|Discovered by||William Herschel|
|Discovery date||August 28, 1789|
|Inclination||0.019° (to Saturn's equator)|
|Dimensions||513.2 × 502.8 × 496.6 km|
|±0.2 km252.1 ( Earths, 0.0395 Moons) 0.1451|
|Mass||22±0.00101)×1020 kg(1.080 (1.8×10−5 Earths)|
|0.113 m/s2 ( 0.0113g)|
|0.239 km/s (860.4 km/h)|
|Albedo||±0.008 ( 1.375geometric at 550 nm) or ±0.04 ( 0.81Bond)|
|trace, significant spatial variability|
|Composition by volume||91% water vapor
3.2% carbon dioxide
Enceladus (pronounced //) is the sixth-largest moon of Saturn. It is about 500 kilometers (310 mi) in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the solar system. Consequently, its surface temperature at noon only reaches −198 °C (−324 °F), far colder than a light-absorbing body would be. 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 on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft passed nearby in the early 1980s. 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. 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 about 200 kilograms (440 lb) per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes are similar in composition to comets. 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 mi).
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.
- 1 History
- 2 Orbit and rotation
- 3 Geology
- 4 Shape and size
- 5 Origin
- 6 Potential habitability
- 7 Exploration
- 8 See also
- 9 Notes and references
- 10 External links
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. Its faint apparent magnitude (HV = +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. 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.
Enceladus is named after the giant Enceladus of Greek mythology. 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. 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. 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.
Orbit and rotation
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. 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.
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. This libration could have provided Enceladus with an additional heat source.
Source of the E ring
Plumes from Enceladus, which are similar in composition to comets, have been shown to be the source of the material in Saturn's E ring. 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.
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. 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. This hypothesis was confirmed by Cassini's first two close flybys in 2005.
Voyager 2 was the first spacecraft to observe Enceladus's 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. In addition, extensive linear cracks 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. 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 and bolometric Bond albedo of ±0.04. 0.81 Because it reflects so much sunlight, its surface only reaches a mean noon temperature of −198 °C (−324 °F), somewhat colder than other Saturnian satellites.
Observations during three flybys by Cassini on February 17, March 9, and July 14, 2005, revealed Enceladus' 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. 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. All of this indicates that Enceladus's interior may be liquid today, even though it should have been frozen long ago.
Impact cratering is a common occurrence on many Solar System bodies. Much of Enceladus' surface is covered with craters at various densities and levels of degradation. This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.
Cassini observations provided a much closer look at the crater distribution and size, showing that many of Enceladus' craters are heavily degraded through viscous relaxation and fracturing. 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.
Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges. 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.
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. 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.
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. 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. 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.
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. 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. 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.[c]
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.
South polar region
Images taken by Cassini during the flyby on July 14, 2005, revealed a distinctive, tectonically deformed region surrounding Enceladus's south pole. This area, reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges. The area has few sizable impact craters, suggesting that it is the youngest surface on Enceladus and on any of the mid-sized icy satellites; modeling of the cratering rate suggests that some regions of the south polar terrain are possibly as young as 500,000 years or less. Near the center of this terrain are four fractures bounded by ridges, unofficially called "tiger stripes". They appear to be the youngest features in this region and are surrounded by mint-green-colored (in false color, UV–green–near IR images), coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls. Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine-grained water ice from the E ring. Results from the visual and infrared spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is chemically distinct from the rest of the surface of Enceladus. VIMS detected crystalline water ice in the stripes, suggesting that they are quite young (likely less than 1,000 years old) or the surface ice has been thermally altered in the recent past. VIMS also detected simple organic (carbon-containing) compounds in the tiger stripes, chemistry not found anywhere else on Enceladus thus far. One of these areas of "blue" ice in the south polar region was observed at high resolution during the July 14 flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.
The boundary of the south polar region is marked by a pattern of parallel, Y- and V-shaped ridges and valleys. The shape, orientation, and location of these features suggest they are caused by changes in the overall shape of Enceladus. As of 2006 there were two theories for what could cause such a shift in shape: the orbit of Enceladus may have migrated inward, leading to an increase in Enceladus's rotation rate. Such a shift would lead to a more oblate shape; or a rising mass of warm, low-density material in Enceladus's interior may have led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole. Consequently, the moon's ellipsoid shape would have adjusted to match the new orientation. One problem of the polar flattening hypothesis is that both polar regions should have similar tectonic deformation histories. However, the north polar region is densely cratered, and has a much older surface age than the south pole. Thickness variations in Enceladus's lithosphere is one explanation for this discrepancy. Variations in lithospheric thickness are supported by the correlation between the Y-shaped discontinuities and the V-shaped cusps along the south polar terrain margin and the relative surface age of the adjacent non-south polar terrain regions. The Y-shaped discontinuities, and the north-south trending tension fractures into which they lead, are correlated with younger terrain with presumably thinner lithospheres. The V-shaped cusps are adjacent to older, more heavily cratered terrains.
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.
Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice. 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. 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. 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. 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. 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.
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. 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). Gravity measurements by Cassini show that the density of the core is low, indicating that the core contains water in addition to silicates.
Subsurface water ocean
Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from its south polar surface, with jets moving 250 kg of water vapor every second at up to 2,189 km/h (1,360 mph) into space. Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring. 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. The plumes' "salty" composition indicates that the source is a salty subsurface ocean. Cassini also found traces of simple organic compounds in some dust grains.
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. The 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.
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. The amount of libration (0.120° ± 0.014°) implies that this global ocean is about 26 to 31 kilometers deep. For comparison, Earth's ocean has an average depth of 3.7 kilometers.
