Everything about Dione Moon totally explained
Dione (
dye-OE-nee, or as 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.
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, he suggested the names of the
Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
Physical characteristics
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're newer. These are now known to be ice cliffs.
Scientists recognise the following types of Dionean
geological feature:
- Chasmata (chasms)
- Lineae (wispy features)
- Craters
The ice cliffs (formerly '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 doesn't 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
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 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.
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; another similarly close fly-by is planned for
2010 April 7.
Further Information
Get more info on 'Dione Moon'.
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