Dione (moon)

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Not to be confused with the asteroid called 106 Dione.
"Saturn IV" redirects here. For the rocket, see Saturn C-4.
Dione
synchronous)
zero
Albedo0.998±0.004 (geometric)[8]
Temperature87 K (−186°C)
10.4 [9]

Dione (

trailing hemisphere is marked by large cliffs and scarps called chasmata
; the trailing hemisphere is also significantly darker compared to the leading hemisphere.

The moon was discovered by Italian astronomer Giovanni Domenico Cassini in 1684 and is named after the Titaness Dione in Greek mythology. Dione was first imaged up-close by the Voyager 1 space probe in 1980. Later, the Cassini spacecraft made multiple flybys of Dione throughout the 2000s and 2010s as part of its campaign to explore the Saturn system.

Name

Titans (sisters and brothers of Cronus) be used.[11]

Orbit

Animation of Helene's orbit relative to Saturn and Dione
  Polydeuces  ·   Helene ·   Dione ·   Saturn

Dione orbits Saturn with a

jets.[12] The resonance also maintains a smaller eccentricity in Dione's orbit (0.0022), tidally heating it as well.[13]

Trojans

Dione has two co-orbital, or

Lagrangian points L4 and L5, 60 degrees ahead of and behind Dione respectively. A leading co-orbital moon twelve degrees ahead of Helene was reported by Stephen P. Synnott in 1982.[14][15]

Dione trojans
Name Diameter (km) Semi-major axis (km) Mass (kg) Discovery date
Dione 1 122 377 396 (1. 096 ± 0.000 0246) × 1021 30 March 1684
Helene 36.2 ± 0.4 377 600 (7.1 ±0.2) × 1015 1 March 1980
Polydeuces 3.06 ± 0.40 377 600 ≈8 × 1012 21 October 2004

Physical characteristics and interior

See also: List of geological features on Dione
Size comparison of Earth, the Moon, and Dione.

At 1,122 km (697 mi) in diameter, Dione is the

15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined.[16] It is also Saturn's fourth-largest moon. Based on its density, Dione’s interior is likely a combination of silicate rock and water ice in nearly equal parts by mass.[17]

Shape and gravity observations collected by

Enceladus).[17][18][19][20] Downward bending of the surface associated with the 1.5 km high ridge Janiculum Dorsa can most easily be explained by the presence of such an ocean.[21][22] Neither moon has a shape close to hydrostatic equilibrium; the deviations are maintained by isostasy. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.[20]

Though somewhat smaller and denser, Dione is otherwise very similar to

leading and trailing hemispheres
. Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, however, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

Dione in true color.

Scientists recognise Dionean geological features of the following types:

  • Chasmata (chasms; long, deep, steep-sided depressions or canyons)
  • Dorsa
    (ridges)
  • Fossae
    (long narrow depressions)
  • Craters
  • Catenae (crater chains)

Ice cliffs (formerly 'wispy terrain')

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.

When the

cryovolcanism
resurfaced much of its surface, with the streaks forming from eruptions along cracks in the Dionean surface that fell back 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 hypothesis was proven wrong by the Cassini probe flyby of 13 December 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 11 October 2005, and captured oblique images of the cliffs, showing that some of them are several hundred metres high.

Linear features

Dione features linear 'virgae' that are up to hundreds of km long but less than 5 km wide. These lines run parallel to the equator and are only apparent at lower latitudes (at less than 45° north or south); similar features are noted on

exogenic origin, as the result of the emplacement of material across the surface by low‐velocity impacts of material sourced from Saturn's rings, co‐orbital moons, or closely approaching comets.[23]

Craters

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

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[24] 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. Because 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.

Atmosphere

Four Saturn's moons: Titan, in the background; Dione, above the rings; Pandora, beyond the rings on the right of the image; and Pan in the Encke Gap of the A ring on the left of the image.

On 7 April 2010, instruments on board the uncrewed Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions (O+
2) around Dione, so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere.[25][26] The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3.[26][27]

The Cassini probe instruments were unable to directly detect water from the exosphere due to high background levels,[26] but it seems that highly charged particles from the planet's powerful radiation belts could split the water in the ice into hydrogen and oxygen.[25]

Exploration

Picture of Dione in front of Saturn, captured by the Cassini orbiter

Dione was first imaged by the Voyager space probes. It has also been probed five times from close distances by the Cassini orbiter. There was a close targeted flyby at a distance of 500 km (310 mi) on 11 October 2005;[28] another flyby was performed on 7 April 2010, also at a distance of 500 km.[29] A third flyby was performed on 12 December 2011 at a distance of 99 km (62 mi). The following flyby was on 16 June 2015 at a distance of 516 km (321 mi),[30] and the last Cassini flyby was performed on 17 August 2015 at a distance of 474 km (295 mi).[31][32]