A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO3) has an alkaline pH of 11 to 12. 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 plumes.
South polar plumes
Following Voyager's encounters with Enceladus in the early 1980s, scientists postulated that it may be geologically active based on its young, reflective surface and location near the core of the E ring. Based on the connection between Enceladus and the E ring, scientists suspected that Enceladus was the source of material in the E ring, perhaps through venting of water vapor. Readings from Cassini's 2005 passage suggested that cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, had been discovered on Enceladus. The first Cassini sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005, though the possibility of a camera artifact delayed an official announcement. Data from the magnetometer instrument during the February 17, 2005, encounter provided a hint that the feature might be real when it found evidence for a planetary atmosphere. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the waves' frequency can be used to identify their composition, in this case ionized water vapor. During the two following encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower. The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.
Fortuitously, Cassini flew through this gas cloud during the July 14 encounter, allowing instruments such as the ion and neutral mass spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as traces of molecular nitrogen, methane, and carbon dioxide. The CDA "detected a large increase in the number of particles near Enceladus", confirming Enceladus as the primary source for the E ring. Analysis of the CDA and INMS data suggest that the gas cloud Cassini flew through during the July encounter, and observed from a distance with its magnetometer and UVIS, was actually a water-rich cryovolcanic plume, originating from vents near the south pole.
Visual confirmation of venting came in November 2005, when ISS imaged geyser-like jets of icy particles rising from Enceladus's south polar region. (Although the plume was imaged before, in January and February 2005, additional studies of the camera's response at high phase angles, when the Sun is almost behind Enceladus, and comparison with equivalent high-phase-angle images taken of other Saturnian satellites, were required before this could be confirmed.) The November 2005 images showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface. The particles have a bulk velocity of 1.25 ±0.1 km/s. Cassini's UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.
Observations during a flyby on March 12, 2008, revealed additional chemicals in the plume, including trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde. The plumes' composition, as measured by the INMS, is similar to that seen at most comets.
The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers, similar to Earth's geysers,. The mechanism that drives and sustains the eruptions is thought to be tidal heating. The intensity of the eruption of the south polar jets varies significantly as a function of the position of Enceladus in its orbit. The plumes are about four times brighter when Enceladus is at apoapsis (the point in its orbit most distant from Saturn) than when it is at periapsis. This is consistent with geophysical calculations which predict the south polar fissures are under compression near periapsis, pushing them shut, and under tension near apoapsis, pulling them open.
Much of the plume activity consists of broad curtain-like eruptions. Optical illusions from a combination of viewing direction and local fracture geometry previously made the plumes look like discrete jets.
Possible heat sources
During the flyby of July 14, 2005, 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. The presence of a subsurface ocean under the south polar region is now accepted, 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.
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, decompression and dissociation of clathrates, and shear heating, 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, 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. A 2007 study predicted the internal heat of Enceladus, if generated by tidal forces, could be no greater than 1.1 gigawatts, 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, and suggest that it is in thermal equilibrium.
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. 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.
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. Tidal dissipation of Enceladus's ice crust is significant because Enceladus has a subsurface ocean. Some 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. A more recent analysis claimed that "a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that eruptions are maintained over geological timescales."  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. The surface of Enceladus indicates that the entire moon has experienced periods of enhanced heat flux in the past.
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. The present-day radiogenic heating rate is 3.2 × 1015 ergs/s (or 0.32 gigawatts), assuming Enceladus has a composition of ice, iron and silicate materials. 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. 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.
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. Reducing the freezing point of water with ammonia would also allow for outgassing and higher gas pressure, and less heat required to power the water plumes. 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.
Shape and size
Enceladus is a relatively small satellite composed of ice and rock. 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. 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).
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 , 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 qtid 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). 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).
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.
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.
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. 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.
Date of formation
In 2016, a study of how the orbits of Saturn's moons should 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.
Enceladus ejects plumes of salt water that are laced with grains of silica-rich sand, nitrogen (in ammonia), and organic molecules, including trace amounts of simple hydrocarbons such as methane (CH
4), propane (C
8), acetylene (C
2) and formaldehyde (CH
2O), which are carbon-bearing molecules. This indicates that hydrothermal activity—an energy source—may be at work in Enceladus's subsurface ocean. In addition, models indicate the large rocky core is porous, allowing water to flow through it to pick up heat.
Molecular hydrogen (H
2), a geochemical source of energy that can be metabolized by methanogen microbes to provide energy for life, could be present if, as models suggest, Enceladus's salty ocean has an alkaline pH from serpentinization of chondritic rock.
The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns, with an energy source and simple organic compounds in contact with Enceladus's rocky core, may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life. 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.
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. Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface. 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.
Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing higher-resolution images to be obtained. These images showed a young surface. 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.
|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. 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. 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. Cassini performed a flyby on October 28, 2015, passing as close as 49 km (30 mi) and through a plume. 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.
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. By contrast, the water thought to be on Jupiter's moon Europa is located under a much thicker layer of ice.
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, a probe flyby (Journey to Enceladus and Titan or JET) to analyze plume contents in-situ, a lander by the German Aerospace Center to study the habitability potential of its subsurface ocean (Enceladus Explorer), and two astrobiology-oriented mission concepts (the Enceladus Life Finder and Life Investigation For Enceladus). 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).
