Dione (moon)

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Not to be confused with the asteroid called 106 Dione.
For other uses, see Dione.
Dione
Dione3 cassini big.jpg
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.
Discovery
Discovered by Giovanni Cassini
Discovery date March 21, 1684
Designations
Saturn IV
Adjectives Dionean
Orbital characteristics
377396 km
Eccentricity 0.0022[1]
2.736915 d[1]
Inclination 0.019° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions 1128.8 × 1122.6 × 1119.2 km[2]
Mean radius
561.4±0.4 km[2]
3964776.51 km2[3]
Mass (1.095452±0.000168)×1021 kg[4] (3.28×10−4 Earths)
Mean density
1.478±0.003 g/cm³[2]
0.233 m/s2
0.51 km/s
2.736915 d
(synchronous)
zero
Albedo 0.998±0.004 (geometric)[5]
Temperature 87 K (−186°C)
10.4 [6]

Dione (/dˈni/;[7] Greek: Διώνη) is a moon of Saturn discovered by Cassini in 1684.[8] It is named after the Titaness Dione of Greek mythology. It is also designated Saturn IV.

Name[edit]

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.[9] 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 Kronos (the Greek Saturn), be used.[10]

Orbit[edit]

Dione orbits Saturn with a semimajor axis about 2% less than that of Earth's Moon. However, reflecting Saturn's greater mass, Dione's orbital period is one tenth that of the Moon. Dione is currently in a 1:2 mean motion orbital resonance with Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus. This resonance helps maintain Enceladus's orbital eccentricity (0.0047), providing a source of heat for Enceladus's extensive geological activity, which shows up most dramatically in its cryovolcanic geyser-like jets.[11]

Dione has two co-orbital, or Trojan, moons, Helene and Polydeuces. They are located within Dione's Lagrangian points L4 and L5, 60 degrees ahead of, and behind Dione respectively.

Physical characteristics[edit]

Cassini enhanced-color composite of Dione, showing the darker, fractured terrain of the trailing hemisphere. The Padua Chasmata trace an arc on the left, interrupted near the top by central peak crater Ascanius. The Janiculum Dorsa extend along the upper right terminator. Near the lower left limb is the small crater Cassandra with its prominent ray system.

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.[12] 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:

Crescent Dione from Cassini, October 11, 2005. The crater near the limb at top is Alcander, with larger crater Prytanis adjacent to its left. At lower right, several of the Palatine Chasmata fractures are visible, one of which can be seen bisecting the smaller craters Euryalus (right) and Nisus.

The ice cliffs (formerly 'wispy terrain')[edit]

Fractures bisecting older craters on Dione. Those running from upper right to lower left are the Carthage Fossae, while Pactolus Catena runs more horizontally at lower right.

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 against the shadow of Saturn's rings.
Map of Dione, as imaged by Cassini

Craters[edit]

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[13] 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.

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.

Exosphere[edit]

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.[14][15] The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3.[15][16]

The Cassini probe instruments were unable to directly detect water from the exosphere due to high background levels,[15] but it seems that highly charged particles from the planets' powerful radiation belts could split the water in the ice into hydrogen and oxygen.[14]

Exploration[edit]

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.

