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Enceladus

January 31, 2023

For other uses, see Enceladus (disambiguation).

Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about 500 kilometers (310 miles) in diameter,[5] 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 (75.1 K; −324.4 °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 terrain.

Enceladus
View of trailing hemisphere in natural color[a]
Discovery
Discovered byWilliam Herschel
Discovery dateAugust 28, 1789[1]
Designations
Designation
Saturn II
Pronunciation/ɛnˈsɛlədəs/[2]
Named after
Ἐγκέλαδος Egkelados
AdjectivesEnceladean /ɛnsəˈleɪdiən/[3][4]
Orbital characteristics
237948 km[5]
Eccentricity0.0047[5][6]
1.370218 d[5]
Inclination0.009° (to Saturn's equator)[5]
Satellite ofSaturn
Physical characteristics
Dimensions513.2 × 502.8 × 496.6 km[5][7]
Mean radius
252.1±0.2 km[5][7] (0.0395 Earths, 0.1451 Moons)
Mass(1.08022±0.00101)×1020 kg[5][8] (1.8×10−5 Earths)
Mean density
1.609±0.005 g/cm3[5][7]
0.113 m/s2 (0.0113 g)
0.3305±0.0025[9]
0.239 km/s (860.4 km/h)[5]
Synchronous
0
Albedo1.375±0.008 (geometric at 550 nm)[10] or 0.81±0.04 (Bond)[11]
Surface temp. min mean max
Kelvin[12] 32.9 K 75 K 145 K
Celsius −240 °C −198 °C −128 °C
11.7[13]
Atmosphere
Surface pressure
Trace, significant spatial variability[15][16]
Composition by volume91% water vapor
4% nitrogen
3.2% carbon dioxide
1.7% methane[14]

Enceladus was discovered on August 28, 1789, by William Herschel,[1][17][18] but little was known about it until the two Voyager spacecraft, Voyager 1 and Voyager 2, flew by Saturn in 1980 and 1981.[19] In 2005, the spacecraft Cassini 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.[20] Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kilograms (440 pounds) per second.[16][19][21] More than 100 geysers have been identified.[22] Some of the water vapor falls back as "snow"; the rest escapes and supplies most of the material making up Saturn's E ring.[23][24] According to NASA scientists, the plumes are similar in composition to comets.[25] In 2014, NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi).[26][27][28] The existence of Enceladus' subsurface ocean has since been mathematically modelled and replicated.[29]

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 currently geologically active. 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 driving the geological activity.[30]

Cassini performed chemical analysis of Enceladus's plumes, finding evidence for hydrothermal activity,[31][32] possibly driving complex chemistry.[33] Ongoing research on Cassini data suggests that Enceladus's hydrothermal environment could be habitable to some of Earth's hydrothermal vent's microorganisms, and that plume-found methane could be produced by such organisms.[34][35]

History

Discovery

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

Enceladus was discovered by William Herschel on August 28, 1789, during the first use of his new 1.2 m (47 in) 40-foot telescope, then the largest in the world, at Observatory House in Slough, England.[18][36] 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.[37] Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel. Only its orbital characteristics were known, with estimations of its mass, density and albedo.

Naming

Enceladus is named after the giant Enceladus of Greek mythology.[1] The name, like the names of each of the first seven satellites of Saturn to be discovered, was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.[38] He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.

Geological features on Enceladus are named by the International Astronomical Union (IAU) after characters and places from Richard Francis Burton's 1885 translation of The Book of One Thousand and One Nights.[39] Impact craters are named after characters, whereas other feature types, such as fossae (long, narrow depressions), dorsa (ridges), planitiae (plains), sulci (long parallel grooves), and rupes (cliffs) are named after places. The IAU has officially named 85 features on Enceladus, most recently Samaria Rupes, formerly called Samaria Fossa.[40][41]

Shape and size

Enceladus is a relatively small satellite composed of ice and rock.[42] 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.[6] 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).[43][44]

Orbit and rotation

 
Enceladus's orbit (red) – Saturn's north pole view

Enceladus is one of the major inner satellites of Saturn along with Dione, Tethys, and Mimas. It orbits at 238,000 km (148,000 mi) from Saturn's center and 180,000 km (110,000 mi) 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.[6] 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.[45]

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 more than 1.5° about its spin axis. However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration.[6] This libration could have provided Enceladus with an additional heat source.[30][46][47]

Source of the E ring

Main article: Rings of Saturn § E Ring
 
Possible origins of methane found in plumes

Plumes from Enceladus, which are similar in composition to comets,[25] have been shown to be the source of the material in Saturn's E ring.[23] 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.[48]

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.[49] Enceladus is orbiting inside the ring, at its narrowest but highest density point. In the 1980s some suspected that Enceladus is the main source of particles for the ring.[50][51][52][53] This hypothesis was confirmed by Cassini's first two close flybys in 2005.[54][55]

The Cosmic Dust Analyzer (CDA) "detected a large increase in the number of particles near Enceladus", confirming it as the primary source for the E ring.[54] 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.[56]

Visual confirmation of venting came in November 2005, when ISS imaged geyser-like jets of icy particles rising from Enceladus's south polar region.[6][24] (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.[57])

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

    Enceladus orbiting within Saturn's E ring

  •  

    Enceladus geyser tendrils - comparison of images ("a";"c") with computer simulations

  •  

    Enceladus south polar region - locations of most active tendril-producing geysers

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

Geology

Surface features

See also: List of geological features on Enceladus
 
South polar view of the anti-Saturn hemisphere, with fractured areas in blue (false color)
 
Enceladus – tilted terminator – north is up

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.[58] In addition, extensive linear cracks[59] 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.[60] The fresh, clean ice that dominates its surface makes Enceladus the most reflective body in the Solar System, with a visual geometric albedo of 1.38[10] and bolometric Bond albedo of 0.81±0.04.[11] 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.[12]

Observations during three flybys on February 17, March 9, and July 14, 2005, revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. The smooth plains, which Voyager 2 had observed, resolved into relatively crater-free regions filled with numerous small ridges and scarps. Numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.[61] 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.[6] All of this indicates that Enceladus's interior is liquid today, even though it should have been frozen long ago.[60]

 
Enceladus – possibility of fresh ice detected (September 18, 2020)
 
Enceladus – Infrared map view (September 29, 2020)
 
 
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.
 
