Dione
What is Dione?
Moons· Diameter 1,123 km. A bright icy moon of Saturn whose trailing side is streaked with ice cliffs hundreds of metres high. Scientists have suggested an ocean may lie beneath its icy crust.
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Orbit and size
| Diameter | 1,123 km |
|---|---|
| Distance from Saturn | 377,400 km |
| Orbit | 2.74 d |
| Note | bright ice-cliff streaks |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Helene and Polydeuces, the moons that share Dione's orbit
Dione does not have its orbit to itself. Sixty degrees ahead of it on the same path travels Helene, 35 km across, and sixty degrees behind it travels Polydeuces, only about 3 km across. These two positions are special points where the gravity of Saturn and Dione balance out. A small moon caught there keeps a steady distance from Dione as it follows along, the same principle that gathers the Trojan asteroids ahead of and behind Jupiter.
Such 'Trojan moons' have so far been found only around Saturn; besides Dione, the moon Tethys has two companions like this. Helene was found with a ground-based telescope in 1980, and Polydeuces in Cassini images in 2004. Neither stays perfectly still. Each drifts slowly back and forth around its balance point, and Polydeuces swings more than 30 degrees ahead and behind.
Cassini made five targeted flybys of Dione. In 2010 it detected oxygen ions around the moon, revealing a very thin atmosphere like Rhea's. In December 2011 it came closest, 99 km above the surface, and gravity data from its last pass, in August 2015, formed the basis for studying whether Dione has an ocean.
The moon in depth
Giovanni Cassini discovered Dione in 1684. It is 1,123 km across and orbits 377,400 km from Saturn once every 2.74 days. With a density 1.48 times that of water, it contains more rock than most of Saturn's mid-sized moons. Dione moves in step with its neighbour Enceladus: while Enceladus goes around twice, Dione goes around once, and these regular tugs help keep Enceladus warm inside.
The first Voyager pictures of Dione showed thin, bright, thread-like streaks on its trailing side, and for years no one knew what they were. Close-up images from the Cassini spacecraft revealed ice cliffs hundreds of metres high, glinting in the sunlight. They formed where the crust was pulled apart and cracked.
Dione may have an ocean too. A 2016 study of Cassini's gravity data suggested there could be an ocean beneath an icy shell about 100 km thick. If confirmed, Dione would join Enceladus as another water world in Saturn's system. Following Saturn in the orbit view shows Dione circling outside Tethys and inside Rhea.
Frequently asked
- How big is Dione?
- The diameter of Dione is 1,123 km.
- How far is Dione from Earth?
- The distance from saturn of Dione is 377,400 km.
- What is notable about Dione?
- The note of Dione is bright ice-cliff streaks.
- Which planet does Dione orbit?
- Dione orbits Saturn.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.