In May of 2013, NASA’s spacecraft Cassini provided scientists with new evidence that the Saturian moon Dione could be more active than previously predicted. It is thought that Dione could be a fossil of the activity that Cassini previously discovered spraying from Enceladus, another one of Saturn’s moons. Other bodies in the solar system that are thought to have a subsurface ocean, like Enceladus and Titan and Jupiter’s moon Europa, are some of the most geologically active worlds in our solar system; and have been targets for geologists and scientists looking for the building blocks of life elsewhere in the solar system. This means that there if there is a subsurface ocean on Dione, it would boost the astrobiological potential of this moon. While Cassini has been exploring Dione since 2004, recently a magnetometer on the spacecraft detected a faint particle stream coming from the moon. Some images show evidence for a possible liquid or slushy layer of rock under the ice crust surface. Other images have shown ancient, inactive cracks in Dione that are similar to the ones seen on Enceladus, further proving the idea that a subsurface ocean would exist on Dione as it does on Enceladus. Other evidence that Dione might have a subsurface ocean comes from an observation of the mountain Janiculum Dorsa, located on Dione. The journal Icarus published a paper that examined the mountain in March of 2013. This mountain ranges in height from about 1 to 2 km (0.6 to 1.2 miles). The moon’s crust seems to pucker 0.5 km (0.3 miles) under the mountain, suggesting the icy crust was warm. Dione also gets warmed by tidal heating as it gets closer to and farther from Saturn in its orbit.[17]

Gallery[edit]

See also[edit]

References[edit]

  1. ^ a b http://exp.arc.nasa.gov/downloads/celestia/data/solarsys.ssc[dead link] Exp.arc.nasa.gov Retrieved on 05-21-07
  2. ^ a b c Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.  edit
  3. ^ Phil Davis? (April 1, 2011). "Solar System Exploration: Planets: Saturn: Moons: Dione: Facts & Figures". NASA. Retrieved March 24, 2013. 
  4. ^ Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; Ionasescu, R.; Jones, J. B.; Mackenzie, R. A.; Meek, M. C.; Parcher, D.; Pelletier, F. J.; Owen, Jr., W. M.; Roth, D. C.; Roundhill, I. M.; Stauch, J. R. (December 2006). "The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data". The Astronomical Journal 132 (6): 2520–2526. Bibcode:2006AJ....132.2520J. doi:10.1086/508812.  edit
  5. ^ Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992. Retrieved 20 December 2011.  (supporting online material, table S1) edit
  6. ^ Observatorio ARVAL (April 15, 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2011-12-17. 
  7. ^ In USA dictionary transcription, US dict: dī·ō′·nē.
  8. ^ Cassini, G. D. (1686–1692). "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 of the Royal Society of London 16 (179–191): 79–85. doi:10.1098/rstl.1686.0013. JSTOR 101844.  edit
  9. ^ Fred William Price – The planet observer's handbook – page 279
  10. ^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
  11. ^ Porco, C. C.; Helfenstein, P.; Thomas, P. C.; Ingersoll, A. P.; Wisdom, J.; West, R.; Neukum, G.; Denk, T.; Wagner, R. (10 March 2006). "Cassini Observes the Active South Pole of Enceladus". Science 311 (5766): 1393–1401. Bibcode:2006Sci...311.1393P. doi:10.1126/science.1123013. PMID 16527964.  edit
  12. ^ See note g Triton (moon)#Notes
  13. ^ 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
  14. ^ a b Ghosh, Pallab (2 March 2012). "Oxygen envelops Saturn's icy moon". BBC News. Retrieved 2012-03-02. 
  15. ^ a b c Robert L. Tokar; Robert E. Johnson; Michelle F. Thomsen; Edward C. Sittler; Andrew J Coates; et al. (10 January 2012 (online)). "Detection of Exospheric O2+ at Saturn's Moon Dione". Geophysical Research Letters. Bibcode:2012GeoRL..3903105T. doi:10.1029/2011GL050452. Retrieved 2012-03-02.  Check date values in: |date= (help)
  16. ^ Sven Simon; Joachim Saur; itz M. Neubauer; Alexandre Wennmacher; Michele K. Dougherty (2011). "Magnetic signatures of a tenuous atmosphere at Dione". Geophysical Research Letters 38 (L15102): 5. Bibcode:2011GeoRL..3815102S. doi:10.1029/2011GL048454. Retrieved 2012-03-02. 
  17. ^ Jia-Rui Cook (May 29, 2013). "Cassini Finds Hints of Activity at Saturn Moon Dione". NASA. Retrieved October 1, 2013. 

External links[edit]