Enceladus

Enceladus (pronounced /ɛnˈsɛlədəs/) is the sixth-largest moon of Saturn. It is approximately 500 kilometers (310 mi) in diameter,^[3] about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, reflecting almost all the sunlight that strikes it, making its surface temperature at noon reach only −198 °C (−324.4 °F). Despite its small size, Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

Enceladus was discovered in 1789 by William Herschel,^[1]^[13]^[14] but little was known about it until the two Voyager spacecraft passed nearby in the early 1980s.^[15] In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region.^[16] Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling approximately 200 kilograms (440 lb) per second.^[15]^[17]^[18] Over 100 geysers have been identified.^[19] Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring.^[20]^[21] According to NASA scientists, the plumes are similar in composition to comets.^[22] In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6.2 mi).^[23]^[24]^[25]

These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is geologically active today. Like many other satellites in the extensive systems of the giant planets, Enceladus is trapped in an orbital resonance. Its resonance with Dione excites its orbital eccentricity, which is damped by tidal forces, tidally heating its interior, and possibly driving the geological activity.

History

Discovery

Voyager 2 view of Enceladus in 1981: Samarkand Sulci vertical grooves (lower center); Ali Baba and Aladdin craters (upper left)

Enceladus was discovered by Fredrick William Herschel on August 28, 1789, during the first use of his new 1.2 m (47 in) telescope, then the largest in the world.^[14]^[26] Its faint apparent magnitude (H_V = +11.7) and its proximity to the much brighter Saturn and Saturn's rings make Enceladus difficult to observe from Earth with smaller telescopes. Like many satellites of Saturn discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane. At such times, the reduction in glare from the rings makes the moons easier to observe.^[2] Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its mass, density and albedo.

Naming

Enceladus is named after the giant Enceladus of Greek mythology.^[1] The name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.^[27] He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.

Features on Enceladus are named by the International Astronomical Union (IAU) after characters and places from Burton's translation of The Book of One Thousand and One Nights.^[28] Impact craters are named after characters, whereas other feature types, such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places. The IAU has officially named 85 features on Enceladus, most recently Samaria Rupes, formerly called Samaria Fossa.^[29]^[30]

Orbit and rotation

View of Enceladus's orbit (highlighted in red) from above Saturn's north pole

Enceladus is one of the major inner satellites of Saturn along with Dione, Tethys, and Mimas. It orbits at 238,000 km from Saturn's center and 180,000 km from its cloud tops, between the orbits of Mimas and Tethys. It orbits Saturn every 32.9 hours, fast enough for its motion to be observed over a single night of observation. Enceladus is currently in a 2:1 mean-motion orbital resonance with Dione, completing two orbits around Saturn for every one orbit completed by Dione. This resonance maintains Enceladus's orbital eccentricity (0.0047), which is known as a forced eccentricity. This non-zero eccentricity results in tidal deformation of Enceladus. The dissipated heat resulting from this deformation is the main heating source for Enceladus's geologic activity.^[4] Enceladus orbits within the densest part of Saturn's E Ring, the outermost of its major rings, and is the main source of the ring's material composition.^[31]

Like most of Saturn's larger satellites, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike Earth's Moon, Enceladus does not appear to librate about its spin axis (more than 1.5°). However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration.^[4] This libration could have provided Enceladus with an additional heat source.^[32]

Source of the E Ring

Plumes from Enceladus, which are similar in composition to comets,^[22] have been shown to be the source of the material in Saturn's E Ring.^[20] The E Ring is the widest and outermost ring of Saturn (except for the tenuous Phoebe ring). It is an extremely wide but diffuse disk of microscopic icy or dusty material distributed between the orbits of Mimas and Titan.^[33]

Mathematical models show that the E ring is unstable, with a lifespan between 10,000 and 1,000,000 years; therefore, particles composing it must be constantly replenished.^[34] Enceladus is orbiting inside the ring, at its narrowest but highest density point, raising suspicion since the 1980s that Enceladus is the main source of particles for the ring.^[35]^[36]^[37]^[38] This hypothesis was confirmed by Cassini's first two close flybys in 2005.^[39]^[40]

