Dione (/daɪˈoʊni/) is the fourth-largestmoon of Saturn. With amean diameter of 1,123 km and a density of about 1.48 g/cm3, Dione is composed of an icy mantle and crust overlying asilicate rocky core, with rock and water ice roughly equal in mass. Itstrailing hemisphere is marked by large cliffs and scarps calledchasmata; the trailing hemisphere is also significantly darker compared to the leading hemisphere.
The moon was discovered by Italian astronomerGiovanni Domenico Cassini in 1684 and is named after the TitanessDione inGreek mythology. Dione was first imaged up-close by theVoyager 1 space probe in 1980. Later, theCassini spacecraft made multiple flybys of Dione throughout the 2000s and 2010s as part of its campaign to explore the Saturn system.
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a Greekdelta (the initial of Dione) combined with the crook of the Saturn symbol as the symbol of Dione (). This symbol is not widely used.[12]
Animation of Helene's orbit relative to Saturn and Dione Polydeuces· Helene· Dione· Saturn
Dione orbits Saturn with asemimajor axis about 2% less than that of Earth'sMoon. However, reflectingSaturn's greater mass (95 times that of Earth), Dione's orbital period is one tenth that of Earth's Moon. Dione is currently in a 1:2 mean-motionorbital resonance with moonEnceladus, completing one orbit of Saturn for every two orbits completed by Enceladus. This resonance maintains Enceladus's orbital eccentricity (0.0047), providing a source of heat for Enceladus's extensive geological activity, which shows up most dramatically in itscryovolcanic geyser-likejets.[13] The resonance also maintains a smaller eccentricity in Dione's orbit (0.0022),tidally heating it as well.[14]
Dione has two co-orbital, ortrojan, moons,Helene andPolydeuces. They are located within Dione'sLagrangian pointsL4 andL5, 60 degrees ahead of and behind Dione respectively. A leading co-orbital moon twelve degrees ahead of Helene was reported byStephen P. Synnott in 1982.[15][16]
At 1,122 km (697 mi) in diameter, Dione is the15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined.[17] 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.[18]
Shape and gravity observations collected byCassini suggest a roughly 400 km radius rocky core surrounded by a roughly 160 km envelope of H2O, mainly in the form of water ice, but with some models suggesting that the lowermost part of this layer could be in the form of aninternal liquid salt water ocean (a situation similar to that of its orbital resonance partner,Enceladus).[18][19][20][21] Downward bending of the surface associated with the 1.5 km high ridgeJaniculum Dorsa can most easily be explained by the presence of such an ocean.[22][23] Neither moon has a shape close tohydrostatic equilibrium; the deviations are maintained byisostasy. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, wheretidal heating of the crust is greatest.[21]
Though somewhat smaller and denser, Dione is otherwise very similar toRhea. They both have similar albedo features and varied terrain, and both have dissimilarleading 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 Dioneangeological features of the following types:
Chasmata (chasms; long, deep, steep-sided depressions or canyons)
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 itsray system are at lower right.
When theVoyager 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, because all that was known was that the material has a highalbedo 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 ascryovolcanism 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 theCassini 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.
TheCassini orbiter performed a closer flyby of Dione at 500 km (310 mi) on 11 October 2005, and capturedoblique images of the cliffs, showing that some of them are several hundred metres high.
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 onRhea. They are brighter than everything around them and appear to overlay other features such as ridges and craters, indicating they are relatively young. It has been proposed that these lines are ofexogenic 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.[24]
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 andWolfe[25] proposed a cratering model for atidally 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.
LikeCallisto, Dione's craters lack the high-relief features seen on theMoon andMercury; this is probably due to slumping of the weak icy crust over geologic time.
Four of 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 uncrewedCassini 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 anexosphere rather than a tenuous atmosphere.[26][27] The density of molecular oxygen ions determined from theCassini plasma spectrometer data ranges from 0.01 to 0.09 per cm3.[27][28]
TheCassini probe instruments were unable to directly detect water from the exosphere due to high background levels,[27] 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.[26]
Picture of Dione in front of Saturn, captured by theCassini orbiter
Dione was first imaged by theVoyager space probes. It has also been probed five times from close distances by theCassini orbiter. There was a close targeted flyby at a distance of 500 km (310 mi) on 11 October 2005;[29] another flyby was performed on 7 April 2010, also at a distance of 500 km.[30] 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),[31] and the lastCassini flyby was performed on 17 August 2015 at a distance of 474 km (295 mi).[32][33]
In May 2013, it was announced that NASA's spacecraftCassini 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.[22][34][35] The ridge Janiculum Dorsa has a height of 1 to 2 km (0.62 to 1.24 mi); Dione's crust seems to pucker 0.5 km (0.31 mi) 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.[36]
^abRoatsch, 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.ISBN978-1-4020-9216-9.
^Collins, G. C. (2010). Collins, G. C. (ed.).Testing Candidate Driving Forces for Faulting on Dione: Implications for Nonsynchronous Rotation and a Freezing Ocean. American Geophysical Union, Fall Meeting 2010, abstract #P24A-08.AGU Fall Meeting Abstracts. Vol. 2010. pp. P24A–08.Bibcode:2010AGUFM.P24A..08C.
^Phillips, C. B.; Hammond, N. P.; Roberts, J. H.; Nimmo, F. (2012).Subsurface Structure and Thermal History of Icy Satellites from Stereo Topography. American Geophysical Union, Fall Meeting 2012, abstract #P22B-03.Bibcode:2012AGUFM.P22B..03P.
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