Enceladus

Key facts

Diameter

504 km15% of Earth's Moon

Mass

1.08 × 10²⁰ kg

Surface gravity

0.113 m/s²0.012 g

Density

1,609 kg/m³

Distance from Saturn

237,948 km≈ 3.9 Saturn radii

Orbital period

32.9 hoursSame as its rotation (tidally locked)

Discovered

1789William Herschel

Surface

Fresh, highly reflective water ice

Surface temperature

About −198 °C on average

Atmosphere

Thin water-vapour plume over the south pole

Notable feature

The four 'tiger stripe' fissures

Named after

Enceladus, a Giant of Greek mythology

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

Surface and geology

Enceladus has a split personality. Its northern regions are pocked with old craters, while large areas elsewhere are smooth, furrowed and almost crater-free, a sign that they have been resurfaced in geologically recent times. Some of the youngest terrain may be only a few million years old or less.

The south polar region is the most active. Four parallel fractures, nicknamed tiger stripes and formally named Alexandria, Cairo, Baghdad and Damascus Sulci, each run for roughly 130 kilometres and are flanked by ridges. They are noticeably warmer than their surroundings, and more than a hundred individual jets have been traced to them.

Snow from the plumes falls back on the surface, coating it in fine, fresh ice. This is why Enceladus is so dazzlingly bright, with a reflectivity higher than fresh snow on Earth.

Interior and composition

Enceladus has a rocky core surrounded by a global ocean and an outer shell of ice. In 2015 Cassini scientists showed that the moon wobbles too much in its orbit for the ice to be frozen to the core, which means the ocean must extend all the way around. The shell is thought to average a few tens of kilometres thick but may be only a few kilometres thick beneath the south pole.

Tiny grains of silica in the plume suggest that hot water reacts with rock on the seafloor, perhaps much like hydrothermal vents in Earth's oceans. Cassini also detected molecular hydrogen, a potential energy source for microbes, along with complex organic molecules, and in 2023 researchers reported phosphates, a crucial building block of DNA and cell membranes.

Orbit and rotation

Enceladus orbits Saturn in about 33 hours and is tidally locked, always showing the same face to the planet. It completes exactly two orbits for every one of Dione, and this resonance keeps its path slightly oval. As Enceladus moves closer to and farther from Saturn, tides squeeze and flex its interior, generating the heat that keeps the ocean liquid.

The plumes even vary with the orbit: they tend to be brightest when Enceladus is farthest from Saturn, when tidal stresses pull the tiger stripes open.

Origin

Enceladus probably formed from ice and rock in the disc of material around young Saturn. How long its ocean has existed is an open question. Some models allow it to be billions of years old, while others suggest the inner moons and their current heating may be far more recent. Either way, Enceladus constantly feeds Saturn's faint, wide E ring with ice particles from its plumes.

Exploration

William Herschel discovered Enceladus in 1789. It remained a point of light until the Voyager flybys, and its true nature emerged only when Cassini made repeated close passes, some of them diving directly through the plumes.

The moons of Saturn

Surprising facts about Enceladus

• Enceladus reflects nearly all of the sunlight that reaches it, making it one of the most reflective bodies in the Solar System.
• The plumes blast out roughly 200 kilograms of water vapour every second, and much of that material ends up forming Saturn's E ring.
• Cassini was never designed to hunt for life, yet it effectively sampled an alien ocean simply by flying through Enceladus' spray.
• The phosphate concentrations inferred from plume grains are estimated to be at least 100 times higher than in Earth's oceans.
• Enceladus is small enough to fit within the length of Great Britain, yet it hosts one of the most promising habitats beyond Earth.

Keep exploring

• Europa, moon of Jupiter: an ocean hidden under ice
• Mimas, moon of Saturn: the death star with a secret
• Dione, moon of Saturn: ice cliffs and wispy streaks
• Saturn: the planet guide
• Watch Enceladus orbit Saturn in the interactive 3D solar system
• All the major moons of the Solar System

Sources

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