View of Enceladus's orbit from the side, showing Enceladus in relation to Saturn's E Ring

Enceladus orbiting within Saturn's E ring
Enceladus geyser tendrils - comparison of images ("a";"c") with computer simulations
Enceladus south polar region - locations of most active tendril-producing geysers

Eruptions on Enceladus look like "discrete" jets, but may be "curtain" eruptions instead
( video animation)

Geology

Surface features

Enceladus - tilted terminator - north is up

Voyager 2 was the first spacecraft to observe the surface in detail, in August 1981. Examination of the resulting highest-resolution imagery revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas.^[41] In addition, extensive linear cracks^[42] and scarps were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old. Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface.^[43] The fresh, clean ice that dominates its surface gives Enceladus probably the most reflective surface of any body in the Solar System with a visual geometric albedo of 1.38.^[7] Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C (−324.4 °F), making it somewhat colder than other Saturnian satellites.^[8]

Observations during three flybys by Cassini on February 17, March 9, and July 14, 2005, revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. The smooth plains, which Voyager 2 had observed, resolved into relatively crater-free regions filled with numerous small ridges and scarps. Numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.^[44] Some areas contain no craters, indicating major resurfacing events in the geologically recent past. There are fissures, plains, corrugated terrain and other crustal deformations. Several additional regions of young terrain were discovered in areas not well-imaged by either Voyager spacecraft, such as the bizarre terrain near the south pole.^[4] All of this indicates that Enceladus's interior may be liquid today, even though it should have been frozen long ago.^[43]

Enhanced-color global map from Cassini images (43.7 MB); leading hemisphere is on right

Enhanced-color maps of the
northern and southern hemispheres of Enceladus

Enhanced-color maps of the
trailing and leading hemispheres of Enceladus

Impact craters

Impact cratering is a common occurrence on many Solar System bodies. Much of Enceladus's surface is covered with craters at various densities and levels of degradation.^[45] This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.^[43]

Cassini observations provided a much closer look at the crater distribution and size, showing that many of Enceladus's craters are heavily degraded through viscous relaxation and fracturing.^[46] Viscous relaxation allows gravity, over geologic time scales, to deform craters and other topographic features formed in water ice, reducing the amount of topography over time. The rate at which this occurs is dependent on the temperature of the ice: warmer ice is easier to deform than colder, stiffer ice. Viscously relaxed craters tend to have domed floors, or are recognized as craters only by a raised, circular rim. Dunyazad crater is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor.^[47]

Tectonic features

View of Enceladus's Europa-like surface with the Labtayt Sulci fractures at center and the Ebony (left) and Cufa dorsa at lower left, imaged by Cassini on February 17, 2005

Enceldaus - Y-shaped discontinuities
(February 15, 2016)

Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges.^[41] Results from Cassini suggest that tectonics is the dominant mode of deformation on Enceladus, including rifts, one of the more dramatic types of tectonic features that were noted. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features are geologically young, because they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.^[48]

Evidence of tectonics on Enceladus is also derived from grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions.^[41] Grooved terrains such as the Samarkand Sulci are reminiscent of grooved terrain on Ganymede. However, unlike those seen on Ganymede, grooved topography on Enceladus is generally more complex. Rather than parallel sets of grooves, these lanes often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands bow upwards with fractures and ridges running the length of the feature. Cassini observations of the Samarkand Sulci have revealed dark spots (125 and 750 m wide) located parallel to the narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.^[46]

In addition to deep fractures and grooved lanes, Enceladus has several other types of tectonic terrain. Many of these fractures are found in bands cutting across cratered terrain. These fractures probably propagate down only a few hundred meters into the crust. Many have probably been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture.^[46]^[49] Another example of tectonic features on Enceladus are the linear grooves first found by Voyager 2 and seen at a much higher resolution by Cassini. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they are among the youngest features on Enceladus. However, some linear grooves have been softened like the craters nearby, suggesting that they are older. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on Europa. These ridges are relatively limited in extent and are up to one kilometer tall. One-kilometer high domes have also been observed.^[46] Given the level of resurfacing found on Enceladus, it is clear that tectonic movement has been an important driver of geology for much of its history.^[48]

