Saturn: Moons: Dione
Dione [Die-OH-nee; adjective: Dionean] is a small moon of 1,123 kilometers (698 miles) in diameter orbiting Saturn every 2.7 days at a distance of 377,400 kilometers (234,000 miles), which is roughly the same distance that the Moon orbits around the Earth.
Dionean Geology
Dione's features include heavily cratered terrain with craters as large as 100 kilometers (62 miles) across, moderately cratered plains, lightly cratered plains, and fractured areas. The heavily cratered areas are most common on the trailing hemisphere. Logically, a moon's leading hemisphere should be the more heavily cratered, so it has been theorized that a more recent impact spun Dione around. It has been calculated that bodies as small as those that made 35-kilometer (22-mile) craters could have spun Dione around. However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.
Fractured areas, seen in Voyager images as bright thin wispy lines, have lengths of tens to hundreds of kilometers, often cutting through plains and craters. Cassini flybys starting in 2005 showed "the wisps" as bright canyon ice walls (some of them several hundred meters high), probably caused by subsidence cracking. The walls are bright because darker material falls off them, exposing bright water ice. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called tiger stripes on Enceladus.
Very fine ice powder (equivalent to cigarette smoke) from Saturn's E-ring constantly bombards Dione. The dust in the E-ring ultimately comes from Enceladus, which has prominent geyser activity.
Dione's density is 1.48 times that of liquid water, suggesting that about a third of Dione is a dense core (probably silicate rock) and the rest is ice. At Dione's average temperature of -186 degrees Celsius (87 kelvin or -121 degrees Fahrenheit), ice is very hard and behaves like rock.
Celestial Mechanics of Dione and Its Near Neighbors
As with Earth's Moon, Dione is phase locked with its parent; one side always faces toward Saturn. Likewise, Dione has gravitationally locked two much smaller moons: Helene orbits Saturn 60 degrees ahead of Dione, and Polydeuces orbits Saturn 60 degrees behind Dione. (These tidally locked moon locations are also referred to as Lagrange points L4 for the leading spot and L5 for the trailing spot, based on calculations of the French astronomer Joseph-Louis Lagrange in 1772
Dione is in resonance with two nearby moons, Mimas and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which helps keep them locked in their positions. Dione keeps Enceladus locked at a period exactly one half of the Dione orbit.
Discovery and Naming
Giovanni Cassini discovered Dione in 1684. John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.
The name Dione comes from the Greek goddess (or titan) Dione, who by some accounts was the daughter of Tethys and Oceanus and who Homer described as the mother of Aphrodite.
Cassini referred to Dione as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Tethys, and Rhea). Other astronomers called the Saturn moons by number in terms of distance from Saturn. Thus, Dione was Saturn IV. Geological features on Dione generally get their names from people and places in Virgil's Aeneid. The International Astronomical Union now controls naming of astronomical bodies.
