Enceladus

Enceladus is the sixth-largest moon of Saturn and the 18th largest in the Solar System. It is small, about 500 kilometres (310 miles) in diameter, but it has some very interesting features. Enceladus is covered in clean, fresh snow, which makes it one of the most reflective objects in space. Because of this, its surface stays very cold, around −198 °C (75.1 K; −324.4 °F).

Enceladus was discovered on August 28, 1789, by William Herschel. For a long time, not much was known about it until the Voyager spacecrafts, _Voyager 1 and _Voyager 2, flew by Saturn in 1980 and 1981. In 2005, the Cassini spacecraft began flying close to Enceladus and discovered something amazing: water-rich plumes coming from its south polar region. These cryovolcanoes shoot out geyser-like jets of water vapour, molecular hydrogen, and ice particles into space.

Scientists found that Enceladus has a large subsurface ocean of liquid water under its surface. This, along with the discovery of possible hydrothermal activity, makes Enceladus a place of great interest for scientists who study places where life might exist beyond Earth.

History

Discovery

Enceladus was discovered by William Herschel on August 28, 1789, using his new 40-foot telescope. At the time, it was the largest telescope in the world and was located at Observatory House in Slough, England. Enceladus is hard to see from Earth because it is very faint and close to the bright planet Saturn and its rings. It was first seen during a time when Earth was in the same plane as Saturn's rings, making the moons easier to spot. Before space missions, not much more was known about Enceladus except for its orbit around Saturn.

Naming

Enceladus is named after a giant from Greek mythology called Enceladus. The name was suggested by William Herschel's son, John Herschel, in 1847. He chose names from Greek myths because Saturn, known as Cronus in Greek stories, was the leader of the Titans.

Features on Enceladus, like valleys, ridges, and plains, are named after characters and places from an old book called The Book of One Thousand and One Nights. The International Astronomical Union gives these names. So far, 85 features on Enceladus have official names.

Orbit and rotation

Enceladus is the second largest moon close to Saturn. It travels around Saturn at a distance of 238,000 kilometres from Saturn's center and completes one orbit every 32.9 hours.

Enceladus is in a special pattern with another moon, Dione, which helps keep its orbit slightly stretched. This stretching creates heat inside Enceladus, which may explain its active geology. Enceladus is within Saturn's E ring and is the main source of material for this ring.

Like most of Saturn's larger moons, Enceladus always shows the same face toward Saturn as it orbits.

Source of the E ring

Main article: Rings of Saturn § E Ring

Plumes from Enceladus are the source of the material in Saturn's E ring. The E ring is the widest and outermost ring of Saturn. It is a wide, thin disk of tiny icy or dusty particles between the orbits of Mimas and Titan.

Models show the E ring does not last forever and needs constant replenishment. Enceladus orbits inside the ring and was confirmed as its main source of particles by the Cassini spacecraft in 2005.

The Cassini spacecraft detected a large increase in particles near Enceladus and later saw geyser-like jets of icy particles rising from Enceladus's south polar region.

Physical characteristics

Shape and size

Enceladus is a small moon made of ice and rock. It has an oval shape, with diameters of 513 km between the poles closest and farthest from Saturn, 503 km between the leading and trailing sides, and 497 km between the north and south poles.

Enceladus is only one-seventh the size of Earth's Moon. It is the sixth largest moon of Saturn, smaller than Titan, Rhea, Iapetus, Dione, and Tethys.

Internal structure

Before the Cassini mission, little was known about what lies inside Enceladus. Cassini’s flybys gave scientists new clues about its interior. Earlier ideas suggested Enceladus was mostly water ice, but Cassini data showed its density is higher, meaning it has more silicates and iron.

Scientists think Enceladus may have a rocky core surrounded by an icy mantle. Heat from radioactive decay and tides from Saturn might keep parts of its core warm enough to melt, creating magma chambers. These could flex and create geological activity we see today.

Subsurface ocean

In 2005, scientists saw water vapor shooting from Enceladus's south pole. By 2006, they learned these plumes supply particles to Saturn’s E Ring. In 2010, Cassini data suggested a liquid water ocean under the ice, possibly 10 km deep at the south pole.

Later, measurements of Enceladus’s “wobble” as it orbits Saturn suggested the entire icy shell floats on a global ocean about 26 to 31 km deep—much deeper than Earth’s oceans.

Composition

Cassini flew through Enceladus’s plumes and studied what they contained. The plumes are salty and include water vapor, traces of nitrogen, carbon dioxide, and simple hydrocarbons like methane and propane. Cassini also found organic compounds, including benzene and larger molecules.

The mass spectrometer detected molecular hydrogen and ammonia. A model suggests Enceladus’s ocean is very alkaline, which could support the formation of organic molecules like those detected in the plumes.