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, 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
- 2008 NASA/ESA: Titan Saturn System Mission (TSSM)
- 2010 PSDS Decadal Survey: Enceladus Studies including the Enceladus Orbiter
- 2010 JPL: Journey to Enceladus and Titan (JET)
- 2012 DLR: Enceladus Explorer (EnEx) lander, employing the IceMole
- 2012 JPL: Life Investigation For Enceladus (LIFE)
- 2015 JPL: Enceladus Life Finder (ELF)
Notes and references
- 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.
- "Planetary Body Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology Science Center. Retrieved January 12, 2015.
- Redd, Nola Taylor (April 5, 2013). "Enceladus: Saturn's Tiny, Shiny Moon". Space.com. Retrieved April 27, 2014.
- "Solar System Exploration – Enceladus: Facts & Figures". NASA. August 12, 2013. Archived from the original on October 16, 2013. Retrieved April 26, 2014.
- Porco, C. C.; Helfenstein, P.; Thomas, P. C.; Ingersoll, A. P.; Wisdom, J.; West, R.; Neukum, G.; Denk, T.; Wagner, R. (March 10, 2006). "Cassini Observes the Active South Pole of Enceladus". Science. 311 (5766): 1393–1401. Bibcode:2006Sci...311.1393P. doi:10.1126/science.1123013. PMID 16527964.
- Roatsch, T.; Jaumann, R.; Stephan, K.; Thomas, P. C. (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763–781. doi:10.1007/978-1-4020-9217-6_24. ISBN 978-1-4020-9216-9.
- Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; Ionasescu, R.; Jones, J. B.; Mackenzie, R. A.; Meek, M. C.; Parcher, D.; Pelletier, F. J.; Owen, Jr., W. M.; Roth, D. C.; Roundhill, I. M.; Stauch, J. R. (December 2006). "The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data". The Astronomical Journal. 132 (6): 2520–2526. Bibcode:2006AJ....132.2520J. doi:10.1086/508812.
- Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (February 9, 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. doi:10.1126/science.1134681. PMID 17289992. Retrieved December 20, 2011. (supporting online material, table S1)
- Howett C. J. A., Spencer J. R., Pearl J., Segura, M. (2010). "Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements". Icarus. 206 (2): 573–593. Bibcode:2010Icar..206..573H. doi:10.1016/j.icarus.2009.07.016.
- Spencer, J. R.; Pearl, J. C.; et al. (2006). "Cassini Encounters Enceladus: Background and the Discovery of a South Polar Hot Spot". Science. 311 (5766): 1401–5. Bibcode:2006Sci...311.1401S. doi:10.1126/science.1121661. PMID 16527965.
- Observatorio ARVAL (April 15, 2007). "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved December 17, 2011.
- Waite, J. H.; Combi, M. R.; et al. (2006). "Cassini Ion and Neutral Mass Spectrometer: Enceladus Plume Composition and Structure". Science. 311 (5766): 1419–22. Bibcode:2006Sci...311.1419W. doi:10.1126/science.1121290. PMID 16527970.
- Dougherty, M. K.; Khurana, K. K.; et al. (2006). "Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer". Science. 311 (5766): 1406–9. Bibcode:2006Sci...311.1406D. doi:10.1126/science.1120985. PMID 16527966.
- Hansen, Candice J.; Esposito, L.; et al. (2006). "Enceladus' Water Vapor Plume". Science. 311 (5766): 1422–5. Bibcode:2006Sci...311.1422H. doi:10.1126/science.1121254. PMID 16527971.
- Herschel, W. (January 1, 1790). "Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; With Remarks on the Construction of Its Ring, Its Atmosphere, Its Rotation on an Axis, and Its Spheroidal Figure". Philosophical Transactions of the Royal Society of London. 80: 1–20. doi:10.1098/rstl.1790.0004.
- Herschel, W. (1795). "Description of a Forty-feet Reflecting Telescope". Philosophical Transactions of the Royal Society of London. 85: 347–409. Bibcode:1795RSPT...85..347H. doi:10.1098/rstl.1795.0021. (reported by Arago, M. (1871). "Herschel". Annual Report of the Board of Regents of the Smithsonian Institution. pp. 198–223.)
- Lovett, Richard A. (September 4, 2012). "Secret life of Saturn's moon: Enceladus". Cosmos Magazine. Retrieved August 29, 2013.
- Chang, Kenneth (March 12, 2015). "Suddenly, It Seems, Water Is Everywhere in Solar System". New York Times. Retrieved March 13, 2015.
- Spencer, J. R.; Nimmo, F. (May 2013). "Enceladus: An Active Ice World in the Saturn System". Annual Review of Earth and Planetary Sciences. 41: 693–717. Bibcode:2013AREPS..41..693S. doi:10.1146/annurev-earth-050212-124025.
- Dyches, Preston; Brown, Dwayne; et al. (July 28, 2014). "Cassini Spacecraft Reveals 101 Geysers and More on Icy Saturn Moon". NASA. Retrieved July 29, 2014.
- "Icy Tendrils Reaching into Saturn Ring Traced to Their Source". NASA News. April 14, 2015. Retrieved April 15, 2015.
- "Ghostly Fingers of Enceladus". NASA/JPL/Space Science Institute. NASA. September 19, 2006. Retrieved April 26, 2014.
- Battersby, Stephen (March 26, 2008). "Saturn's moon Enceladus surprisingly comet-like". New Scientist. Retrieved April 16, 2015.
- Platt, Jane; Bell, Brian (April 3, 2014). "NASA Space Assets Detect Ocean inside Saturn Moon". NASA. Retrieved April 3, 2014.
- Witze, A. (April 3, 2014). "Icy Enceladus hides a watery ocean". Nature. doi:10.1038/nature.2014.14985.