 
Enhanced-color global map from Cassini images (43.7 MB); leading hemisphere is on right
 
 
Enhanced-color maps of the
northern and southern hemispheres of Enceladus
 
 
Enhanced-color maps of the
trailing and leading hemispheres of Enceladus

Impact craters

Impact cratering is a common occurrence on many Solar System bodies. Much of Enceladus's surface is covered with craters at various densities and levels of degradation.[62] This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.[60]

Cassini observations provided a much closer look at the crater distribution and size, showing that many of Enceladus's craters are heavily degraded through viscous relaxation and fracturing.[63] 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.[64]

Tectonic features

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

Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges.[58] 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.[65]

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.[58] 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.[63]

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.[63][66] 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.[63] 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.[65]

Smooth plains

Two regions of smooth plains were observed by Voyager 2. They generally have low relief and have far fewer craters than in the cratered terrains, indicating a relatively young surface age.[62] 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.[63] 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.[6][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 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.[67]

South polar region

See also: Tiger stripes (Enceladus)
 
Close-up of south pole terrain

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.[6][68] 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.[6] Near the center of this terrain are four fractures bounded by ridges, unofficially called "tiger stripes".[69] 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.[68] 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.[70] VIMS also detected simple organic (carbon-containing) compounds in the tiger stripes, chemistry not found anywhere else on Enceladus thus far.[71]

One of these areas of "blue" ice in the south polar region was observed at high resolution during the July 14, 2005, flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.[72]

 
Y-shaped discontinuities, imaged February 15, 2016

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;[6] 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.[67] 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.[6] However, the north polar region is densely cratered, and has a much older surface age than the south pole.[62] 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.[6]

South polar plumes

See also: Cryovolcano
 
One possible scheme for Enceladus's cryovolcanism

Following Voyager's encounters with Enceladus in the early 1980s, scientists postulated it to be geologically active based on its young, reflective surface and location near the core of the E ring.[58] 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.[50][51] 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,[6] though the possibility of a camera artifact delayed an official announcement. Data from the magnetometer instrument during the February 17, 2005, encounter provided evidence for a planetary atmosphere. The magnetometer observed a deflection or "draping" of the magnetic field, consistent with local ionization of neutral gas. In addition, an increase in the power of ion cyclotron waves near the orbit of Enceladus was observed, which was further evidence of the ionization of neutral gas. These waves are produced by the interaction of ionized particles and magnetic fields, and the waves' frequency is close to the gyrofrequency of the freshly produced ions, in this case water vapor.[15] 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.[15] 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.[16]

Cassini flew through this gas cloud on a few encounters, allowing instruments such as the ion and neutral mass spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. (See 'Composition' section.) 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 (310 mi) from the surface.[56] The particles have a bulk velocity of 1.25 ± 0.1 kilometers per second (2,800 ± 220 miles per hour),[73] and a maximum velocity of 3.40 km/s (7,600 mph).[74] 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.

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 or fumaroles.[6] Fumaroles are probably the closer analogy, since periodic or episodic emission is an inherent property of geysers. The plumes of Enceladus were observed to be continuous to within a factor of a few. The mechanism that drives and sustains the eruptions is thought to be tidal heating.[75] 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.[76][77][78] 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.[79]

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.[80][81][82]

The extent to which cryovolcanism really occurs is a subject of some debate. At Enceladus, it appears that cryovolcanism occurs because water-filled cracks are periodically exposed to vacuum, the cracks being opened and closed by tidal stresses.[83][84][85]

  • Enceladus – plume animation (00:48)

  •  

    Enceladus and south polar jets (April 13, 2017).

  •  

    Plumes above the limb of Enceladus feeding the E ring

  •  

    A false-color Cassini image of the jets

Internal structure

 
A model of the interior of Enceladus: silicate core (brown); water-ice-rich mantle (white); a proposed diapir under the south pole (noted in the mantle (yellow) and core (red))[67]

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.[86][87]

Initial mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.[58] 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.[6] This density is higher than those of 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.[88][89] 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.[90] 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.[91][89] 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.[47][92]

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.[6] 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).[91] Gravity measurements by Cassini show that the density of the core is low, indicating that the core contains water in addition to silicates.[93]

Subsurface water ocean

 
Artist's impression of a global subsurface ocean of liquid water[26][28] (updated and better scaled version)

Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from its south polar surface,[6][94] with jets moving 250 kg of water vapor every second[94] at up to 2,189 km/h (1,360 mph) into space.[95] Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring.[6][54] 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.[96]

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.[26][27][28][97] The top of the ocean probably 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.[26][98]

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.[99] The amount of libration (0.120° ± 0.014°) implies that this global ocean is about 26 to 31 kilometers (16 to 19 miles) deep.[100][101][102][103] For comparison, Earth's ocean has an average depth of 3.7 kilometers.[102]

Composition

 
Enceladus – organics on ice grains (artist concept)
 
Chemical composition of Enceladus's plumes

The Cassini spacecraft flew through the southern plumes on several occasions to sample and analyze its composition. As of 2019, the data gathered is still being analyzed and interpreted. The plumes' salty composition (-Na, -Cl, -CO3) indicates that the source is a salty subsurface ocean.[104]

The INMS instrument detected mostly water vapor, as well as traces of molecular nitrogen, carbon dioxide,[14] and trace amounts of simple hydrocarbons such as methane, propane, acetylene and formaldehyde.[105][106] The plumes' composition, as measured by the INMS, is similar to that seen at most comets.[106] Cassini also found traces of simple organic compounds in some dust grains,[96][107] as well as larger organics such as benzene (C
6H
6),[108] and complex macromolecular organics as large as 200 atomic mass units,[33][109] and at least 15 carbon atoms in size.[110]

The mass spectrometer detected molecular hydrogen (H2) which was in "thermodynamic disequilibrium" with the other components,[111] and found traces of ammonia (NH
3).[112]

A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO3) has an alkaline pH of 11 to 12.[113][114] 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 could support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus's plumes.[113][115]

