Dione is an ice moon with a split personality: one hemisphere is bright and densely cratered, while the other is streaked with luminous “wispy terrain”. Those wisps puzzled astronomers for a quarter of a century until Cassini flew close enough to resolve them.
Its “wisps” are ice cliffs hundreds of metres tall lining huge fractures in the crust.
Two tiny Trojan moons, Helene and Polydeuces, share Dione’s orbit around Saturn.
The fractured wispy terrain cuts mostly across the darker trailing hemisphere.
Wisps that turned out to be cliffs
In Voyager’s distant images the bright streaks looked like frost or ash painted on the surface. Cassini revealed the truth: they are sheer ice cliffs hundreds of metres high, the exposed faces of giant tectonic fractures. A small frozen moon had somehow been geologically alive - and gravity data hint it may still hide liquid water at depth.
Physical characteristics
Diameter
1,123 km
Mass
1.100e+21 kg
Surface gravity
0.23 m/s²
Density
1.48 g/cm³
Tidally locked
Yes
Mean temperature
-186 °C
Atmosphere and composition
Atmosphere
None.
Composition
Water ice around a comparatively large rocky core.
Discovery
Discovered by
Giovanni Cassini
Year
1684
How
Aerial telescope at the Paris Observatory
Missions
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.