- Iess, L.; Stevenson, D. J.; et al. (April 4, 2014). "The Gravity Field and Interior Structure of Enceladus". Science. 344 (6179): 78–80. Bibcode:2014Sci...344...78I. doi:10.1126/science.1250551. Retrieved April 3, 2014.
- Frommert, H.; Kronberg, C. "William Herschel (1738–1822)". Retrieved March 11, 2015.
- As reported by Lassell, William (January 14, 1848). "Names". Monthly Notices of the Royal Astronomical Society. 8 (3): 42–3. Bibcode:1848MNRAS...8...42L. doi:10.1093/mnras/8.3.42.
- "Categories for Naming Features on Planets and Satellites". Gazetteer of Planetary Nomenclature. USGS Astrogeology Science Center. Retrieved January 12, 2015.
- "Nomenclature Search Results: Enceladus". Gazetteer of Planetary Nomenclature. USGS Astrogeology Science Center. Retrieved January 13, 2015.
- "Samaria Rupes". Gazetteer of Planetary Nomenclature. USGS Astrogeology Research Program.
- Hillier, J. K.; Green, S. F.; et al. (June 2007). "The composition of Saturn's E ring". Monthly Notices of the Royal Astronomical Society. 377 (4): 1588–96. Bibcode:2007MNRAS.377.1588H. doi:10.1111/j.1365-2966.2007.11710.x.
- Hurford, Terry; Bruce, B. "Implications of Spin-orbit Librations on Enceladus". American Astronomical Society, DPS meeting #40, #8.06. NASA. Retrieved April 27, 2014.
- Hedman, M. M.; Burns, J. A.; et al. (2012). "The three-dimensional structure of Saturn's E ring". Icarus. 217: 322–338. arXiv: . Bibcode:2012Icar..217..322H. doi:10.1016/j.icarus.2011.11.006.
- Vittorio, Salvatore A. (July 2006). "Cassini visits Enceladus: New light on a bright world". Cambridge Scientific Abstracts (CSA). CSA. Retrieved April 27, 2014.
- Baum, W. A.; Kreidl, T. (July 1981). "Saturn's E ring: I. CCD observations of March 1980". Icarus. 47: 84–96. Bibcode:1981Icar...47...84B. doi:10.1016/0019-1035(81)90093-2. Retrieved April 27, 2014.
- Haff, P. K.; Eviatar, A.; et al. (1983). "Ring and plasma: Enigmae of Enceladus". Icarus. 56: 426–438. Bibcode:1983Icar...56..426H. doi:10.1016/0019-1035(83)90164-1.
- Pang, Kevin D.; Voge, Charles C.; et al. (1984). "The E ring of Saturn and satellite Enceladus". Journal of Geophysical Research. 89: 9459. Bibcode:1984JGR....89.9459P. doi:10.1029/JB089iB11p09459.
- Blondel, Philippe; Mason, John (August 23, 2006). Solar System Update. Berlin Heidelberg: Springer Science. pp. 241–3. doi:10.1007/3-540-37683-6. ISBN 978-3-540-37683-5.
- Spahn, F.; Schmidt, J.; et al. (2006). "Cassini Dust Measurements at Enceladus and Implications for the Origin of the E ring". Science. 311 (5766): 1416–18. Bibcode:2006Sci...311.1416S. doi:10.1126/science.1121375. PMID 16527969.
- Cain, Fraser (February 5, 2008). "Enceladus is Supplying Ice to Saturn's A-Ring". NASA. Universe Today. Retrieved April 26, 2014.
- Rothery, David A. (1999). Satellites of the Outer Planets: Worlds in their own right. Oxford University Press. ISBN 978-0-19-512555-9.
- Steigerwald, Bill (May 16, 2007). "Cracks on Enceladus Open and Close under Saturn's Pull". NASA.
- "Satun Moons – Enceladus". Cassini Solstice Mission Team. JPL/NASA. Retrieved April 26, 2014.
- Rathbun, J. A.; Turtle, E. P.; et al. (2005). "Enceladus' global geology as seen by Cassini ISS". Eos Trans. AGU. 82 (52 (Fall Meeting Supplement), abstract P32A–03). Bibcode:2005AGUFM.P32A..03R.
- Smith, B. A.; Soderblom, L.; et al. (1982). "A New Look at the Saturn System: The Voyager 2 Images". Science. 215 (4532): 504–37. Bibcode:1982Sci...215..504S. doi:10.1126/science.215.4532.504. PMID 17771273.
- Turtle, E. P.; et al. (April 28, 2005). "Enceladus, Curiouser and Curiouser: Observations by Cassini's Imaging Science Subsystem" (PDF). Cassini CHARM Teleconference. JPL/NASA. Archived from the original (PDF) on February 1, 2010.
- "Shahrazad (Se-4)". PIA12783: The Enceladus Atlas. NASA/Cassini Imaging Team. Retrieved February 4, 2012.
- Helfenstein, P.; Thomas, P. C.; et al. Patterns of fracture and tectonic convergence near the south pole of Enceladus (PDF). Lunar and Planetary Science XXXVII (2006).
- Barnash, A. N.; et al. (2006). Interactions Between Impact Craters and Tectonic Fractures on Enceladus. Bulletin of the American Astronomical Society. 38 (3, presentation no. 24.06). p. 522.
- Nimmo, F.; Pappalardo, R. T. (2006). "Diapir-induced reorientation of Saturn's moon Enceladus". Nature. 441 (7093): 614–16. Bibcode:2006Natur.441..614N. doi:10.1038/nature04821. PMID 16738654.