In May 2013, it was announced that NASA's spacecraft Cassini had provided scientists with evidence that Dione is more active than previously realized. Using topographic data, NASA teams deduced that crustal depression associated with a prominent mountain ridge on the leading hemisphere is best explained if there was a global subsurface liquid ocean like that of Enceladus.[21][33][34] The ridge Janiculum Dorsa has a height of 1 to 2 km (0.6 to 1.2 miles); Dione's crust seems to pucker 0.5 km (0.3 miles) under it, suggesting that the icy crust was warm when the ridge formed, probably due to the presence of a subsurface liquid ocean, which increases tidal flexing.[35]

See also

References

  1. ^ "Dione". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 22 March 2020.
    "Dione". Merriam-Webster.com Dictionary.
  2. ^ JPL (March 13, 2007) Cassini: Dionean Linea Archived 2021-10-26 at the Wayback Machine
  3. ^ Len Krisak (2011) Virgil's Eclogues, p. 71
  4. ^ a b "Data for our solar system". Exp.arc.nasa.gov. 20 April 2003. Archived from the original on 9 March 2005. Retrieved 21 May 2007.
  5. ^
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  7. ^ Phil Davis? (1 April 2011). "Solar System Exploration: Planets: Saturn: Moons: Dione: Facts & Figures". NASA. Archived from the original on 12 October 2012. Retrieved 24 March 2013.
  8. S2CID 21932253
    . Retrieved 20 December 2011. (supporting online material, table S1)
  9. ^ Observatorio ARVAL (15 April 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Archived from the original on 20 September 2011. Retrieved 17 December 2011.
  10. ISBN 978-0-521-78981-3
    .
  11. ^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
  12. S2CID 6976648
    .
  13. ^ Jia-Rui Cook (29 May 2013). "Cassini Finds Hints of Activity at Saturn Moon Dione". NASA. Retrieved 1 October 2013.
  14. ^ "IAUC 6162: Poss. Sats OF SATURN; AL Com".
  15. ^ Guinness Book of Astronomy, Patrick Moore,
    Guinness Publishing
    , second edition, 1983 pp 110, 114
  16. ^ See note g Triton (moon)#Notes
  17. ^
    S2CID 201103604
    .
  18. ^ "NASA Astrobiology Strategy" (PDF). NASA. 2015. Archived from the original (PDF) on 22 December 2016. Retrieved 26 September 2017.
  19. ^ Howell, E. (5 October 2016). "Another Saturn Moon May Hide Subsurface Ocean". Seeker.com. Discovery Communications, LLC. Retrieved 8 October 2016.
  20. ^
    S2CID 119236092
    .
  21. ^
    doi:10.1016/j.icarus.2012.12.021
    .
  22. ^ Overlooked Ocean Worlds Fill the Outer Solar System. John Wenz, Scientific American. 4 October 2017.
  23. ISSN 1944-8007
    .
  24. ^ 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
  25. ^ a b Ghosh, Pallab (2 March 2012). "Oxygen envelops Saturn's icy moon". BBC News. Retrieved 2 March 2012.
  26. ^
    doi:10.1029/2011GL050452
    .
  27. doi:10.1029/2011GL048454
    .
  28. ^ Martinez, Carolina (17 October 2005). "Cassini Views Dione, a Frigid Ice World". NASA. Archived from the original on 11 May 2021. Retrieved 22 August 2015.
  29. ^ Cassini Doubleheader: Flying By Titan and Dione (April 2010). NASA – Cassini Solstice MIssion.
  30. ^ Landau, Elizabeth; Dyches, Preston (17 June 2015). "Cassini Sends Back Views After Zooming Past Dione". Jet Propulsion Laboratory. Retrieved 21 July 2017.
  31. ^ Dyches, Preston (13 August 2015). "Cassini to Make Last Close Flyby of Saturn Moon Dione". NASA News. Retrieved 19 August 2015.
  32. ^ Spacecraft Makes Final Close Flyby of Saturn Moon Dione Today. Space.com Calla Cofield. August 17, 2015.
  33. Bibcode:2010AGUFM.P24A..08C
    .
  34. Bibcode:2012AGUFM.P22B..03P
    .
  35. ^ "Cassini Finds Hints of Activity at Saturn Moon Dione". NASA News. 29 May 2013. Retrieved 29 May 2015.

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