A 2025 study reported finding organic molecules in samples from the plumes.

Possible heat sources

During a 2005 flyby, instruments found a warm region near Enceladus’s south pole, with temperatures much higher than solar heating could explain. A subsurface ocean under the south pole is accepted, but the heat source remains unclear.

Several ideas exist for the heat, including venting from a liquid water reservoir, sublimation of ice, or tidal heating. Radioactive decay in Enceladus’s core may have given it an initial warm core, and tidal heating from Saturn’s gravity is thought to be a major heat source today.

One study suggested friction in Enceladus’s core could keep its ocean warm for billions of years. Another idea is that past changes in Enceladus’s orbit increased tidal forces, helping maintain the ocean.

The observed heat output is harder to explain with tidal heating alone, so the main source of heat is still a mystery. Most scientists think the heat isn’t enough to keep the ocean warm today, so it might be a remnant of a time when tidal heating was stronger, or another mechanism is at work.

Tidal heating

Tidal heating happens when an object’s shape changes due to gravity, creating heat. For Enceladus, this could happen in its ice crust because of the subsurface ocean. A 2017 simulation suggested friction in Enceladus’s core could keep the ocean warm for billions of years. If Enceladus had a more eccentric orbit in the past, stronger tidal forces could have kept the ocean from freezing.

A 2016 study suggested that the “tiger stripes” on Enceladus’s surface could be cracks that let heat escape, helping explain the ongoing eruptions. Past models suggest that gravitational interactions with Dione could give Enceladus the needed changes in its orbit to keep the ocean warm, especially if the ocean contains ammonia.

Radioactive heating

Today, radioactive decay inside Enceladus adds about 0.3 gigawatts to the heat we see. The thick subsurface ocean suggests much more heat than this alone can provide.

One idea is that Enceladus started with short-lived radioactive materials that gave off huge amounts of heat for about 7 million years. This could have formed a rocky core surrounded by ice. Even as that heat faded, combined with tidal forces from Saturn, it might keep the ocean from freezing.

Chemical factors

Early studies didn’t find ammonia in Enceladus’s plumes, suggesting the water was nearly pure and needed high temperatures to stay liquid. But in 2009, traces of ammonia were found, which could lower the freezing point of water. This would need less heat to power the plumes. Even without ammonia, the observed heat output is enough to explain the cryovolcanism we see.

Surface features

See also: List of geological features on Enceladus

Voyager 2 was the first spacecraft to look closely at Enceladus's surface in August 1981. It found different types of terrain, including areas covered in craters, smooth young areas, and ridged areas bordering the smooth spots. There were also many long cracks and slopes. The smooth areas have fewer craters, meaning they are probably only a few hundred million years old.

Enceladus must have been active recently with processes like "water volcanism" that renew its surface. The clean, fresh ice covering Enceladus makes it one of the most reflective objects in the Solar System. Because it reflects so much sunlight, its surface stays very cold, around −198 °C (−324 °F).

More detailed observations during flybys in 2005 showed Enceladus's surface features more clearly. The smooth areas seen by Voyager 2 were filled with small ridges and slopes. Many cracks were found in the older, cratered areas, showing the surface has changed a lot since the craters formed.

Some areas have almost no craters, meaning they were renewed not too long ago. There are fissures, plains, wavy terrain, and other changes in the crust. Several new young areas were found in places not well seen by Voyager. All of this suggests Enceladus's inside is liquid today, even though it should have frozen long ago.

Snow

Enceladus's surface is covered in snow from its geysers. The snow is several hundred meters deep in most places, and up to about 700 meters thick in the deepest spots. We can tell how deep it is by how it sinks into cracks in the surface. For the snow to be this thick without being more squished, the geysers must have been more active recently than they are now.

Impact craters

Impact cratering happens often on many objects in the Solar System. Much of Enceladus's surface is covered with craters of different amounts and wear. This suggests Enceladus has been covered over many times.

Cassini gave a closer look at the craters, showing many are worn down by movement of the surface over time. Warm ice deforms more easily than cold ice, so craters change shape over millions of years. Some craters have rounded floors or are recognized only by a raised edge. Dunyazad crater is a good example, with a rounded floor.

Tectonic features

See also: Tiger stripes (Enceladus)

Voyager 2 found several types of tectonic features on Enceladus, including troughs, slopes, and lines of grooves and ridges. Cassini suggests tectonics is a big part of how Enceladus changes, including rifts, which are very dramatic. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. These features look young because they cut through other features and have sharp edges.

Evidence of tectonics also comes from grooved areas made of lines of curved grooves and ridges. These were first seen by Voyager 2 and often separate smooth areas from cratered areas. Grooved areas like the Samarkand Sulci look like those on Ganymede but are more complex. Rather than straight lines, they often look like zig-zag patterns.

