A Cassini mosaic of degraded craters, fractures, and disrupted terrain in Enceladus's north polar region. The two prominent craters above the middle terminator are Ali Baba (upper) and Aladdin. The Samarkand Sulci grooves run vertically to their left.
Discovery
Discovered by William Herschel
Discovery date August 28, 1789 [1]
Designations
Pronunciation en-SEL-ə-dəs
Saturn II[2]
Orbital characteristics
237948 km[3]
Eccentricity 0.0047[3][4]
1.370218 d[5]
Inclination 0.019° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions 513.2 × 502.8 × 496.6 km [3][6]
252.1±0.2 km[3][6] (0.0395 Earths, 0.1451 Moons)
Mass (1.08022±0.00101)×1020 kg[3][7] (1.8×10−5 Earths)
Mean density
1.609±0.005 g/cm³[3][6]
0.114 m/s² (0.0113 g)
0.239 km/s (860.4 km/h) [3]
synchronous
zero
Albedo 1.375±0.008 (geometric) or 0.99 (Bond) [8]
Surface temp. min mean max
Kelvin[10] 32.9 K 75 K 145 K
11.7 [9]
Atmosphere
Surface pressure

trace, significant spatial

variability [12][13]
Composition 91% water vapor
4% nitrogen
3.2% carbon dioxide
1.7% methane[11]

Enceladus is the sixth-largest moon of Saturn. It was discovered in 1789 by William Herschel,[14][15][16] but little was known about Enceladus until the two Voyager spacecraft passed near it in the early 1980s.[17] The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi),[3] about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all the sunlight that strikes it. Voyager 1 found that Enceladus orbits in the densest part of Saturn's diffuse E ring, indicating a possible association between the two. Voyager 2 revealed that, despite Enceladus's small size, it has a wide range of surfaces ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago.

In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered a water-rich plume venting from Enceladus's 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 approximately 200 kilograms (440 lb) per second.[17][18][19] Over 100 geysers have been identified.[20] Some of the water vapor falls back as "snow" and the rest escapes. This discovery revealed the mechanism by which Enceladus has released most of the material making up Saturn's E ring.[21]

These 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. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity. Enceladus is in such a resonance with Saturn's fourth largest moon, Dione; proximity to Saturn leads to tidal heating of Enceladus's interior, offering a possible explanation for the activity. In 2014, NASA reported that evidence for a large south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km had been found by Cassini.[22][23][24]

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.[25][26] By contrast, the water thought to be on Jupiter's moon Europa is locked under a very thick layer of surface ice, though recent evidence may show that Europa also erupts water plumes.[27]

Discovery and naming

Voyager 2 view (August 26, 1981) of Enceladus's Saturn-facing hemisphere, showing a young surface. The Samarkand Sulci grooves run vertically just right of center; Ali Baba (lower) and Aladdin craters are at 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.[16][28] Herschel first observed Enceladus in 1787, but in his smaller, 16.5 cm (6.5 in) telescope, the moon was not recognized.[29] Its faint apparent magnitude (+11.7m) and its proximity to much brighter Saturn and its rings make Enceladus difficult to observe from Earth with smaller telescopes.[2] Like many Saturnian satellites 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.

Enceladus is named after the giant Enceladus of Greek mythology.[14] The name Enceladus—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.[30] 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.[31] 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. 57 features have been officially named by the IAU; 22 features were named in 1982 based on the results of the Voyager flybys, and 35 features were officially named in November 2006 based on the results of Cassini's three flybys in 2005.[32]

Orbit

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

Enceladus is one of the major inner satellites of Saturn. It is the fourteenth satellite when ordered by distance from Saturn, and orbits within the densest part of the E Ring, the outermost of Saturn's rings.[33]

Enceladus orbits Saturn at a distance of 238,000 km from its center and 180,000 km from its cloud tops, between the orbits of Mimas and Tethys, requiring 32.9 hours to revolve once (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 of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides some of the heating sources for Enceladus's geologic activity.[4]

Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike the 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.[34]

Role as source of the 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. The E ring is distributed between the orbits of Mimas and Titan.[35]

Numerous mathematical models show that this ring is unstable, with a lifespan between 10,000 and 1,000,000 years, therefore, particles composing it must be constantly replenished.[36] Enceladus is orbiting inside this ring, in a place where it is narrowest but present in its highest density, raising suspicion since the 1980s that Enceladus is the main source of particles for the E ring.[37][38][39][40] This hypothesis was confirmed by Cassini's first two close flybys in during 2005.[41][42]

Physical characteristics

Enceladus is a relatively small satellite, with a mean diameter of 505 kilometers (314 mi),[3][6] only one-seventh the diameter of Earth's Moon. Its mass and diameter make Enceladus the sixth most massive and largest satellite of Saturn, after Titan (5150 km), Rhea (1530 km), Iapetus (1440 km), Dione (1120 km) and Tethys (1050 km).