Smooth plains

Two regions of smooth plains were observed by Voyager 2. They generally have low relief and have far fewer craters than in the cratered terrains, indicating a relatively young surface age.^[45] In one of the smooth plain regions, Sarandib Planitia, no impact craters were visible down to the limit of resolution. Another region of smooth plains to the southwest of Sarandib is criss-crossed by several troughs and scarps. Cassini has since viewed these smooth plains regions, like Sarandib Planitia and Diyar Planitia at much higher resolution. Cassini images show these regions filled with low-relief ridges and fractures, probably caused by shear deformation.^[46] The high-resolution images of Sarandib Planitia revealed a number of small impact craters, which allow for an estimate of the surface age, either 170 million years or 3.7 billion years, depending on assumed impactor population.^[4]^{[lower-alpha 3]}

The expanded surface coverage provided by Cassini has allowed for the identification of additional regions of smooth plains, particularly on Enceladus's leading hemisphere (the side of Enceladus that faces the direction of motion as it orbits Saturn). Rather than being covered in low-relief ridges, this region is covered in numerous criss-crossing sets of troughs and ridges, similar to the deformation seen in the south polar region. This area is on the opposite side of Enceladus from Sarandib and Diyar Planitiae, suggesting that the placement of these regions is influenced by Saturn's tides on Enceladus.^[50]

South polar region

Internal structure

A model of the interior of Enceladus: silicate core (brown); water-ice-rich mantle (white); a proposed diapir under the south pole (noted in the mantle (yellow) and core (red))^[50]

Before the Cassini mission, little was known about the interior of Enceladus. However, flybys by Cassini provided information for models of Enceladus's interior, including a better determination of the mass and shape, high-resolution observations of the surface, and new insights on the interior.^[55]^[56]

Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.^[41] However, based on the effects of Enceladus's gravity on Cassini, its mass was determined to be much higher than previously thought, yielding a density of 1.61 g/cm³.^[4] This density is higher than Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicates and iron.

Castillo et al. (2005) suggested that Iapetus and the other icy satellites of Saturn formed relatively quickly after the formation of the Saturnian subnebula, and thus were rich in short-lived radionuclides.^[57] These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock–mass fraction, given its small size.^[58] Given Enceladus's relatively high rock–mass fraction, the proposed enhancement in ²⁶Al and ⁶⁰Fe would result in a differentiated body, with an icy mantle and a rocky core.^[59] Subsequent radioactive and tidal heating would raise the temperature of the core to 1,000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have also melted, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core and would power the current geological activity.^[60]^[32]

In addition to its mass and modeled geochemistry, researchers have also examined Enceladus's shape to determine if it is differentiated. Porco et al. (2006) used limb measurements to determine that its shape, assuming hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.^[4] However, the current shape also supports the possibility that Enceladus is not in hydrostatic equilibrium, and may have rotated faster at some point in the recent past (with a differentiated interior).^[59] Gravity measurements by Cassini show that the density of the core is low, indicating that the core contains water in addition to silicates.^[61]

Subsurface water ocean

Artist's impression of possible hydrothermal activity on Enceladus's ocean floor^[62]

Artist's impression of a global subsurface ocean of liquid water^[23]^[25] (updated - better scaled version)

Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from its south polar surface,^[4]^[63] with jets moving 250 kg of water vapor every second^[63] at up to 2,189 km/h (1,360 mph) into space.^[64] Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring.^[4]^[39] The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles are heavier and mostly fall back to the surface, whereas the fast "fresh" particles escape to the E Ring, explaining its salt-poor composition of 0.5–2% of sodium salts by mass.^[65] The plumes' "salty" composition indicates that the source is a salty subsurface ocean.^[66] Cassini also found traces of simple organic compounds in some dust grains.^[65]^[67]