- "Enceladus in False Color". Cassini Imaging. July 26, 2005. Retrieved March 22, 2006.
- "Cassini Finds Enceladus Tiger Stripes Are Really Cubs". NASA.gov. NASA. August 30, 2005. Retrieved April 3, 2014.
- Brown, R. H.; Clark, R. N.; et al. (2006). "Composition and Physical Properties of Enceladus' Surface". Science. 311 (5766): 1425–28. Bibcode:2006Sci...311.1425B. doi:10.1126/science.1121031. PMID 16527972.
- "Boulder-Strewn Surface". Cassini Imaging. July 26, 2005. Retrieved March 26, 2006.
- "Icy moon Enceladus has underground sea". ESA. April 3, 2014. Retrieved April 30, 2014.
- Tajeddine, R.; Lainey, V.; et al. (October 2012). Mimas and Enceladus: Formation and interior structure from astrometric reduction of Cassini images. American Astronomical Society, DPS meeting #44, #112.03. Bibcode:2012DPS....4411203T.
- Castillo, J. C.; Matson, D. L.; et al. (2005). "26Al in the Saturnian System – New Interior Models for the Saturnian satellites". Eos Transactions AGU. 82 (52 (Fall Meeting Supplement), abstract P32A–01): 1. Bibcode:2005AGUFM.P32A..01C.
- Castillo, J. C.; Matson, D. L.; et al. (2006). A New Understanding of the Internal Evolution of Saturnian Icy Satellites from Cassini Observations (PDF). 37th Annual Lunar and Planetary Science Conference, Abstract 2200.
- Schubert, G.; Anderson, J.; et al. (2007). "Enceladus: Present internal structure and differentiation by early and long-term radiogenic heating". Icarus. 188 (2): 345–55. Bibcode:2007Icar..188..345S. doi:10.1016/j.icarus.2006.12.012.
- Matson, D. L.; et al. (2006). "Enceladus' Interior and Geysers – Possibility for Hydrothermal Geometry and N2 Production" (PDF). 37th Annual Lunar and Planetary Science Conference, abstract. p. 2219.
- Taubner R. S.; Leitner J. J.; Firneis M. G.; Hitzenberg, R. (April 2014), "Including Cassini gravity measurements from the flyby E9, E12, E19 into interior structure models of Enceladus. Presented at EPSC 2014-676", European Planetary Science Congress 2014, 9: EPSC2014–676, Bibcode:2014EPSC....9..676T
- Hsu, Hsiang-Wen; Postberg, Frank; et al. (March 11, 2015). "Ongoing hydrothermal activities within Enceladus". Nature. 519 (7542): 207–10. Bibcode:2015Natur.519..207H. doi:10.1038/nature14262.
- "Enceladus rains water onto Saturn". European Space Agency. 2011. Retrieved January 14, 2015.
- "Astronomers find hints of water on Saturn moon". News9.com. The Associated Press. November 27, 2008. Retrieved September 15, 2011.
- Postberg, F.; Schmidt, J.; et al. (2011). "A salt-water reservoir as the source of a compositionally stratified plume on Enceladus". Nature. 474 (7353): 620–2. Bibcode:2011Natur.474..620P. doi:10.1038/nature10175. PMID 21697830.
- "Ocean Hidden Inside Saturn's Moon". Space.com. June 24, 2009. Retrieved January 14, 2015.
- "Cassini samples the icy spray of Enceladus' water plumes". ESA. 2011.
- Amos, Jonathan (April 3, 2014). "Saturn's Enceladus moon hides 'great lake' of water". BBC News. Retrieved April 7, 2014.
- Sample, Ian (April 3, 2014). "Ocean discovered on Enceladus may be best place to look for alien life". The Guardian. Retrieved April 3, 2014.
- "Cassini finds global ocean in Saturn's moon Enceladus". Astronomy. Retrieved September 15, 2015.
- Thomas, P. C.; Tajeddine, R.; et al. "Enceladus's measured physical libration requires a global subsurface ocean". Icarus. 264: 37–47. arXiv: . Bibcode:2016Icar..264...37T. doi:10.1016/j.icarus.2015.08.037.
- "Cassini Finds Global Ocean in Saturn's Moon Enceladus". Retrieved September 17, 2015.
- Billings, Lee. "Cassini Confirms a Global Ocean on Saturn's Moon Enceladus". Retrieved September 17, 2015.
- "Under Saturnian moon's icy crust lies a 'global' ocean | Cornell Chronicle". Cornell University. Retrieved September 17, 2015.
- R. Glein, Christopher; Baross, John A.; et al. (April 16, 2015). "The pH of Enceladus' ocean". Geochimica et Cosmochimica Acta. 162: 202–19. arXiv: . Bibcode:2015GeCoA.162..202G. doi:10.1016/j.gca.2015.04.017. Retrieved May 8, 2015.
- Glein, C. R.; Baross, J. A.; et al. (March 26, 2015). The chemistry of Enceladus' ocean from a convergence of Cassini data and theoretical geochemistry (PDF). 46th Lunar and Planetary Science Conference 2015.
- Wall, Mike (May 7, 2015). "Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life". Space.com. Retrieved May 8, 2015.
- "NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon". JPL/NASA. December 5, 2005. Retrieved May 4, 2016.
- "Spray Above Enceladus". Cassini Imaging. Retrieved March 22, 2005.
- Perry, M. E.; Teolis, B. D.; Grimes, J.; et al. (March 21, 2016). Direct Measurement of the Velocity of the Enceladus Vapor Plumes (PDF). 47th Lunar and Planetary Science Conference. The Woodlands, Texas. p. 2846.