Enceladus is a scalene ellipsoid in shape; its diameters, calculated from pictures 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 poles, and 497 km between the north and south poles.[4]

Atmosphere

The first Cassini flybys of Enceladus revealed that it has a significant atmosphere compared to the other moons of Saturn besides Titan. The source of the atmosphere may be volcanism, geysers, or gases escaping from the surface or the interior.[43][44] The atmosphere of Enceladus is composed of 91% water vapor, 4% nitrogen, 3.2% carbon dioxide, and 1.7% methane.[12][13]

Internal structure

Model of the interior of Enceladus from 2006. The inner, silicate core is represented in brown, whereas the outer, water-ice-rich mantle is represented in white. The yellow and red colors in the mantle and core respectively represent a proposed diapir under the south pole.[45]

Before the Cassini mission, relatively little was known about the interior of Enceladus. However, results from recent flybys of Enceladus by the Cassini spacecraft have provided information for models of Enceladus's interior. These include a better determination of the mass, shape, high-resolution observations of the surface, and new insights on Enceladus's interior.[46][47]

Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.[48] 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 and colleagues (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.[49] 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 Enceladus's small size.[50] 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.[51] Subsequent radioactive and tidal heating would raise the temperature of the core to 1000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have melted too, 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.[52][34]

In addition to its mass and modeled geochemistry, researchers have also examined Enceladus's shape to test whether it is differentiated or not. Porco et al. 2006 used limb measurements to determine that Enceladus's shape, assuming it is in 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).[51]

Surface features

Voyager 2, in August 1981, was the first spacecraft to observe the surface in detail. Examination of the resulting highest-resolution mosaic reveals 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.[48] In addition, extensive linear cracks[53] 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.[54] 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.[8] Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C, making it somewhat colder than other Saturnian satellites.[10]

Observations during three flybys by Cassini on February 17, March 9, and July 14 of 2005 revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. For example, the smooth plains observed by Voyager 2 resolved into relatively crater-free regions filled with numerous small ridges and scarps. In addition, numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.[55] Some areas show regions with 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 the interior of the moon may be liquid today, even though it should have been frozen eons ago.[54]

Global basemap of Enceladus from Cassini images (2011)
Northern hemisphere
Southern hemisphere

Impact craters

Craters degraded through viscous relaxation and fracturing, imaged by Cassini, February 17, 2005. The Hamah Sulci can be seen running from left to right along the bottom quarter of the image. Craters from Enceladus's ct2 and cp cratered units are visible above them.
Cassini false-color view of Enceladus's surface, showing several tectonic and crater degradation styles. The crater Dunyazad is at the top; the Misr Sulci run along the bottom.

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.[56] This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.[54]

Recent Cassini observations have provided a much closer look at the crater distribution and size. These high-resolution observations reveal that many of Enceladus's craters are heavily degraded through viscous relaxation and fracturing.[57] 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.[58]

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 17 February 2005

Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges.[48] Recent results from Cassini suggest that tectonism is the dominant deformation style on Enceladus. One of the more dramatic types of tectonic features found on Enceladus are rifts. These canyons can be up to 200 km long, 5–10 km wide, and 1 km deep. Such features appear geologically young, as they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.[59]

Another evidence of tectonism on Enceladus is grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions.[48] 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 much more complex. Rather than parallel sets of grooves, these lanes can often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands appear to bow upwards with fractures and ridges running the length of the feature. Cassini observations of the Samarkand Sulci have revealed intriguing dark spots (125 and 750 m wide), which appear to run parallel to narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.[57]

High-resolution mosaic of Enceladus's surface, showing several tectonic and crater degradation styles, such as narrow fractures only several hundred meters wide. The Misr (upper) and Al-Yaman sulci run horizontally at upper right.