Gravimetric data from Cassini's December 2010 flybys showed that Enceladus likely has a liquid water ocean beneath its frozen surface, but at the time it was thought the subsurface ocean was limited to the south pole.^[23]^[24]^[25]^[68] Top of the ocean proabably lies beneath a 30 to 40 kilometers (19 to 25 mi) thick ice shelf. The ocean may be 10 kilometers (6.2 mi) deep at the south pole.^[23]^[69]

Measurements of Enceladus's "wobble" as it orbits Saturn—called libration—suggests that the entire icy crust is detached from the rocky core and therefore that a global ocean is present beneath the surface.^[70] The amount of libration (0.120° ± 0.014°) implies that this global ocean is about 26 to 31 kilometers deep.^[71]^[72]^[73]^[74] For comparison, Earth's ocean has an average depth of 3.7 kilometers.^[73]

A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO₃) has an alkaline pH of 11 to 12.^[75]^[76] The high pH is interpreted to be a consequence of serpentinization of chondritic rock that leads to the generation of H₂, a geochemical source of energy that can support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus's plume.^[75]^[77]

South polar plumes

Possible heat sources

During the July 14, 2005 flyby, the Composite Infrared Spectrometer (CIRS) found a warm region near the south pole. Temperatures in this region ranged from 85–90 K, with small areas showing as high as 157 K (−116 °C), much too warm to be explained by solar heating, indicating that parts of the south polar region are heated from the interior of Enceladus.^[8] The presence of a subsurface ocean under the south polar region is now accepted,^[91] but it cannot explain the source of the heat, with an estimated heat flux of 200 mW/m², which is about 10 times higher than that from radiogenic heating alone.^[92]

Heat map of the south polar fractures, dubbed 'tiger stripes'

Several explanations for the observed elevated temperatures and the resulting plumes have been proposed, including venting from a subsurface reservoir of liquid water, sublimation of ice,^[93] decompression and dissociation of clathrates, and shear heating,^[94] but a complete explanation of all the heat sources causing the observed thermal power output of Enceladus has not yet been settled.

Heating in Enceladus has occurred through various mechanisms ever since its formation. Radioactive decay in its core may have initially heated it,^[95] giving it a warm core and a subsurface ocean, which is now kept above freezing through an unidentified mechanism. Geophysical models indicate that tidal heating is a main heat source, perhaps aided by radioactive decay and some heat-producing chemical reactions.^[96]^[97]^[98]^[99] A 2007 study predicted the internal heat of Enceladus, if generated by tidal forces, could be no greater than 1.1 gigawatts,^[100] but data from Cassini's infrared spectrometer of the south polar terrain over 16 months, indicate that the internal heat generated power is about 4.7 gigawatts,^[100] and suggest that it is in thermal equilibrium.^[8]^[52]^[101]

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.^[4]^[96] Most scientists think the observed heat flux of Enceladus is not enough to maintain the subsurface ocean, and therefore any subsurface ocean must be a remnant of a period of higher eccentricity and tidal heating, or the heat is produced through another mechanism.^[102]^[103]

Tidal heating

Tidal heating occurs through the tidal friction processes: orbital and rotational energy are dissipated as heat in the crust of an object. In addition, to the extent that tides produce heat along fractures, libration may affect the magnitude and distribution of such tidal shear heating.^[32] Tidal dissipation of Enceladus's ice crust is significant because Enceladus has a subsurface ocean. Scientific models of heating on Enceladus suggest that despite the increased heat from tidal dissipation, the total observed heating of Enceladus is not enough to maintain a subsurface ocean for more than 30 million years (Enceladus is billions of years old), even if the ocean contains other chemical components that lower its freezing point. It is thought that if Enceladus had a more eccentric orbit in the past, the enhanced tidal forces could be sufficient to maintain a subsurface ocean, such that a periodic enhancement in eccentricity could maintain a subsurface ocean that periodically changes in size.^[103] Previous models suggest that resonant perturbations of Dione could provide the necessary periodic eccentricity changes to maintain the subsurface ocean of Enceladus, if the ocean contains a substantial amount of ammonia.^[4] The surface of Enceladus indicates that the entire moon has experienced periods of enhanced heat flux in the past.^[104]

