How Well Do You Know Enceladus?

Enceladus: Saturn's Sixth-Largest Moon

• Enceladus is the sixth-largest moon of Saturn, measuring about 500 km in diameter and mostly covered by fresh, clean ice.

• Its surface temperature at noon only reaches -198°C, far colder than a light-absorbing body would be.

• Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrain.

• Discovered in 1789 by William Herschel, little was known about Enceladus until the Voyager and Cassini spacecrafts provided greater detail about its surface and environment.

• Cassini discovered water-rich plumes venting from the south polar region and more than 100 geysers have been identified.

• The plumes are similar in composition to comets and the escaping internal heat and lack of impact craters in the south polar region show that Enceladus is currently geologically active.

• Enceladus is trapped in an orbital resonance with Dione, exciting its orbital eccentricity and tidally heating its interior to drive geological activity.

• Cassini also found evidence for hydrothermal activity, possibly driving complex chemistry, and ongoing research suggests Enceladus's hydrothermal environment could be habitable to some microorganisms.

• Enceladus is named after the giant Enceladus of Greek mythology, and geological features are named after characters and places from The Book of One Thousand and One Nights.

• Enceladus is a scalene ellipsoid in shape and rotates synchronously with its orbital period, keeping one face pointed toward Saturn.

• Plumes from Enceladus have been shown to be the source of the material in Saturn's E ring, the widest and outermost ring of Saturn.

• Enceladus's surface features include several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas.Geology, Surface Features, and Composition of Enceladus

• Enceladus has ridges and domes, but they are not as extensive as those on Europa.

• Smooth plains on Enceladus have low relief and fewer craters, indicating a young surface age.

• The south polar region of Enceladus is covered in tectonic fractures and ridges and has few sizable impact craters, suggesting it is the youngest surface on Enceladus and any mid-sized icy satellite.

• The tiger stripes near the center of the south polar region are the youngest features and surrounded by mint-green-colored coarse-grained water ice, and VIMS detected simple organic compounds in the stripes.

• Cryovolcanism at Enceladus occurs because water-filled cracks are periodically exposed to vacuum, opened and closed by tidal stresses.

• Enceladus's mass is higher than previously thought, yielding a density of 1.61 g/cm3, higher than Saturn's other mid-sized icy satellites, indicating it contains a greater percentage of silicates and iron.

• Enceladus's shape is consistent with an undifferentiated interior, but its current shape also supports the possibility that it may have rotated faster at some point in the recent past.

• Gravimetric data from Cassini's flybys showed that Enceladus likely has a liquid water ocean beneath its frozen surface, and measurements of its "wobble" suggest that the entire icy crust is detached from the rocky core.

• The plumes' composition indicates that the source is a salty subsurface ocean, and the INMS instrument detected mostly water vapor, molecular nitrogen, carbon dioxide, and trace amounts of simple hydrocarbons.

• Cassini found traces of simple organic compounds in some dust grains, larger organics such as benzene, and complex macromolecular organics as large as 200 atomic mass units and at least 15 carbon atoms in size.

• Enceladus's south polar plumes are the source of Saturn's E Ring, and the sources of salty particles are uniformly distributed along the tiger stripes.

• The intensity of the eruption of the south polar jets varies significantly as a function of the position of Enceladus in its orbit, being about four times brighter when Enceladus is at apoapsis than when it is at periapsis.Enceladus: A Moon with Potential for Life

• Enceladus has a subsurface ocean that is heated from the interior, with temperatures ranging from 85 to 90 K, much too warm to be explained by solar heating.

• The salty ocean has an alkaline pH of 11 to 12, and is thought to be a consequence of serpentinization of chondritic rock that leads to the generation of H2, a geochemical source of energy that could support both abiotic and biological synthesis of organic molecules.

• Nitrogen-bearing and oxygen-bearing amines were found in Enceladus's erupting plumes, with significant implications for the availability of amino acids in the internal ocean. The compounds on Enceladus could be precursors for "biologically relevant organic compounds".

• The source of heat causing the observed thermal power output of Enceladus has not yet been settled, and several explanations have been proposed, including venting from a subsurface reservoir of liquid water, sublimation of ice, decompression and dissociation of clathrates, and shear heating.

• Heating in Enceladus has occurred through various mechanisms ever since its formation, including radioactive decay in its core and tidal heating, aided by radioactive decay and some heat-producing chemical reactions.

• The observed power output of 4.7 gigawatts is challenging to explain from tidal heating alone, so the main source of heat remains a mystery.

• Enceladus's subsurface ocean may be habitable, with an energy source and complex organic compounds in contact with Enceladus's rocky core, which may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life.

• The presence of ample hydrogen in Enceladus's ocean means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water.

• Voyager 1 and 2 made the first close-up images of Enceladus, with Voyager 2 passing closer and revealing a young surface with different regions of vastly different surface ages.

• The presence of hydrothermal activity on Enceladus's sub-surface ocean floor was discovered in 2017, indicating a potential oasis of habitability.

• Several robotic missions have been proposed to further explore Enceladus and assess its habitability, including Journey to Enceladus and Titan (JET), Enceladus Explorer (En-Ex), Enceladus Life Finder (ELF), Life Investigation For Enceladus (LIFE), and Enceladus Life Signatures and Habitability (ELSAH).

• Enceladus is losing mass at a rate of 200 kg/second, and if mass loss at this rate continued for 4.5 Gyr, the satellite would have lost approximately 30% of its initial mass.

• In 2016, a study suggested that all of Saturn's satellites inward of Titan, including Enceladus, may have formed as little as 100 million years ago, while a later study from 2019 estimated that the ocean is around one billion years old.Exploration of Enceladus: From Discovery to Proposed Missions

• Enceladus is a small, icy moon of Saturn, discovered in 1789 by William Herschel.

• In 1981, the Voyager 2 spacecraft provided the first detailed images of Enceladus, revealing a heavily cratered surface and a young, geologically active region near its south pole.

• In 2004, the Cassini spacecraft began orbiting Saturn and conducted numerous flybys of Enceladus, discovering water vapor and simple hydrocarbons venting from the south polar region.

• Cassini also provided evidence that Enceladus has a subsurface ocean with an energy source, nutrients, and organic molecules, making it a potential habitat for extraterrestrial life.

• Proposed mission concepts for further exploration of Enceladus include a probe flyby to analyze plume contents in situ, a lander to study the habitability potential of its subsurface ocean, and two astrobiology-oriented missions.

• The Enceladus Life Finder and Life Investigation For Enceladus (LIFE) missions are examples of the astrobiology-oriented missions that would study Enceladus for evidence of extraterrestrial life.

• In 2008, the European Space Agency (ESA) assessed concepts to send a probe to Enceladus in a mission to be combined with studies of Titan.

• In 2017, Russian billionaire Yuri Milner expressed interest in funding a "low-cost, privately funded mission to Enceladus which can be launched relatively soon."

• In 2018, NASA and the Breakthrough Initiatives signed a cooperation agreement for the mission's initial concept phase.

• In 2022, the Planetary Science Decadal Survey recommended that NASA prioritize its newest probe concept, the Enceladus Orbilander, as a Flagship-class mission.

• The Enceladus Orbilander would be launched on a similarly affordable rocket, but would cost about $5 billion, and be designed to endure eighteen months in orbit inspecting Enceladus' plumes before landing and spending two Earth years conducting surface astrobiology research.

• Enceladus is considered one of the best places for the study of potentially habitable environments for extraterrestrial life.