Further analysis in 2019 was done of the spectral characteristics of ice grains in Enceladus's erupting plumes. The study found that nitrogen-bearing and oxygen-bearing amines were likely present, with significant implications for the availability of amino acids in the internal ocean. The researchers suggested that the compounds on Enceladus could be precursors for "biologically relevant organic compounds".[116][117]

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 to 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.[12] The presence of a subsurface ocean under the south polar region is now accepted,[118] 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.[119]

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

Several explanations for the observed elevated temperatures and the resulting plumes have been proposed, including venting from a subsurface reservoir of liquid water, sublimation of ice,[120] decompression and dissociation of clathrates, and shear heating,[121] 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,[122] giving it a warm core and a subsurface ocean, which is now kept above freezing through unidentified mechanisms. Geophysical models indicate that tidal heating is a main heat source, perhaps aided by radioactive decay and some heat-producing chemical reactions.[123][124][125][126] A 2007 study predicted the internal heat of Enceladus, if generated by tidal forces, could be no greater than 1.1 gigawatts,[127] 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,[127] and suggest that it is in thermal equilibrium.[12][70][128]

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.[6][123] 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.[129][130]

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.[47] Tidal dissipation of Enceladus's ice crust is significant because Enceladus has a subsurface ocean. A computer simulation that used data from Cassini was published in November 2017, and it indicates that friction heat from the sliding rock fragments within the permeable and fragmented core of Enceladus could keep its underground ocean warm for up to billions of years.[131][132][133] 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.[130] 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."[75] 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.[6] The surface of Enceladus indicates that the entire moon has experienced periods of enhanced heat flux in the past.[134]

Radioactive heating

The "hot start" model of heating suggests Enceladus began as ice and rock that contained rapidly decaying short-lived radioactive isotopes of aluminium, iron and manganese. Enormous amounts of heat were then produced as these isotopes decayed for about 7 million years, resulting in the consolidation of rocky material at the core surrounded by a shell of ice. Although the heat from radioactivity would decrease over time, the combination of radioactivity and tidal forces from Saturn's gravitational tug could prevent the subsurface ocean from freezing.[122] 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.[6] Heating from long-lived radioactive isotopes uranium-238, uranium-235, thorium-232 and potassium-40 inside Enceladus would add 0.3 gigawatts to the observed heat flux.[123] 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.[73]

Chemical factors

Because no ammonia was initially found in the vented material by INMS or UVIS, which could act as an antifreeze, 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.[112][135] Reducing the freezing point of water with ammonia would also allow for outgassing and higher gas pressure,[136] and less heat required to power the water plumes.[137] 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.[127][137]

Origin

Mimas–Enceladus paradox

Mimas, the innermost of the round moons of Saturn and directly interior to Enceladus, is a geologically dead body, even though it should experience stronger tidal forces than Enceladus. This apparent paradox can be explained in part by temperature-dependent properties of water ice (the main constituent of the interiors of Mimas and Enceladus). The tidal heating per unit mass is given by the formula

 

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).[138] 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).[139]

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.[140]

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.[141]

Proto-Enceladus hypothesis

Enceladus is losing mass at a rate of 200 kg/second. If mass loss at this rate continued for 4.5 Gyr, the satellite would have lost approximately 30% of its initial mass. A similar value is obtained by assuming that the initial densities of Enceladus and Mimas were equal.[141] 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.[142]

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.[143] A later study from 2019 estimated that the ocean is around one billion years old.[144]

Potential habitability

 
Enceladus (artist concept; February 24, 2020)

Enceladus ejects plumes of salted water laced with grains of silica-rich sand,[145] nitrogen (in ammonia),[146] and organic molecules, including trace amounts of simple hydrocarbons such as methane (CH
4), propane (C
3H
8), acetylene (C
2H
2) and formaldehyde (CH
2O), which are carbon-bearing molecules.[105][106][147] This indicates that hydrothermal activity —an energy source— may be at work in Enceladus's subsurface ocean.[145][148] In addition, models indicate[149] that the large rocky core is porous, allowing water to flow through it, transferring heat and chemicals. It was confirmed by observations and other research.[150][151][152] 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.[113][114][115]

The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns,[150] with an energy source and complex organic compounds[33] in contact with Enceladus's rocky core,[27][28][153] may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life.[26][97][98][154][155][156] Geochemical modeling results concerning not-yet-detected phosphorus indicate the moon meets potential abiogenesis-requirements.[157][158] The presence of a wide range of organic compounds and ammonia indicates their source may be similar to the water/rock reactions known to occur on Earth and that are known to support life.[159] 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 (JET), Enceladus Explorer (En-Ex), Enceladus Life Finder (ELF), Life Investigation For Enceladus (LIFE), and Enceladus Life Signatures and Habitability (ELSAH).

Hydrothermal vents

 
Artist's impression of possible hydrothermal activity on Enceladus's ocean floor[32]

On April 13, 2017, NASA announced the discovery of possible hydrothermal activity on Enceladus's sub-surface ocean floor. In 2015, the Cassini probe made a close fly-by of Enceladus's south pole, flying within 48.3 km (30.0 mi) of the surface, as well as through a plume in the process. A mass spectrometer on the craft detected molecular hydrogen (H2) from the plume, and after months of analysis, the conclusion was made that the hydrogen was most likely the result of hydrothermal activity beneath the surface.[31] It has been speculated that such activity could be a potential oasis of habitability.[160][161][162]

The presence of ample hydrogen in Enceladus's ocean means that microbes – if any exist there – could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water. The chemical reaction is known as "methanogenesis" because it produces methane as a byproduct, and is at the root of the tree of life on Earth, the birthplace of all life that is known to exist.[163][164]

Exploration

Voyager missions

Main article: Voyager program

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.[165] Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.[166] 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.[166]

Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing higher-resolution images to be obtained.[165] These images showed a young surface.[58] 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

Main article: Cassini–Huygens
Enceladus – close flyby (October 28, 2015)[167]
 
Before
 
Up close
 
Plumes
 
After
Enceladus – final flyby (December 19, 2015)[168]
 