In other places, these lines curve upward with cracks and ridges along them. Cassini saw dark spots along some of these lines, which might be small pits.

In addition to big cracks and grooved lines, Enceladus has other types of tectonic terrain. Many cracks are found in bands cutting through cratered areas. These cracks probably go down only a few hundred meters. Many seem to have been affected by soft material from impact craters, changing the direction of the crack.

Another type of tectonic feature are linear grooves first seen by Voyager 2 and seen more clearly by Cassini. These cut across other terrain types and are among the youngest features. Some have softened like nearby craters, meaning they are older. Ridges have also been seen, though not as much as on Europa. These ridges are not very big and can be up to one kilometer tall. One-kilometer high domes have also been seen. With all the surface changes, tectonic movement has been important for much of Enceladus's history.

Smooth plains

Two areas of smooth plains were seen by Voyager 2. They have low height changes and far fewer craters than cratered areas, meaning a younger surface. In one smooth area, Sarandib Planitia, no craters were seen at all. Another smooth area southwest of Sarandib has many troughs and slopes. Cassini saw these smooth areas, like Sarandib Planitia and Diyar Planitia, in much more detail. Cassini images show these areas filled with low ridges and cracks, probably from shifting movement. High-resolution images of Sarandib Planitia showed small impact craters, helping estimate the surface age at either 170 million years or 3.7 billion years.

Cassini's wider view found more smooth plains, especially on Enceladus's leading side (the side facing its direction of motion around Saturn). Instead of low ridges, this area has many crossing sets of troughs and ridges, similar to the south pole area. This area is on the opposite side from Sarandib and Diyar Planitiae, suggesting Saturn's pull on Enceladus affects where these smooth areas are.

South polar region

Images from Cassini on July 14, 2005, showed a special, changed area around Enceladus's south pole. This area, going up to 60° south latitude, is covered in tectonic cracks and ridges. It has few big craters, meaning it is the youngest surface on Enceladus and on mid-sized icy moons. Studies suggest some parts of this area could be as young as 500,000 years or less.

Near the center of this area are four cracks bounded by ridges, called "tiger stripes". They seem to be the youngest features here and are surrounded by a special kind of ice that looks green in some images. This ice is on a flat surface, showing the area is young enough not to have been covered by finer ice from Enceladus's E ring.

Tools on Cassini suggest the material around the tiger stripes is chemically different from the rest of Enceladus. It found crystals of water ice in the stripes, meaning they are very young (likely less than 1,000 years old) or the ice has been warmed recently. It also found simple organic compounds in the tiger stripes, which is unique on Enceladus.

One area of this special ice was seen up close during the July 14, 2005, flyby, showing extreme changes and rocky terrain, with some rocks 10–100 m across.

The edge of the south polar region is marked by patterns of parallel, Y- and V-shaped ridges and valleys. These shapes and positions suggest they are caused by changes in Enceladus's overall shape. As of 2006, there were two ideas about what caused this change: Enceladus's orbit may have moved inward, making it spin faster and become more stretched; or a rising mass of warm, less dense material inside Enceladus may have moved the south polar terrain from the mid-latitudes to the pole.

Because of this, the moon's shape adjusted to match the new position. One problem with the idea that the poles flattened is that both poles should have similar patterns of changes. However, the north pole is covered in craters and has a much older surface than the south pole. Differences in how thick the outer layer is on Enceladus might explain this. Thinner outer layers are linked to younger terrain with fewer craters, and thicker layers to older, cratered terrain.

South polar plumes

See also: Cryovolcano

After Voyager's visits in the early 1980s, scientists thought Enceladus might be active because of its young, bright surface and place near the E ring's core. They thought Enceladus might supply material to the E ring, perhaps through water vapor coming out. The first Cassini images of a plume of tiny particles above Enceladus's south pole came in January and February 2005, though it was first thought to be a camera effect.

Data from the magnetometer during the February 17, 2005, visit showed evidence of a atmosphere. It saw a change in the magnetic field, suggesting nearby gas. In later visits, the magnetometer team found gases are concentrated over the south pole, with much less away from the pole. Unlike the magnetometer, the Ultraviolet Imaging Spectrograph did not see an atmosphere over the equator but did see water vapor over the south pole during the July visit. Cassini flew through this gas cloud on several visits, letting tools like the ion and neutral mass spectrometer and the cosmic dust analyzer sample the plume directly. (See 'Composition' section.) November 2005 images showed the plume's fine structure, with many jets (perhaps from many different vents) within a larger, faint part stretching nearly 500 km (310 mi) from the surface. The particles move at about 1.25 ± 0.1 kilometers per second (2,800 ± 220 miles per hour), with a top speed of 3.40 km/s (7,600 mph). Cassini's UVIS later saw gas jets matching the dust jets seen by the Imaging Science Subsystem during a non-targeted visit with Enceladus in October 2007.

