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Moon of Saturn
Enceladus
Saturn's tiny white moon that sprays a hidden ocean into space
Key facts
- Diameter
- About 311 miles / 500 km
- Distance from Saturn
- About 148,000 miles / 238,000 km
- One orbit
- About 1.37 Earth days (32.9 hours)
- Discovered
- 28 August 1789, by William Herschel
- Named after
- Enceladus, a giant in Greek mythology
- Surface
- Bright water ice, the most reflective in the solar system
- Surface temperature
- About minus 201 °C
- Density
- About 1.61 g/cm³
- Hidden ocean
- Global salty ocean, about 6.2 miles / 10 km deep under the south pole
- Plume speed
- About 400 metres per second
- Visited by
- Voyager 1 and 2 (1980s), Cassini (2005 to 2015 flybys)
About Enceladus
Enceladus is a small moon of Saturn. It is white and shiny like snow. It is the shiniest world we know. Its surface is very, very cold. But deep inside there is a salty sea. Warm water hides under the thick ice. Near the south pole, the ice has cracks. Water sprays out of the cracks. The spray shoots high into space. It makes a big soft ring round Saturn. A spacecraft called Cassini flew through the spray. Maybe tiny living things could live in that sea. Nobody knows that yet. Scientists want to go back and look.
Enceladus is one of Saturn's most studied moons, even though it is small. It is only about 500 kilometres across, about as wide as the American state of Arizona. It is a bright white sphere, the most reflective body in the solar system. Enceladus orbits Saturn at about 238,000 kilometres, closer than the Moon is to Earth.
One orbit takes only about 33 hours, so its year is shorter than two Earth days. The surface is frozen hard, at around minus 201 degrees Celsius. Yet Saturn's gravity squeezes and stretches the moon, and that warms the inside. Under the icy shell lies a global ocean of salty water.
Near the south pole there are four long cracks called tiger stripes. Jets of water vapour and ice burst out of them at about 400 metres per second. The spray drifts into space and forms Saturn's faint E ring. In 2005 NASA's Cassini spacecraft discovered these plumes and later flew right through them.
It found salts, carbon dioxide, methane and organic molecules in the water. On the sea floor, hot vents may reach more than 90 degrees Celsius. That makes Enceladus one of the best places to search for life beyond Earth.
Enceladus is Saturn's sixth-largest moon, about 500 km across (JPL gives a mean radius of 252.1 km). It orbits at about 238,000 km from Saturn with a period of 1.37 days, embedded within the diffuse E ring, which it actually supplies. William Herschel discovered it on 28 August 1789; his son John proposed the name in 1847, after a giant from Greek mythology, in keeping with Saturn's identification with the Titan leader Cronus. Enceladus has the most reflective surface known in the solar system, and its surface temperature is about minus 201 degrees Celsius.
Its mean density of about 1.61 grams per cubic centimetre indicates a mix of rock and water ice. The two Voyager spacecraft imaged Enceladus in the 1980s and revealed a puzzling, partly crater-free surface. NASA's Cassini mission (in orbit around Saturn from 2004 to 2017) made the moon a headline target. In 2005 it discovered plumes of water vapour and ice grains erupting from four fractures near the south pole, nicknamed the tiger stripes, each roughly 135 km long.
Material leaves at about 400 metres per second. Cassini flew directly through the plume on several passes, the last in October 2015, sampling water vapour, carbon dioxide, methane, salts, silica and organic compounds that were about 20 times denser than expected. Gravity measurements confirmed a global saltwater ocean beneath the ice, about 10 km deep under the south polar region, where the ice shell may thin to between 1 and 5 km. Silica nanoparticles in the plume point to hydrothermal vents on the sea floor at temperatures above about 90 degrees Celsius.
The heat comes from tidal flexing driven by Saturn's gravity. Two points are often misstated. First, Cassini did not find life; it found liquid water, chemical energy and organic molecules, which are the ingredients life needs, not life itself. Second, Enceladus does not have a thick atmosphere; the plume creates only a tenuous, localised envelope of gas.
No spacecraft has visited since Cassini, but several proposals aim to return and sample the plume more thoroughly.
Did you know?
- Enceladus makes its own ring. Ice sprayed from its south pole spreads around its orbit to form Saturn's wide, faint E ring.
- Its surface is the whitest and most reflective in the solar system, bouncing back almost all the sunlight that reaches it.
- Cassini flew straight through the plume and tasted salt, methane and organic molecules from an ocean hidden under the ice.
Neighbours around Saturn
Questions children ask
Is there really an ocean on Enceladus?
Yes. Cassini measured tiny wobbles in the moon's gravity and found a global ocean of salty liquid water under the icy shell. Near the south pole it is about 10 km deep, and water from it sprays out into space.
Why is Enceladus warm inside when it is so cold outside?
Saturn's gravity squeezes and stretches Enceladus as it orbits. This flexing, called tidal heating, warms the rock and ice inside. The surface stays at about minus 201 degrees Celsius, but the ocean below can be liquid.
Has anyone found life on Enceladus?
No. Cassini found the ingredients life needs, including liquid water, heat, salts and organic molecules, but it was not built to detect living things. Scientists hope a future mission will fly through the plume to look more closely.
What are the tiger stripes?
They are four long cracks near the south pole, each about 135 km long. They look like stripes in photographs. Warm ice and water vapour escape through them, feeding the giant plume that Cassini discovered in 2005.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- What sprays out of the cracks at the south pole of Enceladus?
- Which ring of Saturn is made from material sprayed by Enceladus?
- Which spacecraft discovered the plumes of Enceladus in 2005?
Europa
Titan
Mimas