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 appear to propagate down only a few hundred meters into the crust. Many appear to have been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture.[57][60] 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 appear to be among the youngest features on Enceladus. However, some linear grooves appear to be softened like the craters nearby, suggesting an older age. 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.[57] Given the level of resurfacing found on Enceladus, it is clear that tectonism has been an important driver of geology for much of its history.[59]

Smooth plains

The Samarkand Sulci (vertical, right of center), with the northwest portion of Sarandib Planitia to their right; Daryabar Fossa extends up and right from the image bottom

Two regions of smooth plains were observed by Voyager 2. These plains generally have low relief and have far fewer craters than in the cratered terrains, indicating a relatively young surface age.[56] 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 smooth plain regions to be filled with low-relief ridges and fractures. These features are currently interpreted as being caused by shear deformation.[57] The high-resolution images of Sarandib Planitia have 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][b]

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 the moon 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.[45]

South polar region

False-color mosaic of Enceladus taken by the Cassini–Huygens probe on July 14, 2005. It shows the south polar region, as demarcated by the circumpolar set of ridges and troughs in the bottom half of the mosaic

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.[4][61] 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 (SPT) are possibly as young as 500,000 years, or younger.[4] Near the center of this terrain are four fractures bounded on either side by ridges, unofficially called "tiger stripes". These fractures 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.[61] 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 1000 years old) or the surface ice has been thermally altered in the recent past.[62] VIMS also detected simple organic (carbon-containing) compounds in the tiger stripes, chemistry not found anywhere else on Enceladus thus far.[63] One of these areas of "blue" ice in the south polar region was observed at very 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.[64]

South pole terrain - close-up.
Composite map of Enceladus' south polar region (to 65° S latitude) cracks dubbed 'tiger stripes' where the geysers are located (2007)

Cryovolcanism

One possible scheme for Enceladus's cryovolcanism

Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring.[48] 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 from Enceladus's interior.[37][38]

Enceladus - South Pole - Geyser basin (August 10, 2014).[20]
Plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Enceladus. From right to left, the four major stripes, Damascus, Baghdad, Cairo, and Alexandria sulci are being backlit by the Sun.

Thanks to data from a number of instruments on the Cassini spacecraft in 2005, cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has 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,[4] though the possibility of the plume being a camera artifact stalled 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 an atmosphere at Enceladus. 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 frequency of the waves can be used to identify the composition, in this case ionized water vapor.[12] During the next two 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.[12] 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 for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.[13]

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 minor components like molecular nitrogen, methane, and carbon dioxide.[11] CDA "detected a large increase in the number of particles near Enceladus", confirming Enceladus as the primary source for the E ring.[41] 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.[65]

Visual confirmation of venting came in November 2005, when ISS (Imaging Science Subsystem) imaged geyser-like jets of icy particles rising from the moon's south polar region.[4][21] (As stated above, the plume was imaged before, in January and February 2005, but 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.[66]) The images taken in November 2005 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, thus making Enceladus the fourth body in the Solar System to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.[65] 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.

Additional observations were acquired during a flyby on March 12, 2008. Data on this flyby revealed additional chemicals in the plume, including trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.[67][68] The composition of Enceladus's plume as measured by the INMS instrument on Cassini is similar to that seen at most comets.[68]

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized subsurface chambers, similar to geysers on Earth.[4] 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.[69][70][71] This is consistent with geophysical calculations that predict that the south polar fissures will be under compression near periapsis, pushing them shut, and under tension near apoapsis, pulling them open.[72]

Internal water ocean

More evidence of an underground ocean of liquid water on Enceladus was reported on April 3, 2014 (artist's impression)[22][24]

Gravimetric data revealed the existence of a water ocean beneath the icy southern crust about the volume of Lake Superior in North America, or 245 times the water volume of Lake Garda in Italy.[73]

Evidence began to accumulate in 2005, when scientists observed water vapor spewing from Enceladus's south polar surface,[4][74] with jets moving 250 kg of water vapour every second[74] at up to 2,189 km/h (1,360 mph) into space.[75]

Eventually it was determined that in Saturn's E-ring, about 6% of particles contain 0.5–2% of sodium salts by mass, which is a significant amount. 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 onto the surface, whereas the fast "fresh" particles escape to the E-ring, explaining its salt-poor composition.[76] The "salty" composition of the plumes indicates that their source is a salty subsurface ocean.[21][77] Additionally, Cassini found traces of organic compounds in some dust grains.[76][78]