Studies of images, mass spectrometry, and magnetospheric data suggest the south polar plume comes from pressurized underground chambers, like Earth's geysers or fumaroles. Fumaroles are probably a better comparison, since geysers often shoot out water in a repeating way. The plumes on Enceladus were seen to be steady within a few times difference. The mechanism that powers and keeps these eruptions going is thought to be tidal heating.

How strong the south polar jets are changes a lot depending on where Enceladus is in its orbit. The plumes are about four times brighter when Enceladus is at apoapsis (farthest from Saturn) than when it is at periapsis (closest to Saturn). This matches predictions that the south polar cracks are squeezed shut near periapsis and pulled open near apoapsis. Strike-slip tectonics may also control jet activity in these areas by creating pull-apart basins along the Tiger Stripes.

Much of the plume activity looks like wide curtains. Earlier views made the plumes look like separate jets because of how they were seen and the local crack shapes.

How much cryovolcanism really happens is debated. On Enceladus, it seems to happen because water-filled cracks are sometimes exposed to empty space, opening and closing due to tidal forces.

Origin

Mimas–Enceladus paradox

Mimas, a moon of Saturn that is closer to the planet than Enceladus, seems unusual because it is not geologically active. Normally, Mimas should experience stronger forces from Saturn than Enceladus, which might make it more active. However, Enceladus shows more activity.

The difference can be explained by the temperature of the ice inside these moons. Enceladus might have areas that are warmer, allowing it to stay active. Mimas, being smaller, cools down faster and does not stay warm enough to be active.

Proto-Enceladus hypothesis

Enceladus is losing mass, and if this continued for a very long time, it would have lost a lot of its original material. This loss of mass might be linked to changes in its surface, especially around the south pole.

Date of formation

Studies suggest that Enceladus and other moons of Saturn close to the planet may have formed relatively recently, perhaps only 100 million years ago. Other research indicates that the ocean on Enceladus could be about one billion years old.

Potential habitability

Enceladus shoots out water with tiny particles and special molecules, including some that contain carbon. This suggests there might be warm water activity deep inside Enceladus, which could give energy for life. Scientists think Enceladus has a salty ocean inside, with rocks that could let water move through them, sharing heat and chemicals. This ocean might have the right conditions for tiny life to exist.

In June 2023, scientists found a new important chemical on Enceladus that is needed for life. In December 2023, they found more special molecules in the water that could help support life or create it. Many space missions have been suggested to learn more about Enceladus and see if it could be a place where life might exist.

Exploration

Voyager missions

Main article: Voyager program

The two Voyager spacecraft were the first to take close pictures of Enceladus. Voyager 1 flew by Enceladus on November 12, 1980, from a distance of 202,000 km. The pictures were not very clear, but they showed that Enceladus has a bright surface without many craters, meaning it is young. Voyager 1 also found that Enceladus is inside a ring of particles around Saturn called the E ring, and scientists think these particles come from Enceladus's surface.

Voyager 2 flew even closer to Enceladus on August 26, 1981, getting within 87,010 km. The better pictures showed that Enceladus has a young surface, but some parts look much older than others. This was a big surprise because scientists did not think such a small and cold moon could show signs of activity.

Cassini

Main article: Cassini–Huygens

The Cassini spacecraft arrived at Saturn on July 1, 2004, and gave us many answers about Enceladus. Cassini flew close to Enceladus several times and found water vapor and some simple carbon-containing molecules coming from its southern area.

These discoveries led Cassini to fly even closer to Enceladus. In March 2008, it flew just 48 km from the surface. Later, Cassini flew through a plume of material from Enceladus and found molecular hydrogen, which suggests there might be hydrothermal activity on the ocean floor. This makes Enceladus a good place to look for places where life might exist.

In December 2023, scientists found hydrogen cyanide and other organic molecules in the plumes of Enceladus. These molecules could be important for supporting tiny living things or helping create life.

Proposed mission concepts

The discoveries made by Cassini have led to ideas for new missions to Enceladus. These include a probe to study the plume up close, a lander to check on the possibility of life in its ocean, and several other ideas focused on searching for life.

The European Space Agency was looking at ideas in 2008 for a mission to Enceladus and another moon, Titan. In 2022, NASA suggested a new mission called the Enceladus Orbilander, which would orbit Enceladus and study its plumes before landing to look for signs of life on the surface.

In 2024, the European Space Agency chose a mission to Enceladus as its top priority. This mission, called the L4 mission, plans to launch in 2042 and arrive at Enceladus in 2053.