Dione

Key facts

Diameter

1,123 km32% of Earth's Moon

Mass

1.09 × 10²¹ kg

Surface gravity

0.232 m/s²0.024 g

Density

1,478 kg/m³

Distance from Saturn

377,396 km≈ 6.3 Saturn radii

Orbital period

2.74 daysSame as its rotation (tidally locked)

Discovered

1684Giovanni Cassini

Surface

Cratered ice with bright fracture cliffs

Surface temperature

About −186 °C (87 K)

Atmosphere

Extremely thin trace of oxygen ions

Trojan moons

Helene (leading) and Polydeuces (trailing)

Named after

Dione, a Titaness of Greek mythology

Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.

Surface and geology

Voyager images showed Dione's trailing hemisphere covered in bright, feathery markings known as wispy terrain. Cassini's close-up views revealed that these are networks of chasmata: long fractures and ice cliffs hundreds of metres high, where tectonic stresses have cracked the crust and exposed fresh, bright ice. Named systems include Eurotas, Palatine and Padua Chasmata.

The rest of Dione is cratered, with some areas far more heavily battered than others. The largest crater, Evander, is about 350 kilometres wide, and long ridges such as Janiculum Dorsa may record the crust flexing over a warmer interior in the past.

Curiously, the hemisphere that leads in Dione's orbit is less heavily cratered than expected. One suggested explanation is that a large impact once spun the moon around before tides locked it again, though this remains speculative.

Interior and composition

Dione is roughly half ice and half rock by mass, with a rocky core beneath a thick icy mantle. Analyses of Cassini gravity data published in 2016 suggested that an ocean, perhaps tens of kilometres deep, might lie about 100 kilometres beneath the surface. This interpretation is debated, but if correct it would make Dione another potential ocean world.

Cassini also detected a very tenuous exosphere containing molecular oxygen ions, likely produced as charged particles and sunlight break apart surface ice.

Orbit and rotation

Dione orbits Saturn in about 2.7 days and is tidally locked. For every orbit it makes, Enceladus completes exactly two, and this resonance helps keep Enceladus' orbit slightly eccentric and its interior warm.

Two small trojan moons share Dione's path: Helene, about 60 degrees ahead, and Polydeuces, about 60 degrees behind. Polydeuces swings back and forth around its Lagrange point in a surprisingly wide loop.

Origin

Like Saturn's other mid-sized moons, Dione likely formed from ice and rock orbiting the young planet. Its relatively high density and evidence of past tectonics suggest its rocky interior supplied enough radioactive heat to keep it active for a long time, and possibly still today.

Exploration

Giovanni Domenico Cassini discovered Dione in March 1684, at the same time as Tethys. Most detailed knowledge comes from the Voyager and Cassini missions.

The moons of Saturn

Surprising facts about Dione

• The bright wisps on Dione are not frost deposits, as once thought, but sheer ice cliffs exposed by fractures in the crust.
• Dione has two trojan companions, Helene and Polydeuces, making it one of only two Saturnian moons known to share its orbit this way.
• Dione's gravitational resonance with Enceladus may be one reason Enceladus is warm enough to have an ocean.
• Cassini detected oxygen ions near Dione, evidence of an exosphere so thin that its molecules rarely collide.

Keep exploring

• Tethys, moon of Saturn: an icy moon split by a canyon
• Enceladus, moon of Saturn: ice fountains from a hidden sea
• Rhea, moon of Saturn: saturn's ancient frozen giant
• Saturn: the planet guide
• Watch Dione orbit Saturn in the interactive 3D solar system
• All the major moons of the Solar System

Sources

• NASA Science — Dione
• JPL Solar System Dynamics — Planetary satellite physical parameters
• Wikipedia — Dione