Radioactive heating

The "hot start" model of heating suggests Enceladus began as ice and rock that contained rapidly decaying short-lived radioactive isotopes of aluminium, iron and manganese. Enormous amounts of heat were then produced as these isotopes decayed for about 7 million years, resulting in the consolidation of rocky material at the core surrounded by a shell of ice. Although the heat from radioactivity would decrease over time, the combination of radioactivity and tidal forces from Saturn's gravitational tug could prevent the subsurface ocean from freezing.^[95] The present-day radiogenic heating rate is 3.2 × 10¹⁵ ergs/s, assuming Enceladus has a composition of ice, iron and silicate materials.^[4] Heating from long-lived radioactive isotopes uranium U238, U235, thorium Th232 and potassium K40 inside Enceladus would add 0.3 gigawatts to the observed heat flux.^[96] The presence of Enceladus's regionally thick subsurface ocean suggests a heat flux ~10 times higher than that from radiogenic heating in the silicate core.^[105]

Chemistry

Because no ammonia was initially found in the vented material by INMS or UVIS, which could act as an anti-freeze, it was thought such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least 270 K (−3 °C), because pure water requires more energy to melt. In July 2009 it was announced that traces of ammonia had been found in the plumes during flybys in July and October 2008.^[106]^[107] Reducing the freezing point of water with ammonia would also allow for outgassing and higher gas pressure,^[108] and less heat required to power the water plumes.^[109] The subsurface layer heating the surface water ice could be an ammonia–water slurry at temperatures as low as 170 K (−103 °C), and thus less energy is required to produce the plume activity. However, the observed 4.7 gigawatts heat flux is enough to power the cryovolcanism without the presence of ammonia.^[100]^[109]

Shape and size

Enceladus is a relatively small satellite composed of ice and rock.^[110] It is a scalene ellipsoid in shape; its diameters, calculated from images taken by Cassini's ISS (Imaging Science Subsystem) instrument, are 513 km between the sub- and anti-Saturnian poles, 503 km between the leading and trailing hemispheres, and 497 km between the north and south poles.^[4] Enceladus is only one-seventh the diameter of Earth's Moon. It ranks sixth in both mass and size among the satellites of Saturn, after Titan (5,150 km), Rhea (1,530 km), Iapetus (1,440 km), Dione (1,120 km) and Tethys (1,050 km).^[111]^[112]

Enceladus transiting the moon Titan
Size comparison of Earth, the Moon, and Enceladus
A size comparison of Enceladus against the British Isles

Origin

Mimas–Enceladus paradox

Mimas, the innermost of the round moons of Saturn and directly interior to Enceladus, is a geologically dead body, even though it should experience stronger tidal forces than Enceladus. This apparent paradox can be explained in part by temperature-dependent properties of water ice (the main constituent of the interiors of Mimas and Enceladus). The tidal heating per unit mass is given by the formula $q_{tid}=\frac{63\rho n^{5} r^{4} e^{2}}{38 \mu Q }$ , where ρ is the (mass) density of the satellite, n is its mean orbital motion, r is the satellite's radius, e is the orbital eccentricity of the satellite, μ is the shear modulus and Q is the dimensionless dissipation factor. For a same-temperature approximation, the expected value of q_tid for Mimas is about 40 times that of Enceladus. However, the material parameters μ and Q are temperature dependent. At high temperatures (close to the melting point), μ and Q are low, so tidal heating is high. Modeling suggests that for Enceladus, both a 'basic' low-energy thermal state with little internal temperature gradient, and an 'excited' high-energy thermal state with a significant temperature gradient, and consequent convection (endogenic geologic activity), once established, would be stable. For Mimas, only a low-energy state is expected to be stable, despite its being closer to Saturn. So the model predicts a low-internal-temperature state for Mimas (values of μ and Q are high) but a possible higher-temperature state for Enceladus (values of μ and Q are low).^[113] Additional historical information is needed to explain how Enceladus first entered the high-energy state (e.g. more radiogenic heating or a more eccentric orbit in the past).^[114]

