Dione

larger version with caption
Discovery
Discovered by	Giovanni Cassini
Discovered in	March 21, 1684
Orbital characteristics
Semimajor axis	377,396 km
Eccentricity	0.0022 [1]
Orbital period	2.736915 d [2]
Inclination	0.019° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Mean diameter	1118 km
Mass	1.096x1021 kg
Mean density	1.50 g/cm3
Surface gravity	0.022 m/s2
Escape velocity	0.5 km/s
Rotation period	2.736915 d (synchronous)
Axial tilt	zero
Albedo	0.55
Surface temperature	min mean max   -186°C
Atmosphere	none

Dione (dye-oe'-nee, Greek Διώνη) is a moon of Saturn discovered by Giovanni Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV. Image File history File links Dione11. ... Image File history File links Dione11. ... Giovanni Domenico (Jean-Dominique) Cassini Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian-French astronomer and engineer. ... March 21 is the 80th day of the year in the Gregorian Calendar (81st in leap years). ... Events France under Louis XIV makes Truce of Ratisbon separately with the Empire and Spain. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... The orbital period is the time it takes a planet (or another object) to make one full orbit. ... Inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit and is the angular distance of the orbital plane from the plane of the reference (usually planets equator or the ecliptic), stated in degrees. ... Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... For the geometric term, see diameter. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... Density (symbol: ρ - Greek: rho) is a measure of mass per unit of volume. ... The gram or gramme, symbol g, is a unit of mass, and is defined as one one-thousandth of the SI base unit kilogram (i. ... It has been suggested that Law of universal gravitation be merged into this article or section. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the gradient of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... In physics, for a given gravitational field and a given position, the escape velocity is the minimum speed an object without propulsion, at that position, needs to have to move away indefinitely from the source of the field, as opposed to falling back or staying in an orbit within a... On a prograde planet like the Earth, the sidereal day is shorter than the solar day. ... In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting. ... Axial tilt is an astronomical term regarding the inclination angle of a planets rotational axis in relation to its orbital plane. ... The albedo is a measure of reflectivity of a surface or body. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... The degree Celsius (°C or ℃ (Unicode 0x2103)) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... Moons of solar system scaled to Earths Moon The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. ... Atmospheric characteristics Atmospheric pressure 140 kPa Hydrogen >93% Helium >5% Methane 0. ... Giovanni Domenico (Jean-Dominique) Cassini Giovanni Domenico Cassini (June 8, 1625 - September 14, 1712) was an Italian-French astronomer and engineer. ... Events France under Louis XIV makes Truce of Ratisbon separately with the Empire and Spain. ... In Greek mythology, the Titans (Greek Τιταν, plural Τιτανες) are among a series of gods, some of whom opposed Zeus and the Olympian gods in their ascent to power. ... Dione in Greek mythology is a vague goddess presence who has her most concrete form in Book V of Homers Iliad as the mother of Aphrodite: Aphrodite journeys to Diones side after she has been wounded in battle while protecting her favorite son Aeneas. ... Greek mythology comprises the collected narratives of Greek gods, goddesses, heroes, and heroines, originally created and spread within an oral-poetic tradition. ...

There is also an asteroid called 106 Dione. An asteroid is a small, solid object in our Solar System, orbiting the Sun. ... 106 Dione is a large main belt asteroid. ...

Name

Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Lodicea Sidera ("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. Atmosphere none Tethys (tee-this or teth-is, Greek Τηθύς) is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684. ... Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ... Iapetus (eye-ap-i-tus, Greek Ιαπετός) (British spelling: Japetus) is the third-largest moon of Saturn (see: Saturns natural satellites), discovered by Giovanni Domenico Cassini in 1671. ... Louis XIV King of France and Navarre By Hyacinthe Rigaud (1701) Louis XIV (Louis-Dieudonné) (September 5, 1638–September 1, 1715) reigned as King of France and King of Navarre from May 14, 1643 until his death. ... Titan (tye-tun, Greek Τιτάνας) is the largest moon of Saturn and the second largest moon in the solar system[1], after Jupiters moon Ganymede. ... Mimas (mye-mus) is a moon of Saturn that was discovered in 1789 by William Herschel. ... [4]; [5] Atmospheric characteristics Pressure trace, signficant spatial variability [6] Water Vapor 65% [7] Hydrogen 20% [8] Other CO2, CO, N2 [9] Enceladus (en-sel-a-dus, Greek Εγκέλαδος) is a moon of Saturn discovered in 1789 by William Herschel. ... 1789 was a common year starting on Thursday (see link for calendar). ...

The names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope ([3]), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used. John Herschel John Frederick William Herschel (7 March 1792 – 11 May 1871) was an English mathematician and astronomer. ... Sir Wilhelm Friedrich Herschel (Hanover, November 15, 1738 – August 25, 1822 Slough, then in Buckinghamshire now in Berkshire) was a German-born British astronomer and composer who became famous for discovering the planet Uranus, and made many other astronomical discoveries. ... 1847 was a common year starting on Friday (see link for calendar). ... This page is a candidate for speedy deletion, because: it is patent nonsense. ...