- Mosher, Dave (March 26, 2014). "Seeds of Life Found Near Saturn". Space.com. Retrieved April 9, 2014.
- "Cassini Tastes Organic Material at Saturn's Geyser Moon". NASA. March 26, 2008. Retrieved March 26, 2008.
- Kite, Edwin S.; Rubin, Allan M. (January 29, 2016). "Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes". Proceedings of the National Academy of Sciences of the United States of America. 113 (15): 3972–3975. doi:10.1073/pnas.1520507113.
- Spotts, P. (July 31, 2013). "What's going on inside Saturn moon? Geysers offer intriguing new clue". The Christian Science Monitor. Retrieved August 3, 2013.
- Lakdawalla, E. (March 11, 2013). "Enceladus huffs and puffs: plumes vary with orbital longitude". Planetary Society blogs. The Planetary Society. Retrieved January 26, 2014.
- Spencer, J. (July 31, 2013). "Solar system: Saturn's tides control Enceladus' plume". Nature. 500 (7461): 155–6. Bibcode:2013Natur.500..155S. doi:10.1038/nature12462. ISSN 0028-0836. PMID 23903653.
- Hedman, M. M.; Gosmeyer, C. M.; et al. (July 31, 2013). "An observed correlation between plume activity and tidal stresses on Enceladus". Nature. 500 (7461): 182–4. Bibcode:2013Natur.500..182H. doi:10.1038/nature12371. ISSN 0028-0836. PMID 23903658.
- Spitale, Joseph N.; Hurford, Terry A.; et al. (May 7, 2015). "Curtain eruptions from Enceladus' south-polar terrain". Nature. 521 (7550): 57–60. Bibcode:2015Natur.521...57S. doi:10.1038/nature14368. ISSN 0028-0836. Retrieved May 8, 2015.
- Charles Q. Choi (May 6, 2015). "'Jets' on Saturn Moon Enceladus May Actually Be Giant Walls of Vapor and Ice". Space.com. Retrieved May 8, 2015.
- "Long 'curtains' of material may be shooting off Saturn's moon Enceladus". Los Angeles Times. ISSN 0458-3035. Retrieved May 8, 2015.
- Showman, Adam P.; Han, Lijie; et al. (November 2013). "The effect of an asymmetric core on convection in Enceladus' ice shell: Implications for south polar tectonics and heat flux". Geophysical Research Letters. 40 (21): 5610–14. Bibcode:2013GeoRL..40.5610S. doi:10.1002/2013GL057149. Retrieved December 17, 2014.
- Kamata, S.; Nimmo, F. (March 21, 2016). INTERIOR THERMAL STATE OF ENCELADUS INFERRED FROM THE VISCOELASTIC STATE OF ITS ICY SHELL (PDF). 47th Lunar and Planetary Science Conference. Lunar and Planetary Institute.
- Howell, Robert R.; Goguen, J. D.; et al. (2013). "Enceladus Near-Fissure Surface Temperatures". American Astronomical Society. 45. Bibcode:2013DPS....4541601H.
- Abramov, O.; Spencer, J. R. (March 17–21, 2014). New Models of Endogenic Heat from Enceladus' South Polar Fractures (PDF). 45th Lunar and Planetary Science Conference 2014. LPSC.
- "A Hot Start on Enceladus". Astrobio.net. March 14, 2007. Retrieved March 21, 2010.
- "Cassini Finds Enceladus is a Powerhouse". NASA. March 7, 2011. Retrieved April 7, 2014.
- Shoji, D.; Hussmann, H.; et al. (March 14, 2014). "Non-steady state tidal heating of Enceladus". Icarus. 235: 75–85. Bibcode:2014Icar..235...75S. doi:10.1016/j.icarus.2014.03.006. Retrieved April 7, 2014.
- Spencer, John R.; Nimmo, Francis (May 2013). "Enceladus: An Active Ice World in the Saturn System". Annual Review of Earth and Planetary Sciences. 41: 693–717. Bibcode:2013AREPS..41..693S. doi:10.1146/annurev-earth-050212-124025. Retrieved April 7, 2014.
- Běhounková, Marie; Tobie, Gabriel; et al. (September–October 2013). "Impact of tidal heating on the onset of convection in Enceladus's ice shell". Icarus. 226 (1): 898–904. Bibcode:2013Icar..226..898B. doi:10.1016/j.icarus.2013.06.033. Retrieved April 7, 2014.
- Spencer, J. R. (2013). Enceladus Heat Flow from High Spatial Resolution Thermal Emission Observations (PDF). European Planetary Science Congress 2013. EPSC Abstracts.
- Spitale, J. N.; Porco, Carolyn C. (2007). "Association of the jets of Enceladus with the warmest regions on its south-polar fractures". Nature. 449 (7163): 695–7. Bibcode:2007Natur.449..695S. doi:10.1038/nature06217. PMID 17928854.
- Meyer, J.; Wisdom, Jack (2007). "Tidal heating in Enceladus". Icarus. 188 (2): 535–9. Bibcode:2007Icar..188..535M. doi:10.1016/j.icarus.2007.03.001.
- Roberts, J. H.; Nimmo, Francis (2008). "Tidal heating and the long-term stability of a subsurface ocean on Enceladus". Icarus. 194 (2): 675–689. Bibcode:2008Icar..194..675R. doi:10.1016/j.icarus.2007.11.010.
- Kite, Edwin S.; Rubin, Allan M. (January 29, 2016). "Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes". Proceedings of the National Academy of Sciences of the United States of America. 113 (15): 3972–3975. doi:10.1073/pnas.1520507113.