The significantly higher density of Enceladus relative to Mimas (1.61 vs. 1.15 g/cm³), implying a larger content of rock and more radiogenic heating in its early history, has also been cited as an important factor in resolving the Mimas paradox.^[115]

It has been suggested that for an icy satellite the size of Mimas or Enceladus to enter an 'excited state' of tidal heating and convection, it would need to enter an orbital resonance before it lost too much of its primordial internal heat. Because Mimas, being smaller, would cool more rapidly than Enceladus, its window of opportunity for initiating orbital resonance-driven convection would have been considerably shorter.^[116]

Proto-Enceladus hypothesis

Enceladus is losing mass at a rate of 200 kg/second. If mass loss at this rate continued for 4.5 Gyr, the satellite would have lost approximately 30% of its initial mass. A similar value is obtained by assuming that the initial densities of Enceladus and Mimas were equal.^[116] It suggests that tectonics in the south polar region is probably mainly related to subsidence and associated subduction caused by the process of mass loss.^[116]

Date of formation

In 2016, a study of how the moons of Saturn's orbits have changed due to tidal effects suggested that all of Saturn's satellites inward of Titan, including Enceladus (whose geologic activity was used to derive the strength of tidal effects on Saturn's satellites), may have formed as little as 100 million years ago. ^[117]

Potential habitability

Enceladus ejects plumes of salt water that are laced with grains of silica-rich sand,^[118] nitrogen (in ammonia),^[119] nutrients and organic molecules, including trace amounts of simple hydrocarbons such as methane (CH
4), propane (C
3H
8), acetylene (C
2H
2) and formaldehyde (CH
2O), which are carbon-bearing molecules.^[81]^[82]^[120] This indicates that hydrothermal activity—an energy source—may be at work in Enceladus's subsurface ocean.^[118]^[121] In addition, models indicates the large rocky core is porous, allowing water to flow through it to pick up heat.^[122]^[123]

A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO₃) has an alkaline pH of 11 to 12.^[75] This high pH (alkaline) is interpreted to be a consequence of serpentinization of chondritic rock that leads to the generation of molecular hydrogen (H
2), a geochemical source of energy that can be metabolized by methanogen microbes to provide energy for life.^[75]^[76]^[77]

The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns,^[122] with an energy source and simple organic compounds in contact with Enceladus's rocky core,^[24]^[25]^[124] may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life.^[23]^[68]^[69]^[125]^[126]^[127] Therefore, several robotic missions have been proposed to further explore Enceladus and assess its habitability; some of the proposed missions are: Journey to Enceladus and Titan, Enceladus Explorer, Enceladus Life Finder, and Life Investigation For Enceladus.

Exploration

Voyager missions

The two Voyager spacecraft made the first close-up images of Enceladus. Voyager 1 was the first to fly past Enceladus, at a distance of 202,000 km on November 12, 1980.^[128] Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.^[129] Voyager 1 also confirmed that Enceladus was embedded in the densest part of Saturn's diffuse E-Ring. Combined with the apparent youthful appearance of the surface, Voyager scientists suggested that the E-Ring consisted of particles vented from Enceladus's surface.^[129]

Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing higher-resolution images to be obtained.^[128] These images showed a young surface.^[41] They also revealed a surface with different regions with vastly different surface ages, with a heavily cratered mid- to high-northern latitude region, and a lightly cratered region closer to the equator. This geologic diversity contrasts with the ancient, heavily cratered surface of Mimas, another moon of Saturn slightly smaller than Enceladus. The geologically youthful terrains came as a great surprise to the scientific community, because no theory was then able to predict that such a small (and cold, compared to Jupiter's highly active moon Io) celestial body could bear signs of such activity.