Old and new terrain
 
Northern features
 
Frozen fractures
 
Dark spots
 
Ice and atmosphere
 
Animated 3D model of the Cassini–Huygens spacecraft
Cassini flybys of Enceladus[169]
Date
Distance (km)
February 17, 2005 1,264
March 9, 2005 500
July 14, 2005 175
December 24, 2005 94,000
March 12, 2008 48
August 11, 2008 54
October 9, 2008 25
October 31, 2008 200
November 2, 2009 103
November 21, 2009 1,607
April 28, 2010 103
May 18, 2010 201
August 13, 2010 2,554
November 30, 2010 48
December 21, 2010 50
October 1, 2011 99
October 19, 2011 1,231
November 6, 2011 496
March 27, 2012 74
April 14, 2012 74
May 2, 2012 74
October 14, 2015 1,839
October 28, 2015 49
December 19, 2015 4,999

The answers to many remaining mysteries of Enceladus had to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it entered orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000 km of Enceladus. 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.[170] 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.[171] Cassini performed a flyby on October 28, 2015, passing as close as 49 km (30 mi) and through a plume.[172] 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.[115]

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.[173][174][175] By contrast, the water thought to be on Jupiter's moon Europa is located under a much thicker layer of ice.[176]

Proposed mission concepts

The discoveries Cassini made at Enceladus have prompted studies into follow-up mission concepts, including a probe flyby (Journey to Enceladus and Titan or JET) to analyze plume contents in situ,[177][178] a lander by the German Aerospace Center to study the habitability potential of its subsurface ocean (Enceladus Explorer),[179][180][181] and two astrobiology-oriented mission concepts (the Enceladus Life Finder[182][183] and Life Investigation For Enceladus (LIFE)).[146][173][184][185]

The European Space Agency (ESA) was assessing concepts in 2008 to send a probe to Enceladus in a mission to be combined with studies of Titan: Titan Saturn System Mission (TSSM).[186] TSSM was a joint NASA/ESA flagship-class proposal for exploration of Saturn's moons, with a focus on Enceladus, and it 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,[187] although TSSM will continue to be studied and evaluated.

In November 2017, Russian billionaire Yuri Milner expressed interest in funding a "low-cost, privately funded mission to Enceladus which can be launched relatively soon."[188][189] In September 2018, NASA and the Breakthrough Initiatives, founded by Milner, signed a cooperation agreement for the mission's initial concept phase.[190] The spacecraft would be low-cost, low mass, and would be launched at high speed on an affordable rocket. The spacecraft would be directed to perform a single flyby through Enceladus' plumes in order to sample and analyze its content for biosignatures.[191][192] NASA provided scientific and technical expertise through various reviews, from March 2019 to December 2019.[193]

In 2022, the Planetary Science Decadal Survey by the National Academy of Sciences recommended that NASA prioritize its newest probe concept, the Enceladus Orbilander, as a Flagship-class mission, alongside its newest concepts for a Mars sample-return mission and the Uranus Orbiter and Probe. The Enceladus Orbilander would be launched on a similarly affordable rocket, but would cost about $5 billion, and be designed to endure eighteen months in orbit inspecting Enceladus' plumes before landing and spending two Earth years conducting surface astrobiology research.[194]

Year proposed Proponent Project name Status References
2006 GSFC NASA Academy EAGLE study Cancelled [195]
2006 NASA 'Titan and Enceladus $1B Mission Feasibility' Study Cancelled [196][197]
2007 NASA 'Enceladus Flagship' study Cancelled [197]
2007 ESA Titan and Enceladus Mission (TandEM) Cancelled [186]
2007 NASA JPL Enceladus RMA Study Cancelled [198]
2008 NASA/ESA TandEM became Titan Saturn System Mission (TSSM) Cancelled [186]
2010 PSDS Decadal Survey Enceladus Orbiter Cancelled [199]
2011 NASA JPL Journey to Enceladus and Titan (JET) Under study [200]
2012 DLR Enceladus Explorer (EnEx) lander, employing the IceMole Under study [201]
2012 NASA JPL Life Investigation For Enceladus (LIFE) Cancelled [184][202][203]
2015 NASA JPL Enceladus Life Finder (ELF) Under study [204]
2017 ESA/NASA Explorer of Enceladus and Titan (E2T) Under study [205]
2017 NASA Enceladus Life Signatures and Habitability (ELSAH) Under study [206][207]
2017 Breakthrough Initiatives Breakthrough Enceladus mission Under study [188]
2022 PSDS Decadal Survey Enceladus Orbilander Under study [194]

See also

References

Informational notes

  1. ^ Photograph of Enceladus, taken by the narrow-angle camera of the Imaging Science Subsystem (ISS) aboard Cassini, during the spacecraft’s October 28, 2015 flyby. It shows the younger terrain of Sarandib and Diyar Planitia, populated with many grooves (sulci) and depressions (fossae). Older, cratered terrain can be seen towards Enceladus's north pole. The prominent feature visible near the south pole is Cashmere Sulci.
  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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Further reading

  • Lorenz, Ralph (2017). NASA/ESA/ASI Cassini-Huygens: 1997–2017: (Cassini orbiter, Huygens probe and future exploration concepts : owners' workshop manual. Yeovil England: Haynes Publishing. ISBN 9781785211119. OCLC 982381337.
  • Schenk, Paul M.; Clark, Roger N.; Verbiscer, Anne J.; Howett, Carly J. A. Jr.; Waite, Jack Hunter; Dotson, Renée (2018). Enceladus and the icy moons of Saturn. Tucson, AZ: The University of Arizona Press. doi:10.2307/j.ctv65sw2b. ISBN 9780816537075. OCLC 1055049948.