- Bland, M. T.; Singer, Kelsi N.; et al. (2012). "Enceladus' extreme heat flux as revealed by its relaxed craters". Geophysical Research Letters. 39 (17). Bibcode:2012GeoRL..3917204B. doi:10.1029/2012GL052736.
- JPL (July 22, 2009). "Saturnian Moon Shows Evidence of Ammonia". NASA. Retrieved March 21, 2010.
- Waite Jr., J. H.; Lewis, W. S.; et al. (July 23, 2009). "Liquid water on Enceladus from observations of ammonia and 40Ar in the plume". Nature. 460: 487–490. Bibcode:2009Natur.460..487W. doi:10.1038/nature08153. Retrieved November 8, 2014.
- Fortes, A. D. (2007). "Metasomatic clathrate xenoliths as a possible source for the south polar plumes of Enceladus". Icarus. 191 (2): 743–8. Bibcode:2007Icar..191..743F. doi:10.1016/j.icarus.2007.06.013.
- Shin, Kyuchul; Kumar, Rajnish; et al. (September 11, 2012). "Ammonia clathrate hydrates as new solid phases for Titan, Enceladus, and other planetary systems". Proceedings of the National Academy of Sciences of the USA. 109 (37): 14785–90. Bibcode:2012PNAS..10914785S. doi:10.1073/pnas.1205820109. Retrieved April 8, 2014.
- "A Hot Start Might Explain Geysers on Enceladus". NASA/Jet Propulsion Laboratory. March 12, 2007. Retrieved January 12, 2015.
- "Saturnian Satellite Fact Sheet". NASA. October 13, 2015. Retrieved July 15, 2016.
- Thomas, P. C.; Burns, J. A.; et al. (2007). "Shapes of the saturnian icy satellites and their significance". Icarus. 190 (2): 573–584. Bibcode:2007Icar..190..573T. doi:10.1016/j.icarus.2007.03.012.
- Czechowski, Leszek (2006). "Parameterized model of convection driven by tidal and radiogenic heating". Advances in Space Research. 38 (4): 788–93. Bibcode:2006AdSpR..38..788C. doi:10.1016/j.asr.2005.12.013.
- Lainey, Valery; Karatekin, Ozgur; et al. (May 22, 2012). "Strong tidal dissipation in Saturn and constraints on Enceladus' thermal state from astrometry". The Astrophysical Journal. 752 (1): 14. arXiv: . Bibcode:2012ApJ...752...14L. doi:10.1088/0004-637X/752/1/14. Retrieved April 28, 2014.
- Cowen, Ron (April 15, 2006). "The Whole Enceladus: A new place to search for life in the outer solar system". Science News. 169 (15): 282. Retrieved April 8, 2014.
- Czechowski, L. (December 2014). "Some remarks on the early evolution of Enceladus". Planetary and Space Science. 104: 185–99. Bibcode:2014P&SS..104..185C. doi:10.1016/j.pss.2014.09.010.
- Moons of Saturn may be younger than the dinosaurs | Astronomy.com
- Tobie, Gabriel (March 12, 2015). "Planetary science: Enceladus' hot springs". Nature. 519: 162–3. Bibcode:2015Natur.519..162T. doi:10.1038/519162a. PMID 25762276.
- McKay, Christopher P.; Anbar, Ariel D.; et al. (April 15, 2014). "Follow the Plume: The Habitability of Enceladus". Astrobiology. 14 (4): 352–355. Bibcode:2014AsBio..14..352M. doi:10.1089/ast.2014.1158. PMID 24684187. Retrieved May 4, 2014.
- Wall, Mike (May 7, 2015). "Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life". Space.com. Retrieved August 15, 2015.
- O' Neill, Ian (March 12, 2015). "Enceladus Has Potentially Life-Giving Hydrothermal Activity". Discovery News. Retrieved August 15, 2015.
- Spotts, Peter (September 16, 2015). "Proposed NASA mission to Saturn moon: If there's life, we'll find it". The Christian Science Monitor. Retrieved September 27, 2015.
- Taubner, R.-S.; Leitner, J. J.; Firneis, M. G.; Hitzenberger, R. (September 7, 2014). Including Cassini’s Gravity Measurements from the Flybys E9, E12, E19 into Interior Structure Models of Enceladus (PDF). European Planetary Science Congress 2014. EPSC Abstracts.
- "A Perspective on Life on Enceladus: A World of Possibilities". NASA. March 26, 2008. Retrieved September 15, 2011.
- McKie, Robin (July 29, 2012). "Enceladus: home of alien lifeforms?". The Guardian. Retrieved August 16, 2015.
- Coates, Andrew (March 12, 2015). "Warm Oceans on Saturn's Moon Enceladus Could Harbor Life". Discover Magazine. Retrieved August 15, 2015.
- Habitability of Enceladus: Planetary Conditions for Life. (PDF) Christopher D. Parkinson, Mao-Chang Liang, Yuk L. Yung, and Joseph L. Kirschivnk. Origins of Life and Evolution of Biospheres April 10, 2008. doi:10.1007/s11084-008-9135-4
- "Voyager Mission Description". SETI. February 19, 1997. Retrieved May 29, 2006.
- Terrile, R. J.; Cook, A. F. (1981). "Enceladus: Evolution and Possible Relationship to Saturn's E-ring". 12th Annual Lunar and Planetary Science Conference, Abstract. p. 428.