Cassini

Enceladus – Close Flyby (October 28, 2015)^[130]

Approach

Before

Up-Close

Plumes

After

49 km (30 mi) above surface

Enceladus – Final Flyby (December 19, 2015)^[131]

Old and new terrain

Northern features

Frozen fractures

Dark Spots

Ice and atmosphere

4,999 km (3,106 mi) above surface

Animated 3D model of the Cassini–Huygens spacecraft

*Cassini* flybys of Enceladus^[132]
Date	Distance (km)
February 17, 2005	1,264
March 9, 2005	500
July 14, 2005	175
December 24, 2005	94,000
March 12, 2008	48
August 11, 2008	54
October 9, 2008	25
October 31, 2008	200
November 2, 2009	103
November 21, 2009	1,607
April 28, 2010	103
May 18, 2010	201
August 13, 2010	2,554
November 30, 2010	48
December 21, 2010	50
October 1, 2011	99
October 19, 2011	1,231
November 6, 2011	496
March 27, 2012	74
April 14, 2012	74
May 2, 2012	74
October 14, 2015	1,839
October 28, 2015	49
December 19, 2015	4,999

The answers to many remaining mysteries of Enceladus had to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it entered orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000 km of Enceladus. These encounters are listed on the right. The flybys have yielded significant information concerning Enceladus's surface, as well as the discovery of water vapor with traces of simple hydrocarbons venting from the geologically active south polar region. These discoveries prompted the adjustment of Cassini's flight plan to allow closer flybys of Enceladus, including an encounter in March 2008 that took it to within 48 km of the surface.^[133] Cassini's extended mission included seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50 km in the later half of 2008.^[134] Cassini performed a flyby on October 28, 2015, passing as close as 48 km (30 mi) and through a plume.^[135] Confirmation of molecular hydrogen (H
2) would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus seafloor, increasing its habitability.^[77]

Cassini has provided strong evidence that Enceladus has an ocean with an energy source, nutrients and organic molecules, making Enceladus one of the best places for the study of potentially habitable environments for extraterrestrial life.^[136]^[137] By contrast, the water thought to be on Jupiter's moon Europa is located under a much thicker layer of ice.^[138]

Proposed mission concepts

The discoveries Cassini has made at Enceladus have prompted studies into follow-up mission concepts, including a flyby plume sample-return mission by NASA,^[119]^[136] a probe flyby (Journey to Enceladus and Titan or JET) to analyze plume contents in-situ,^[139]^[140] a lander by the German Aerospace Center to study the habitability potential of its subsurface ocean (Enceladus Explorer),^[141]^[142]^[143] and two astrobiology-oriented mission concepts (the Enceladus Life Finder^[144]^[145] and Life Investigation For Enceladus).^[146]^[147] The European Space Agency (ESA) is also assessing concepts to send a probe to Enceladus in a mission to be combined with studies of Titan: TandEM (Titan and Enceladus Mission).^[148]

Additionally, the Titan Saturn System Mission (TSSM) was a joint NASA/ESA flagship-class proposal for exploration of Saturn's moons, with a focus on Enceladus. TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009, it was announced that NASA/ESA had given the EJSM mission priority ahead of TSSM,^[149] although TSSM will continue to be studied and evaluated.

2006 GSFC NASA Academy: 'EAGLE' study
2006 NASA: 'Billion Dollar Box' study
2007 NASA: 'Enceladus Flagship' study
2007 ESA: Titan and Enceladus Mission (TandEM)
2007 JPL: Enceladus RMA Study^[150]
2008 NASA/ESA: Titan Saturn System Mission (TSSM)
2010 PSDS Decadal Survey: Enceladus Studies
2010 JPL: Journey to Enceladus and Titan (JET)^[151]
2012 DLR: Enceladus Explorer (EnEx) lander, employing the IceMole^[152]
2012 JPL: Life Investigation For Enceladus (LIFE)^[146]^[153]^[154]
2015 JPL: Enceladus Life Finder (ELF)^[155]

