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Dione: Saturn's Turned-Around Moon

Saturn and Dione
Saturn and Dione appear askew in this Cassini spacecraft view, with the north poles rotated to the right, as if they were threaded along on the thin diagonal line of the planet's rings.
Credit: NASA/JPL-Caltech/Space Science Institute

Dione is the fourth-largest moon of Saturn, with a diameter of 698 miles (1,123 km). It is the 15th largest moon in the solar system. Dione orbits around Saturn once every 2.7 Earth days, at a distance of 234,000 miles (377,400 km), approximately the same distance from Earth to its moon.

With its wispy terrain and mysterious cratering, Dione presents an intriguing puzzle. One of the 53 named satellites orbiting the ringed planet, the icy moon has a number of features that make it stand out.

Discovery and naming

Dione was the fourth of Saturn's moons to be discovered, the third by Giovanni Cassini. The Italian astronomer found the moon in 1684 while studying Saturn from the Paris Observatory. Cassini referred to it as one of the Sidera Lodoicea, the "Stars of Louis," named after King Louis XIV. Other astronomers named the moons numerically, based on their distance from the planet, making Dione Saturn IV.

Faults and Craters on Dione
This view highlights tectonic faults and craters on Saturn's moon, Dione, an icy world that has undoubtedly experienced geologic activity since its formation. It is based on images from the the Cassini spacecraft taken on Dec. 24, 2005.
Credit: NASA/JPL/Space Science Institute

The icy satellite did not receive a formal name until 1847, when John Herschel (son of British astronomer William Herschel) suggested the moons of Saturn be named for the Titans, brothers and sisters of the Greek god Cronus, who was known to the Romans as Saturn. In Greek mythology, Dione (pronounced "die-OH-nee") was often described as the daughter of Tethys and Oceanus, and resembled the earth goddess Gaia. In his epic poem, "The Iliad," the Greek poet Homer described her as the mother of Aphrodite, goddess of love, and suggested that she was the mother of many others.

Geological features, such as craters and cliffs, take their names from people and places in Virgil's "Aeneid."

Wisps and cliffs

Dione's surface contains both heavily and moderately cratered regions, with some craters as large as 62 miles (100 kilometers). But Dione is something of a puzzle due to the location of its heavily cratered areas. Generally, the leading hemisphere of a body traveling through space plows face-on through debris, and is more heavily cratered. Dione, however, has a more heavily cratered backside than front. Scientists have speculated that a collision could have spun the moon around — an impactor that created one of the 22-mile (35 km) craters could have had the force to turn the moon — but the fact that it seems to have spun exactly 180 degrees is unusual.

Dione is 1.48 times as dense as water, suggesting that the moon has a dense core surrounded by ice. The temperature on the moon averages minus 302 degrees F (minus 186 degrees C). The exceptionally cold temperatures cause the icy surface to behave much like rock.

Dione Up Close
This raw, unprocessed image was taken by NASA's Cassini spacecraft on May 2, 2012. The camera was pointing toward Dione at approximately 14,835 miles (23,875 kilometers) away.
Credit: NASA/JPL/Space Science Institute

When Voyager 1 surveyed the moon in 1980, it unveiled wispy features on the trailing side. NASA's Cassini-Huygens probe took close-up images more than 20 years later that revealed the wisps were ice cliffs. As dark material fell from the cliffs, they revealed brightly shining ice underneath. Created by tectonic activity, these fractured areas crisscross the moon, running tens to hundreds of kilometers in length.

Cassini also detected a wispy oxygen atmosphere on Dione. It is very thin; there is just one oxygen ion for every 0.67 cubic inches (11 cubic centimeters). It is equivalent to conditions 300 miles (480 km) above Earth.

Like many moons, including Earth's, Dione is tidally locked, with the same side always facing its parent planet. The gravity of the moon keeps two smaller moons, Helene and Polydeuces, locked into the same orbital position as the three travel around Saturn. Dione also interacts with two of the larger moons of Saturn, Mimas and Enceladus, influencing their orbits.

Other Dione facts

  • Mass: 1.10 x 1021 kilograms
  • Mean Radius: 349 miles (561 km)
  • Mean Density: 1.478 g/cm3
  • Escape Velocity: 0.51 km/s
  • Albedo: 0.998
  • Temperature: minus 302 degrees F (minus 186 degrees C)

— Nola Taylor Redd, SPACE.com contributor

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