- Dyches, Preston; Brown, Dwayne; Cantillo, Laurie (October 30, 2015). "Saturn's Geyser Moon Shines in Close Flyby Views". NASA. Retrieved October 31, 2015.
- Dyches, Preston (December 21, 2015). "Cassini Completes Final Close Enceladus Flyby". NASA. Retrieved December 22, 2015.
- "Enceladus". Cassini Solstice Mission, NASA JPL. Retrieved January 14, 2015.
- "Cassini's Tour of the Saturn System". Planetary Society. Retrieved March 11, 2015.
- Moomaw, B. (February 5, 2007). "Tour de Saturn Set For Extended Play". Spacedaily. Retrieved February 5, 2007.
- "Deepest-Ever Dive Through Enceladus Plume Completed". Jet Propulsion Laboratory. October 28, 2015. Retrieved October 29, 2015.
- Tsou, P.; Brownlee, D. E.; et al. (June 18–20, 2013). Low Cost Enceladus Sample Return Mission Concept (PDF). Low Cost Planetary Missions Conference (LCPM) # 10.
- "Cassini Images of Enceladus Suggest Geysers Erupt Liquid Water at the Moon's South Pole". Cassini Imaging. Retrieved March 22, 2006.
- "Signs of Europa Plumes Remain Elusive in Search of Cassini Data". NASA. December 17, 2014. Retrieved January 12, 2015.
- Sotin, C.; Altwegg, K.; et al. (2011). JET: Journey to Enceladus and Titan (PDF). 42nd Lunar and Planetary Science Conference. Lunar and Planetary Institute.
- "Cost Capped Titan-Enceladus Proposal". Future Planetary Exploration. March 21, 2011. Retrieved April 9, 2014.
- Konstantinidis, Konstantinos; Flores Martinez, Claudio L.; Dachwald, Bernd; Ohndorf, Andreas; Dykta, Paul (February 2015). "A lander mission to probe subglacial water on Saturn's moon Enceladus for life". Acta Astronautica. 106: 63–89. Bibcode:2015AcAau.106...63K. doi:10.1016/j.actaastro.2014.09.012. Retrieved April 11, 2015.
- Anderson, Paul Scott (February 29, 2012). "Exciting New 'Enceladus Explorer' Mission Proposed to Search for Life". Universe Today. Retrieved April 9, 2014.
- "Searching for life in the depths of Enceladus". German Aerospace Center (DLR). February 22, 2012. Retrieved April 9, 2014.
- Lunine, J. I.; Waite, J. H.; Postberg, F.; Spilker, L. (2015). Enceladus Life Finder: The Search for Life in a Habitable Moon (PDF). 46th Lunar and Planetary Science Conference (2015). Houston, Texas.: Lunar and Planetary Institute.
- Clark, Stephen (April 6, 2015). "Diverse destinations considered for new interplanetary probe". Space Flight Now. Retrieved April 7, 2015.
- Wall, Mike (December 6, 2012). "Saturn Moon Enceladus Eyed for Sample-Return Mission". Space.com. Retrieved April 10, 2015.
- Tsou, Peter; Brownlee, D. E.; McKay, Christopher; Anbar, A. D.; Yano, H. (August 2012). "LIFE: Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life.". Astrobiology. 12 (8): 730–742. Bibcode:2012AsBio..12..730T. doi:10.1089/ast.2011.0813. PMID 22970863.
- "TandEM (Titan and Enceladus Mission) Workshop". European Space Agency. February 7, 2008. Retrieved March 2, 2008.
- Rincon, Paul (February 18, 2009). "Jupiter in space agencies' sights". Science & Environment. BBC News. Retrieved March 13, 2009.
- Adler, M.; Moeller, R. C.; et al. (March 5–12, 2011). Rapid Mission Architecture (RMA) study of possible missions to Saturn's moon Enceladus. Aerospace Conference. IEEE. doi:10.1109/AERO.2011.5747289. ISBN 978-1-4244-7350-2. ISSN 1095-323X.
- Spencer, John (May 2010). "Planetary Science Decadal Survey Enceladus Orbiter" (PDF). NASA. Retrieved 23 June 2016.
- Kane, Van (April 3, 2014). "Discovery Missions for an Icy Moon with Active Plumes". The Planetary Society. Retrieved April 9, 2015.
- Brabaw, Kasandra (April 7, 2015). "IceMole Drill Built to Explore Saturn's Icy Moon Enceladus Passes Glacier Test". Space.com. Retrieved April 9, 2015.
- Tsou, Peter; Anbar, Ariel; Atwegg, Kathrin; Porco, Carolyn; Baross, John; McKay, Christopher (2014). "LIFE – Enceladus Plume Sample Return via Discovery" (PDF). 45th Lunar and Planetary Science Conference. Retrieved April 10, 2015.
- Tsou, Peter (2013). "LIFE: Life Investigation For Enceladus – A Sample Return Mission Concept in Search for Evidence of Life.". Jet Propulsion Laboratory. Archived from the original (.doc) on September 1, 2015. Retrieved April 10, 2015.
- Enceladus Life Finder 2015, PDF.
- Enceladus Profile at NASA's Solar System Exploration site
- Calvin Hamilton's Enceladus page
- The Planetary Society: Enceladus blogs
- CHARM: Cassini–Huygens Analysis and Results from the Mission page, contains presentations on Enceladus results
- Paul Schenk's 3D images and flyover videos of Enceladus and other outer solar system satellites
- Habitability of Enceladus: Planetary Conditions for Life
- Cassini images of Enceladus
- Images of Enceladus at JPL's Planetary Photojournal
- Movie of Enceladus's 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