External links

Listen to this article (45 minutes)
 
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  • 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
Images
  • Cassini images of Enceladus August 13, 2011, at the Wayback Machine
  • Images of Enceladus at JPL's Planetary Photojournal
  • Movie of from the National Oceanic and Atmospheric Administration
  • Enceladus global March 8, 2018, at the Wayback Machine and polar March 8, 2018, at the Wayback Machine 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
  • Google Enceladus 3D, interactive map of the moon
  • Image album by Kevin M. Gill

January 31, 2023
enceladus, other, uses, disambiguation, sixth, largest, moon, saturn, 19th, largest, solar, system, about, kilometers, miles, diameter, about, tenth, that, saturn, largest, moon, titan, mostly, covered, fresh, clean, making, most, reflective, bodies, solar, sy. For other uses see Enceladus disambiguation Enceladus is the sixth largest moon of Saturn 19th largest in the Solar System It is about 500 kilometers 310 miles in diameter 5 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 75 1 K 324 4 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 terrain EnceladusView of trailing hemisphere in natural color a DiscoveryDiscovered byWilliam HerschelDiscovery dateAugust 28 1789 1 DesignationsDesignationSaturn IIPronunciation ɛ n ˈ s ɛ l e d e s 2 Named afterἘgkelados EgkeladosAdjectivesEnceladean ɛ n s e ˈ l eɪ d i e n 3 4 Orbital characteristicsSemi major axis237948 km 5 Eccentricity0 0047 5 6 Orbital period sidereal 1 370218 d 5 Inclination0 009 to Saturn s equator 5 Satellite ofSaturnPhysical characteristicsDimensions513 2 502 8 496 6 km 5 7 Mean radius252 1 0 2 km 5 7 0 0395 Earths 0 1451 Moons Mass 1 08022 0 00101 1020 kg 5 8 1 8 10 5 Earths Mean density1 609 0 005 g cm3 5 7 Surface gravity0 113 m s2 0 0113 g Moment of inertia factor0 3305 0 0025 9 Escape velocity0 239 km s 860 4 km h 5 Synodic rotation periodSynchronousAxial tilt0Albedo1 375 0 008 geometric at 550 nm 10 or 0 81 0 04 Bond 11 Surface temp min mean maxKelvin 12 32 9 K 75 K 145 KCelsius 240 C 198 C 128 CApparent magnitude11 7 13 AtmosphereSurface pressureTrace significant spatial variability 15 16 Composition by volume91 water vapor4 nitrogen3 2 carbon dioxide1 7 methane 14 Enceladus was discovered on August 28 1789 by William Herschel 1 17 18 but little was known about it until the two Voyager spacecraft Voyager 1 and Voyager 2 flew by Saturn in 1980 and 1981 19 In 2005 the spacecraft Cassini 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 20 Cryovolcanoes near the south pole shoot geyser like jets of water vapor molecular hydrogen other volatiles and solid material including sodium chloride crystals and ice particles into space totaling about 200 kilograms 440 pounds per second 16 19 21 More than 100 geysers have been identified 22 Some of the water vapor falls back as snow the rest escapes and supplies most of the material making up Saturn s E ring 23 24 According to NASA scientists the plumes are similar in composition to comets 25 In 2014 NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km 6 mi 26 27 28 The existence of Enceladus subsurface ocean has since been mathematically modelled and replicated 29 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 currently geologically active 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 driving the geological activity 30 Cassini performed chemical analysis of Enceladus s plumes finding evidence for hydrothermal activity 31 32 possibly driving complex chemistry 33 Ongoing research on Cassini data suggests that Enceladus s hydrothermal environment could be habitable to some of Earth s hydrothermal vent s microorganisms and that plume found methane could be produced by such organisms 34 35 Contents 1 History 1 1 Discovery 1 2 Naming 2 Shape and size 3 Orbit and rotation 3 1 Source of the E ring 4 Geology 4 1 Surface features 4 1 1 Impact craters 4 1 2 Tectonic features 4 1 3 Smooth plains 4 1 4 South polar region 4 1 5 South polar plumes 4 2 Internal structure 4 2 1 Subsurface water ocean 4 2 2 Composition 4 3 Possible heat sources 4 3 1 Tidal heating 4 3 2 Radioactive heating 4 3 3 Chemical factors 5 Origin 5 1 Mimas Enceladus paradox 5 2 Proto Enceladus hypothesis 5 3 Date of formation 5 4 Potential habitability 5 5 Hydrothermal vents 6 Exploration 6 1 Voyager missions 6 2 Cassini 6 3 Proposed mission concepts 7 See also 8 References 8 1 Informational notes 8 2 Citations 9 Further reading 10 External linksHistory EditDiscovery Edit Voyager 2 view of Enceladus in 1981 Samarkand Sulci vertical grooves lower center Ali Baba and Aladdin craters upper left Enceladus was discovered by William Herschel on August 28 1789 during the first use of his new 1 2 m 47 in 40 foot telescope then the largest in the world at Observatory House in Slough England 18 36 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 37 Prior to the Voyager missions the view of Enceladus improved little from the dot first observed by Herschel Only its orbital characteristics were known with estimations of its mass density and albedo Naming Edit Enceladus is named after the giant Enceladus of Greek mythology 1 The name like the names of each of the first seven satellites of Saturn to be discovered was suggested by William Herschel s son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope 38 He chose these names because Saturn known in Greek mythology as Cronus was the leader of the Titans Geological features on Enceladus are named by the International Astronomical Union IAU after characters and places from Richard Francis Burton s 1885 translation of The Book of One Thousand and One Nights 39 Impact craters are named after characters whereas other feature types such as fossae long narrow depressions dorsa ridges planitiae plains sulci long parallel grooves and rupes cliffs are named after places The IAU has officially named 85 features on Enceladus most recently Samaria Rupes formerly called Samaria Fossa 40 41 Shape and size EditEnceladus is a relatively small satellite composed of ice and rock 42 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 6 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 43 44 Enceladus transiting the moon Titan Size comparison of Earth the Moon and Enceladus A size comparison of Enceladus against the British Isles Orbit and rotation Edit Enceladus s orbit red Saturn s north pole view Enceladus is one of the major inner satellites of Saturn along with Dione Tethys and Mimas It orbits at 238 000 km 148 000 mi from Saturn s center and 180 000 km 110 000 mi 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 6 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 45 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 more than 1 5 about its spin axis However analysis of the shape of Enceladus suggests that at some point it was in a 1 4 forced secondary spin orbit libration 6 This libration could have provided Enceladus with an additional heat source 30 46 47 Source of the E ring Edit Main article Rings of Saturn E Ring Possible origins of methane found in plumes Plumes from Enceladus which are similar in composition to comets 25 have been shown to be the source of the material in Saturn s E ring 23 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 48 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 49 Enceladus is orbiting inside the ring at its narrowest but highest density point In the 1980s some suspected that Enceladus is the main source of particles for the ring 50 51 52 53 This hypothesis was confirmed by Cassini s first two close flybys in 2005 54 55 The Cosmic Dust Analyzer CDA detected a large increase in the number of particles near Enceladus confirming it as the primary source for the E ring 54 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 56 Visual confirmation of venting came in November 2005 when ISS imaged geyser like jets of icy particles rising from Enceladus s south polar region 6 24 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 57 View of Enceladus s