (formerly 'wispy terrain')

Image:Dione Carthage Linea.jpg

Fractures bisecting older craters on Dione

Until the Cassini probe flyby of December 13, 2004, the origin of the bright wispy material was obscure, in part because the only photographs of it had been taken from a great distance. 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.

However, the latest images from Cassini show that this hypothesis is incorrect, and that the wisps are in fact not ice deposits at all, but rather the bright ice cliffs created by tectonic fractures; Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

The Cassini orbiter performed a closer flyby of Dione (500 km) on October 11, 2005, and captured oblique images of the cliffs, showing that some of them are several hundred metres high.

Craters

Image:Dione and Saturn.jpg

Dione against the shadow of Saturn's rings.

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 kilometers in diameter. The plains areas tends to have craters less than 30 kilometers 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^[10] 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 kilometers, 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.

Like Callisto, Dione's craters lack the high relief features seen on the Moon and Mercury; this is probably due to slumping of the weak icy crust over geologic time.

Map

Image:Dione 2006 PIA08341.jpg

composite image of Dione's surface

Exploration

Dione has been imaged several times from moderate distances by the Cassini orbiter. There was one close targeted fly-by, at a distance of 500 km on 2005 October 11; there are no plans for any others.

Gallery

Dione color south.jpg

Bottom right: A huge, shallow impact basin is visible near Dione's south pole

Dionean Linea PIA08256.jpg

The wispy terrain

Dione as seen by Voyager 1

References

^ [1]
^ http://exp.arc.nasa.gov/downloads/celestia/data/solarsys.ssc Exp.arc.nasa.gov Retrieved on 05-21-07
^ http://exp.arc.nasa.gov/downloads/celestia/data/solarsys.ssc Exp.arc.nasa.gov Retrieved on 05-21-07
^ Thomas, P. C.; Veverka, J.; Helfenstein, P.; Porco, C.; Burns, J.; Denk, T.; Turtle, E.; Jacobson, R. A. (March 13-17 2006). "Shapes of the Saturnian Icy Satellites". 37th Annual Lunar and Planetary Science Conference.
^ ^a ^b ^c Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et al. (December 2006). "The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data". The Astronomical Journal 132: 2520-2526.
^ Verbiscer, A.; French, R.; Showalter, M.; and Helfenstein, P.; Enceladus: Cosmic Graffiti Artist Caught in the Act, Science, Vol. 315, No. 5813 (February 9, 2007), p. 815 (supporting online material, table S1)
^ Classic Satellites of the Solar System. Observatorio ARVAL. Retrieved on 2007-09-28.
^ Cassini, G. D.; An Extract of the Journal Des Scavans. of April 22 st. N. 1686. Giving an account of two new Satellites of Saturn, discovered lately by Mr. Cassini at the Royal Observatory at Paris, Philosophical Transactions, Vol. 16 (1686-1692), pp. 79-85.
^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 1848 January 14)
^ Shoemaker, E. M.; and Wolfe, R. F.; Cratering time scales for the Galilean satellites, in Morrison, D., editor; Satellites of Jupiter, University of Arizona Press, Tucson (AZ) (1982), pp. 277-339

External links

v • d • e Moons of Saturn
Generally listed in increasing distance from Saturn. Temporary names in italics.
Ring shepherds (moonlets)	Pan · Daphnis · Atlas · Prometheus · ?S/2004 S 6 · ?S/2004 S 4 · ?S/2004 S 3 · Pandora
Co-orbitals	Epimetheus · Janus
Inner large (and Trojan)	Mimas · Methone · Anthe · Pallene · Enceladus · Tethys (Telesto • Calypso) · Dione (Helene • Polydeuces)
Outer large	Rhea (rings) · Titan · Hyperion · Iapetus
Inuit group	Kiviuq · Ijiraq · Paaliaq · Siarnaq · Tarqeq
Norse group	Phoebe · Skathi · S/2007 S 2 · Skoll · S/2004 S 13 · Greip · Hyrrokkin · Mundilfari · Jarnsaxa · S/2006 S 1 · S/2004 S 17 · Narvi · Bergelmir · Aegir · Suttungr · S/2004 S 12 · Bestla · Farbauti · Hati · S/2004 S 7 · Thrymr · S/2007 S 3 · S/2006 S 3 · Surtur · Kari · Fenrir · Ymir · Loge · Fornjot
Gallic group	Albiorix · Bebhionn · Erriapus · Tarvos
Rings of Saturn · Cassini-Huygens · Themis · Chiron

v • d • e Natural satellites of the Solar System
Planetary satellites	Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian
Other satellite systems	Plutonian · Eridian · Asteroid satellites
Largest satellites	Ganymede · Titan · Callisto · Io · Moon · Europa · Triton Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys · Enceladus · Miranda · Proteus · Mimas
Inner satellites · Trojans · Irregulars · List · List by diameter · Timeline of discovery · Naming