Physical characteristics

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 densest of Saturn's moons (aside from Titan, whose density is increased by gravitational compression) it must have a considerable fraction of denser material like silicate rock in its interior. Image File history File links Download high resolution version (1000x1024, 134 KB)Enhanced color composite of Saturns moon Dione, based on infrared, green, ultraviolet, and clear-filter images taken by the Cassini spacecraft December 14, 2004. ... Image File history File links Download high resolution version (1000x1024, 134 KB)Enhanced color composite of Saturns moon Dione, based on infrared, green, ultraviolet, and clear-filter images taken by the Cassini spacecraft December 14, 2004. ... Titan (tye-tun, Greek Τιτάνας) is the largest moon of Saturn and the second largest moon in the solar system[1], after Jupiters moon Ganymede. ... In chemistry, a silicate is a compound consisting of silicon and oxygen (SixOy), one or more metals, and possibly hydrogen. ... Sedimentary, volcanic, plutonic, metamorphic rock types of North America. ...

Though somewhat smaller, Dione is otherwise very similar to Rhea. They both have similar compositions, 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. Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ...

Scientists recognise the following types of Dionean geological feature: Geology (from Greek γη- (ge-, the earth) and λογος (logos, word, reason)) is the science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it. ...

• Chasmata (chasms)
• Lineae (wispy features)
• Craters

Chasma (pl. ... Linea is Latin for line. In planetary geology it is used to refer to any long marking, dark or bright, on a planet or moons surface. ... The word crater may refer to A landform resembling a pit or depression in the topography that can be formed in several ways: a meteorite impact with another body can cause an impact crater, an electrical discharge such as lightning may form a crater-like pit, volcanic activity may form...

Cassini image of the bright cliffs on Dione that were mistaken for wispy streaks

Download high resolution version (1000x1000, 180 KB)The fractures and cliffs on Dione Original caption released with image: This very detailed image taken during the Cassini spacecrafts closest approach to Saturns moon Dione on December 14, 2004 is centered on the wispy terrain of the moon. ... Download high resolution version (1000x1000, 180 KB)The fractures and cliffs on Dione Original caption released with image: This very detailed image taken during the Cassini spacecrafts closest approach to Saturns moon Dione on December 14, 2004 is centered on the wispy terrain of the moon. ...

The 'wispy terrain'

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 theory 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 is an artists concept of Cassini during the Saturn Orbit Insertion (SOI) maneuver, just after the main engine has begun firing. ... December 13 is the 347th day of the year (348th in leap years) in the Gregorian calendar. ... 2004(MMIV) is a leap year starting on Thursday of the Gregorian calendar. ... The albedo is a measure of reflectivity of a surface or body. ...

However, the latest images from Cassini show that this theory 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. Results are forthcoming. October 11 is the 284th day of the year (285th in Leap years). ... 2005(MMV) is a common year starting on Saturday of the Gregorian calendar. ...

Craters

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 area tends to have craters less than 30 kilometers in diameter. Some of the plains are heavily cratered while others are not. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This is opposite from 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 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. Download high resolution version (756x744, 35 KB)This picture of Dione was take by Voyager 1 from a range of 162,000 kilometers (100,600 miles) on November 12, 1980. ... Eugene Shoemaker at a stereoscopic microscope used for asteroid discovery Eugene Merle Shoemaker (or Gene Shoemaker) (April 28, 1928 – July 18, 1997) was one of the founders of the fields of planetary science and is best known for co-discovering the Comet Shoemaker-Levy 9 with his wife Carolyn Shoemaker... Wolfe is a surname. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ...

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. Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (IPA: , ka-lis-toe, Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... Atmospheric characteristics Atmospheric pressure trace Potassium 31. ...

See also

• List of geological features on Dione
• The moon Helene, which orbits in Dione's leading Lagrangian point, L₄.
• The moon Polydeuces orbits in Dione's trailing Lagrangian point, L₅.

This is a list of named geological features on Dione. ... Helene (hel-e-nee, Greek ‘Ελένη) is a moon of Saturn. ... The Lagrangian points (IPA: ; also Lagrange point, L-point, or libration point), are the five positions in space where a small object can be stationary with respect to two larger objects (such as a satellite with respect to the Earth and Moon). ... Atmosphere none Polydeuces (pol-ee-dew-seez, Greek Πολυδεύκης) is a natural satellite of Saturn that was discovered on images taken on October 21, 2004 by the Cassini imaging team, led by Carolyn C. Porco et al, and given the temporary designation S/2004 S 5. ...

External links

• The Planetary Society: Dione

Atmosphere none Telesto (te-les-toe, Greek Τελεστώ) is a moon of Saturn. ... Atmosphere none Tethys (tee-this or teth-is, Greek Τηθύς) is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684. ... Atmosphere none Calypso (ka-lip-soe, Greek Καλυψώ) is a moon of Saturn. ... Atmosphere none Polydeuces (pol-ee-dew-seez, Greek Πολυδεύκης) is a natural satellite of Saturn that was discovered on images taken on October 21, 2004 by the Cassini imaging team, led by Carolyn C. Porco et al, and given the temporary designation S/2004 S 5. ... Helene (hel-e-nee, Greek ‘Ελένη) is a moon of Saturn. ... Atmosphere none Rhea (ree-a, Greek ‘Ρέα) is the second largest moon of Saturn and was discovered in 1672 by Giovanni Domenico Cassini. ...