orbit from the side showing Enceladus in relation to Saturn s E ring Enceladus orbiting within Saturn s E ring Enceladus geyser tendrils comparison of images a c with computer simulations Enceladus south polar region locations of most active tendril producing geysers Eruptions on Enceladus look like discrete jets but may be curtain eruptions instead 1 video animation Geology EditSurface features Edit See also List of geological features on Enceladus South polar view of the anti Saturn hemisphere with fractured areas in blue false color Enceladus tilted terminator north is up 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 58 In addition extensive linear cracks 59 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 60 The fresh clean ice that dominates its surface makes Enceladus the most reflective body in the Solar System with a visual geometric albedo of 1 38 10 and bolometric Bond albedo of 0 81 0 04 11 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 12 Observations during three flybys on February 17 March 9 and July 14 2005 revealed Enceladus s surface features in much greater detail than the Voyager 2 observations The smooth plains which Voyager 2 had observed resolved into relatively crater free regions filled with numerous small ridges and scarps Numerous fractures were found within the older cratered terrain suggesting that the surface has been subjected to extensive deformation since the craters were formed 61 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 6 All of this indicates that Enceladus s interior is liquid today even though it should have been frozen long ago 60 Enceladus possibility of fresh ice detected September 18 2020 Enceladus Infrared map view September 29 2020 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 Enhanced color global map from Cassini images 43 7 MB leading hemisphere is on right Enhanced color maps of thenorthern and southern hemispheres of Enceladus Enhanced color maps of thetrailing and leading hemispheres of Enceladus Impact craters Edit 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 62 This subdivision of cratered terrains on the basis of crater density and thus surface age suggests that Enceladus has been resurfaced in multiple stages 60 Cassini observations provided a much closer look at the crater distribution and size showing that many of Enceladus s craters are heavily degraded through viscous relaxation and fracturing 63 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 64 Tectonic features Edit View of Enceladus s Europa like surface with the Labtayt Sulci fractures at center and the Ebony and Cufa dorsa at lower left imaged by Cassini on February 17 2005 Voyager 2 found several types of tectonic features on Enceladus including troughs scarps and belts of grooves and ridges 58 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 65 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 58 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 63 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 63 66 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 63 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 65 Smooth plains Edit 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 62 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 63 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 6 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 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 67 South polar region Edit See also Tiger stripes Enceladus Close up of south pole terrain 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 6 68 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 6 Near the center of this terrain are four fractures bounded by ridges unofficially called tiger stripes 69 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 68 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 70 VIMS also detected simple organic carbon containing compounds in the tiger stripes chemistry not found anywhere else on Enceladus thus far 71 One of these areas of blue ice in the south polar region was observed at high resolution during the July 14 2005 flyby revealing an area of extreme tectonic deformation and blocky terrain with some areas covered in boulders 10 100 m across 72 Y shaped discontinuities imaged February 15 2016 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 6 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 67 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 6 However the north polar region is densely cratered and has a much older surface age than the south pole 62 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 6 South polar plumes Edit See also Cryovolcano One possible scheme for Enceladus s cryovolcanism Following Voyager s encounters with Enceladus in the early 1980s scientists postulated it to be geologically active based on its young reflective surface and location near the core of the E ring 58 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 50 51 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 6 though the possibility of a camera artifact delayed an official announcement Data from the magnetometer instrument during the February 17 2005 encounter provided evidence for a planetary atmosphere The magnetometer observed a deflection or draping of the magnetic field consistent with local ionization of neutral gas In addition an increase in the power of ion cyclotron waves near the orbit of Enceladus was observed which was further evidence of the ionization of neutral gas These waves are produced by the interaction of ionized particles and magnetic fields and the waves frequency is close to the gyrofrequency of the freshly produced ions in this case water vapor 15 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 15 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 16 Cassini flew through this gas cloud on a few encounters allowing instruments such as the ion and neutral mass spectrometer INMS and the cosmic dust analyzer CDA to directly sample the plume See Composition section 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 310 mi from the surface 56 The particles have a bulk velocity of 1 25 0 1 kilometers per second 2 800 220 miles per hour 73 and a maximum velocity of 3 40 km s 7 600 mph 74 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 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 or fumaroles 6 Fumaroles are probably the closer analogy since periodic or episodic emission is an inherent property of geysers The plumes of Enceladus were observed to be continuous to within a factor of a few The mechanism that drives and sustains the eruptions is thought to be tidal heating 75 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 76 77 78 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 79 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 80 81 82 The extent to which cryovolcanism really occurs is a subject of some debate At Enceladus it appears that cryovolcanism occurs because water filled cracks are periodically exposed to vacuum the cracks being opened and closed by tidal stresses 83 84 85 source source source source source source source source source source source source Enceladus plume animation 00 48 Enceladus and south polar jets April 13 2017 Plumes above the limb of Enceladus feeding the E ring A false color Cassini image of the jetsInternal structure Edit A model of the interior of Enceladus silicate core brown water ice rich mantle white a proposed diapir under the south pole noted in the mantle yellow and core red 67 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 86 87 Initial mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice 58 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 6 This density is higher than those of 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 88 89 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 90 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 91 89 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 47 92 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 6 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 