Using gravimetric data from Cassini flybys obtained from 2010–12, scientists were able to confirm that Enceladus likely has a liquid water ocean beneath its frozen surface. The conclusion was announced to the public on April 3, 2014 and published in the journal Science the following day.[22][23][24][73] The technique involved measuring tiny changes in the speed of Cassini as it passed through Enceladus's gravitational field. This causes a Doppler shift in the frequency of the data sent by the craft when received on Earth.[22] A smaller than expected effect was observed when Cassini flew over a depression at the south pole of Enceladus, suggesting a feature more dense than ice, but less dense than rock, was present below the moon's surface to weaken the expected topographic signal.[23] The most likely explanation for this anomaly is the presence of a body of liquid water, which would also explain previously observed water plumes. Calculations suggest that the top of the ocean lies beneath a 30 to 40 kilometres (19 to 25 mi) thick ice shelf. The ocean is about 10 kilometres (6.2 mi) deep.[22] It is not clear if the ocean exists only in the moon's south polar region, stretches to the equator, or into the northern hemisphere,[79] but it appears to be thickest in the south polar region.[24]

Heat sources

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

During the July 14, 2005 flyby, the Composite Infrared Spectrometer (CIRS) found a warm region near the south pole. Temperatures found in this region range from 85–90 K, to small areas with temperatures 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.[10] The presence of a subsurface ocean under the south polar region is now accepted, and it explains the why the thermal output is confined to that area,[80] but it cannot explain the source of the heat.

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,[81] decompression and dissociation of clathrates, and shear heating,[82] but identification of all the internal heat sources causing the observed thermal power output of Enceladus are still undetermined.

Heating in Enceladus has occurred through various mechanisms ever since its formation. Radioactive decay in its core may have initially heated it,[83] giving it a warm core and a subsurface ocean, which is now kept above freezing through an unknown mechanism. Geophysical models indicate that tidal heating is one of the main heat sources, perhaps aided by radioactive decay and some heat-producing chemical reactions.[84][85][86][87] A 2007 study predicted the internal heat of Enceladus, if generated by tidal forces, could be no greater than 1.1 gigawatts,[88] 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,[88] and suggest that it is in thermal equilibrium.[10][62][89]

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][84] 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.[90][91]

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.[34] 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 which 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.[91] 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.[92]

The "hot start" model of heating suggests Enceladus began as ice and rock that contained rapidly decaying radioactive isotopes of aluminum and iron. Enormous amounts of heat were then produced by the decay of those isotopes over a period of 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.[83] 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 natural radioactivity inside Enceladus would add 0.3 gigawatts to the observed heat flux.[84]

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), since pure water would require more energy to melt. In July 2009 it was announced that traces of ammonia had been discovered in the plumes during flybys in July and October 2008.[93] Reducing the freezing point of water with ammonia, would also allow for outgassing and higher gas pressure,[94] and less heat required to power the water plumes.[95] 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 not as much energy is required to produce the plume activity. However, the observed 4.7 gigawatts heat flux is enough to power the cryovolcanism without the aid of ammonia.[88][95]

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 $Q_{tid}=\frac{63\rho n^{5} r^{4} e^{2}}{38 u q }$, 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 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 it 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).[96] Additional historical information is needed to explain how Enceladus first entered the high-energy state (e.g., greater radiogenic heating or a more eccentric orbit in the past).[97]

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 paradox.[98]

Assessment of habitability

The Cassini mission has provided strong evidence that Enceladus has a liquid water ocean with an energy source, nitrogen (in ammonia),[99] nutrients and organic molecules, including trace amounts of simple hydrocarbons such as methane (CH
4
), propane (C
3
H
8
), acetylene (C
2
H
2
) and formaldehyde (CH
2
O
), which are carbon-bearing molecules.[67][68] The presence of an internal salty ocean with an energy source and simple organic compounds in contact with the moon's rocky core,[23][100] may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life.[17][22][26][73][79][98]

Exploration

Voyager missions

The two Voyager spacecraft obtained 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.[101] Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.[102] 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.[102]

Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing much higher-resolution images of this satellite.[101] These images showed a young surface.[48] 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

Animated 3D model of the spacecraft
Date
Distance (km)
February 17, 2005 1,264
March 9, 2005 500
July 14, 2005 175
December 24, 2005 94,000
March 12, 2008 52
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[104] 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 which took the probe to within 52 km of the moon's surface.[105] The extended mission for Cassini 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.[106]

Proposed mission concepts

The discoveries Cassini has made at Enceladus have prompted studies into follow-up missions, including a flyby plume sample return mission by NASA,[26][99] a probe flyby (Journey to Enceladus and Titan or JET) to analyze plume contents in-situ,[107] and a lander by the German Aerospace Center called Enceladus Explorer.[108][109] 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).[110]

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,[111] 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[112]
• 2008 NASA/ESA Titan Saturn System Mission (TSSM)
• 2010 JPL JET Discovery Proposal
• 2012 DLR Enceladus Explorer project

Notes and references

Explanatory notes

1. ^ Used with roughly equal frequency
2. ^ 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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