Notes and references

Explanatory notes

↑ This image of Enceladus by Cassini, taken during its 28 October 2015 flyby, shows the younger terrain of Sarandib and Diyar Planitia, populated with many grooves (sulci) and depressions (fossae). Older, cratered terrain can be seen towards Enceladus' north pole. The prominent feature visible near the south pole is Cashmere Sulci.
↑ Used with roughly equal frequency
↑ Without samples to provide absolute age determinations, crater counting is currently the only method for determining surface age on most planetary surfaces. Unfortunately, there is currently disagreement in the scientific community regarding the flux of impactors in the outer Solar System. These competing models can significantly alter the age estimate even with the same crater counts. For the sake of completeness, both age estimates from Porco, Helfenstein et al. 2006 are provided.

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↑ Moons of Saturn may be younger than the dinosaurs | Astronomy.com
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Images

Cassini images of Enceladus
Images of Enceladus at JPL's Planetary Photojournal
Movie of Enceladus' rotation from the National Oceanic and Atmospheric Administration
Enceladus global and polar basemaps (December 2011) from Cassini images
Enceladus atlas (May 2010) from Cassini images
Enceladus nomenclature and Enceladus map with feature names from the USGS planetary nomenclature page

Moons of Saturn

Temporary names in italics.

Ring shepherds	S/2009 S 1 Propeller moonlets Pan Daphnis Atlas Prometheus ?(S/2004 S 6 S/2004 S 4 S/2004 S 3) Pandora

Co-orbitals	Epimetheus Janus

G Ring	Aegaeon

Inner large (with trojans)	Mimas Enceladus Tethys Telesto Calypso Dione Helene Polydeuces

Alkyonides	Methone Anthe Pallene

Outer large	Rhea Titan Hyperion Iapetus

Inuit group	Kiviuq Ijiraq Paaliaq Siarnaq Tarqeq

Gallic group	Albiorix Bebhionn Erriapus Tarvos

Norse group	Phoebe Skathi S/2007 S 2 Skoll S/2004 S 13 Greip Hyrrokkin Mundilfari Jarnsaxa S/2006 S 1 S/2004 S 17 Narvi Bergelmir Aegir Suttungr S/2004 S 12 Bestla Farbauti Hati S/2004 S 7 Thrymr S/2007 S 3 S/2006 S 3 Surtur Kari Fenrir Ymir Loge Fornjot

Rings of Saturn Cassini–Huygens Themis Chiron In fiction

Natural satellites of the Solar System

Planetary
satellites

Other
satellites

Largest
satellites
(mean radius
≥ 100 km)

size	Ganymede largest / 2634 km / 0.413 Earths Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys [Dysnomia] Enceladus Miranda Proteus Mimas [Hi'iaka] Nereid Hyperion Phoebe smallest / 106 km / 0.017 Earths

abc's	Ariel Callisto Charon Dione [Dysnomia] Enceladus Europa Ganymede [Hi'iaka] Hyperion Iapetus Io Mimas Miranda Moon Nereid Oberon Phoebe Proteus Rhea Tethys Titan Titania Triton Umbriel

Discovery timeline
Inner moons
Irregular moons
List
Naming
Regular moons
Trojan moons
NOTE: Italicized moons are not close to being in hydrostatic equilibrium; [bracketed] moons may or may not be close to being in hydrostatic equilibrium.

Voyager program

Probes	Voyager 1 Voyager 2

Concepts	Gravity assist Radioisotope thermoelectric generator Specific orbital energy of Voyager 1

Work	Pale Blue Dot Family Portrait Voyager Golden Record (Contents)

Voyager team	Stamatios Krimigis Carolyn Porco Raymond Heacock Jim Blinn Edward C. Stone Timothy Ferris

Observations	Enceladus Masubi (volcano) Exploration of Jupiter Exploration of Saturn Exploration of Uranus Prometheus (volcano) Exploration of Io Rings of Neptune Dunyazad (crater) Surt (volcano) Shahrazad (crater) Pele (volcano)

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