91 Gravity measurements by Cassini show that the density of the core is low indicating that the core contains water in addition to silicates 93 Subsurface water ocean Edit Artist s impression of a global subsurface ocean of liquid water 26 28 updated and better scaled version Evidence of liquid water on Enceladus began to accumulate in 2005 when scientists observed plumes containing water vapor spewing from its south polar surface 6 94 with jets moving 250 kg of water vapor every second 94 at up to 2 189 km h 1 360 mph into space 95 Soon after in 2006 it was determined that Enceladus s plumes are the source of Saturn s E Ring 6 54 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 96 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 26 27 28 97 The top of the ocean probably 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 26 98 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 99 The amount of libration 0 120 0 014 implies that this global ocean is about 26 to 31 kilometers 16 to 19 miles deep 100 101 102 103 For comparison Earth s ocean has an average depth of 3 7 kilometers 102 Composition Edit Enceladus organics on ice grains artist concept Chemical composition of Enceladus s plumes The Cassini spacecraft flew through the southern plumes on several occasions to sample and analyze its composition As of 2019 the data gathered is still being analyzed and interpreted The plumes salty composition Na Cl CO3 indicates that the source is a salty subsurface ocean 104 The INMS instrument detected mostly water vapor as well as traces of molecular nitrogen carbon dioxide 14 and trace amounts of simple hydrocarbons such as methane propane acetylene and formaldehyde 105 106 The plumes composition as measured by the INMS is similar to that seen at most comets 106 Cassini also found traces of simple organic compounds in some dust grains 96 107 as well as larger organics such as benzene C6 H6 108 and complex macromolecular organics as large as 200 atomic mass units 33 109 and at least 15 carbon atoms in size 110 The mass spectrometer detected molecular hydrogen H2 which was in thermodynamic disequilibrium with the other components 111 and found traces of ammonia NH3 112 A model suggests that Enceladus s salty ocean Na Cl CO3 has an alkaline pH of 11 to 12 113 114 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 could support both abiotic and biological synthesis of organic molecules such as those that have been detected in Enceladus s plumes 113 115 Further analysis in 2019 was done of the spectral characteristics of ice grains in Enceladus s erupting plumes The study found that nitrogen bearing and oxygen bearing amines were likely present with significant implications for the availability of amino acids in the internal ocean The researchers suggested that the compounds on Enceladus could be precursors for biologically relevant organic compounds 116 117 Possible heat sources Edit 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 to 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 12 The presence of a subsurface ocean under the south polar region is now accepted 118 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 119 Heat map of the south polar fractures dubbed tiger stripes Several explanations for the observed elevated temperatures and the resulting plumes have been proposed including venting from a subsurface reservoir of liquid water sublimation of ice 120 decompression and dissociation of clathrates and shear heating 121 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 122 giving it a warm core and a subsurface ocean which is now kept above freezing through unidentified mechanisms Geophysical models indicate that tidal heating is a main heat source perhaps aided by radioactive decay and some heat producing chemical reactions 123 124 125 126 A 2007 study predicted the internal heat of Enceladus if generated by tidal forces could be no greater than 1 1 gigawatts 127 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 127 and suggest that it is in thermal equilibrium 12 70 128 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 6 123 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 129 130 Tidal heating Edit 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 47 Tidal dissipation of Enceladus s ice crust is significant because Enceladus has a subsurface ocean A computer simulation that used data from Cassini was published in November 2017 and it indicates that friction heat from the sliding rock fragments within the permeable and fragmented core of Enceladus could keep its underground ocean warm for up to billions of years 131 132 133 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 130 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 75 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 6 The surface of Enceladus indicates that the entire moon has experienced periods of enhanced heat flux in the past 134 Radioactive heating Edit 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 122 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 6 Heating from long lived radioactive isotopes uranium 238 uranium 235 thorium 232 and potassium 40 inside Enceladus would add 0 3 gigawatts to the observed heat flux 123 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 73 Chemical factors Edit Because no ammonia was initially found in the vented material by INMS or UVIS which could act as an antifreeze 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 112 135 Reducing the freezing point of water with ammonia would also allow for outgassing and higher gas pressure 136 and less heat required to power the water plumes 137 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 127 137 Origin EditMimas Enceladus paradox Edit 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 t i d 63 r n 5 r 4 e 2 38 m Q displaystyle q tid frac 63 rho n 5 r 4 e 2 38 mu Q where r 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 m 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 m and Q are temperature dependent At high temperatures close to the melting point m 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 m and Q are high but a possible higher temperature state for Enceladus values of m and Q are low 138 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 139 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 140 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 141 Proto Enceladus hypothesis Edit 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 141 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 142 Date of formation Edit 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 143 A later study from 2019 estimated that the ocean is around one billion years old 144 Potential habitability Edit Enceladus artist concept February 24 2020 Enceladus ejects plumes of salted water laced with grains of silica rich sand 145 nitrogen in ammonia 146 and organic molecules including trace amounts of simple hydrocarbons such as methane CH4 propane C3 H8 acetylene C2 H2 and formaldehyde CH2 O which are carbon bearing molecules 105 106 147 This indicates that hydrothermal activity an energy source may be at work in Enceladus s subsurface ocean 145 148 In addition models indicate 149 that the large rocky core is porous allowing water to flow through it transferring heat and chemicals It was confirmed by observations and other research 150 151 152 Molecular hydrogen H2 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 113 114 115 The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns 150 with an energy source and complex organic compounds 33 in contact with Enceladus s rocky core 27 28 153 may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life 26 97 98 154 155 156 Geochemical modeling results concerning not yet detected phosphorus indicate the moon meets potential abiogenesis requirements 157 158 The presence of a wide range of organic compounds and ammonia indicates their source may be similar to the water rock reactions known to occur on Earth and that are known to support life 159 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 JET Enceladus Explorer En Ex Enceladus Life Finder ELF Life Investigation For Enceladus LIFE and Enceladus Life Signatures and Habitability ELSAH Hydrothermal vents Edit Artist s impression of possible hydrothermal activity on Enceladus s ocean floor 32 On April 13 2017 NASA announced the discovery of possible hydrothermal activity on Enceladus s sub surface ocean floor In 2015 the Cassini probe made a close fly by of Enceladus s south pole flying within 48 3 km 30 0 mi of the surface as well as through a plume in the process A mass spectrometer on the craft detected molecular hydrogen H2 from the plume and after months of analysis the conclusion was made that the hydrogen was most likely the result of hydrothermal activity beneath the surface 31 It has been speculated that such activity could be a potential oasis of habitability 160 161 162 The presence of ample hydrogen in Enceladus s ocean means that microbes if any exist there could use it to obtain energy by combining the hydrogen with carbon dioxide dissolved in the water The chemical reaction is known as methanogenesis because it produces methane as a byproduct and is at the root of the tree of life on Earth the birthplace of all life that is known to exist 163 164 Exploration EditVoyager missions Edit Main article Voyager program 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 165 Images acquired from this distance had very poor spatial resolution but revealed a highly reflective surface devoid of impact craters indicating a youthful surface 166 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 166 Voyager 2 passed closer to Enceladus 87 010 km on August 26 1981 allowing higher resolution images to be obtained 165 These images showed a young surface 58 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 Edit Main article Cassini Huygens Enceladus close flyby October 28 2015 167 Before Up close Plumes After Enceladus final flyby December 19 2015 168 Old and new terrain Northern features Frozen fractures Dark spots Ice and atmosphere Animated 3D model of the Cassini Huygens spacecraft Cassini flybys of Enceladus 169 Date Distance km February 17 2005 1 264March 9 2005 500July 14 2005 175December 24 2005 94 000March 12 2008 48August 11 2008 54October 9 2008 25October 31 2008 200November 2 2009 103November 21 2009 1 607April 28 2010 103May 18 2010 201August 13 2010 2 554November 30 2010 48December 21 2010 50October 1 2011 99October 19 2011 1 231November 6 2011 496March 27 2012 74April 14 2012 74May 2 2012 74October 14 2015 1 839October 28 2015 49December 19 2015 4 999The 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 170 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 171 Cassini performed a flyby on October 28 2015 passing as close as 49 km 30 mi and through a plume 172 Confirmation of molecular hydrogen H2 would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus seafloor increasing its habitability 115 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 173 174 175 By contrast the water thought to be on Jupiter s moon Europa is located under a much thicker layer of ice 176 Proposed mission concepts Edit The discoveries Cassini made at Enceladus have prompted studies into follow up mission concepts including a probe flyby Journey to Enceladus and Titan or JET to analyze plume contents in situ 177 178 a lander by the German Aerospace Center to study the habitability potential of its subsurface ocean Enceladus Explorer 179 180 181 and two astrobiology oriented mission concepts the Enceladus Life Finder 182 183 and Life Investigation For Enceladus LIFE 146 173 184 185 The European Space Agency ESA was assessing concepts in 2008 to send a probe to Enceladus in a mission to be combined with studies of Titan Titan Saturn System Mission TSSM 186 TSSM was a joint NASA ESA flagship class proposal for exploration of Saturn s moons with a focus on Enceladus and it 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 187 although TSSM will continue to be studied and evaluated In November 2017 Russian billionaire Yuri Milner expressed interest in funding a low cost privately funded mission to Enceladus which can be launched relatively soon 188 189 In September 2018 NASA and the Breakthrough Initiatives founded by Milner signed a cooperation agreement for the mission s initial concept phase 190 The spacecraft would be low cost low mass and would be launched at high speed on an affordable rocket The spacecraft would be directed to perform a single flyby through Enceladus plumes in order to sample and analyze its content for biosignatures 191 192 NASA provided scientific and technical expertise through various reviews from March 2019 to December 2019 193 In 2022 the Planetary Science Decadal Survey by the National Academy of Sciences recommended that NASA prioritize its newest probe concept the Enceladus Orbilander as a Flagship class mission alongside its newest concepts for a Mars sample return mission and the Uranus Orbiter and Probe The Enceladus Orbilander would be launched on a similarly affordable rocket but would cost about 5 billion and be designed to endure eighteen months in orbit inspecting Enceladus plumes before landing and spending two Earth years conducting surface astrobiology research 194 Year proposed Proponent Project name Status References2006 GSFC NASA Academy EAGLE study Cancelled 195 2006 NASA Titan and Enceladus 1B Mission Feasibility Study Cancelled 196 197 2007 NASA Enceladus Flagship study Cancelled 197 2007 ESA Titan and Enceladus Mission TandEM Cancelled 186 2007 NASA JPL Enceladus RMA Study Cancelled 198 2008 NASA ESA TandEM became Titan Saturn System Mission TSSM Cancelled 186 2010 PSDS Decadal Survey Enceladus Orbiter Cancelled 199 2011 NASA JPL Journey to Enceladus and Titan JET Under study 200 2012 DLR Enceladus Explorer EnEx lander employing the IceMole Under study 201 2012 NASA JPL Life Investigation For Enceladus LIFE Cancelled 184 202 203 2015 NASA JPL Enceladus Life Finder ELF Under study 204 2017 ESA NASA Explorer of Enceladus and Titan E2T Under study 205 2017 NASA Enceladus Life Signatures and Habitability ELSAH Under study 206 207 2017 Breakthrough Initiatives Breakthrough Enceladus mission Under study 188 2022 PSDS Decadal Survey Enceladus Orbilander Under study 194 See also EditEnceladus in fiction List of extraterrestrial volcanoes List of geological features on Enceladus List of natural satellitesReferences EditInformational notes Edit Photograph of Enceladus taken by the narrow angle camera of the Imaging Science Subsystem ISS aboard Cassini during the spacecraft s October 28 2015 flyby It shows the younger terrain of 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Enceladus and the icy moons of Saturn Tucson AZ The University of Arizona Press doi 10 2307 j ctv65sw2b ISBN 9780816537075 OCLC 1055049948 External links EditListen to this article 45 minutes source source This audio file was created from a revision of this article dated 24 October 2011 2011 10 24 and does not reflect subsequent edits Audio help More spoken articles Enceladus at Wikipedia s sister projects Definitions from Wiktionary Media from Commons News from Wikinews Quotations from Wikiquote Texts from Wikisource Textbooks from Wikibooks Resources from Wikiversity 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 LifeImagesCassini images of Enceladus Archived August 13 2011 at the Wayback Machine Images of Enceladus at JPL s Planetary Photojournal Movie of Enceladus s rotation from the National Oceanic and Atmospheric Administration Enceladus global Archived March 8 2018 at the Wayback Machine and polar Archived March 8 2018 at the Wayback Machine 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 Google Enceladus 3D interactive map of the moon Image album by Kevin M Gill Portals Astronomy Stars Spaceflight Outer space Solar System Retrieved from https en wikipedia org w index php title Enceladus amp oldid 1133970030, wikipedia, wiki, book, books, library,

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