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Enceladus (moon)

                   
Enceladus
PIA08409 North Polar Region of Enceladus.jpg
Degraded craters, fractures and disrupted terrain in Cassini mosaic of Enceladus's north polar region. The two prominent craters above the middle terminator are Ali Baba (upper) and Aladdin. The Samarkand Sulci grooves run vertically to their left.
Discovery
Discovered by William Herschel
Discovery date August 28, 1789 [1]
Designations
Pronunciation /ɛnˈsɛlədəs/[a]
Alternative names Saturn II
Adjective Enceladean, Enceladan [b]
Semi-major axis 237,948 km
Eccentricity 0.004 7 [2]
Orbital period 1.370 218 days[3]
Inclination 0.019° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions 513.2×502.8×496.6 km [4]
Mean radius 252.1 ± 0.2 km [4] (0.0395 Earths)
Mass (1.080 22 ± 0.001 01)×1020 kg [5] (1.8×10−5 Earths)
Mean density 1.609 ± 0.005 g/cm³ [4]
Equatorial surface gravity 0.114 m/s² (0.011 3 g)
Escape velocity 0.239 km/s (860.4 km/h)
Rotation period synchronous
Axial tilt zero
Albedo 1.375 ± 0.008 (geometric) or 0.99 (Bond) [6]
Surface temp.
   Kelvin[8]
min mean max
32.9 K 75 K 145 K
Apparent magnitude 11.7 [7]
Atmosphere
Surface pressure trace, significant spatial variability [9][10]
Composition 91% Water vapour
4% Nitrogen
3.2% Carbon dioxide
1.7% Methane[11]
This article is about the moon of Saturn; for the mythological giant, see Enceladus (mythology).

Enceladus is the sixth-largest of the moons of Saturn.[12] It was discovered in 1789 by William Herschel.[13] Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi), about a tenth of that of Saturn's largest moon, Titan, and that it reflects almost all of the sunlight that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, indicating a possible association between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.

In 2005 the Cassini spacecraft performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer Solar System bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.[14] The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring.

In May 2011 NASA scientists at an Enceladus Focus Group Conference reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".[15][16]

Contents

  Name

  Size comparison of Earth and Enceladus

Enceladus is named after the Giant Enceladus of Greek mythology. The name Enceladus—like the names of each of the first seven satellites of Saturn to be discovered—was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.[17] He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.

Features on Enceladus are named by the International Astronomical Union (IAU) after characters and places from the Arabian Nights.[18] Impact craters are named after characters, while other feature types, such as fossae (long, narrow depressions), dorsa (ridges), planitia (plains), and sulci (long parallel grooves), are named after places. 57 features have been officially named by the IAU; 22 features were named in 1982 based on the results of the Voyager flybys, and 35 features were approved in November 2006 based on the results of Cassini's three flybys in 2005.[19]

  Exploration

  Figure 1: Voyager 2 view (August 26, 1981) of Enceladus's Saturn-facing hemisphere. The Samarkand Sulci grooves run vertically just right of center; Ali Baba (lower) and Aladdin craters are at upper left.
Previous Cassini encounters with Enceladus[20][21]
Date Distance (km)
February 17, 2005 1,264
March 9, 2005 500
March 29, 2005 64,000
May 21, 2005 93,000
July 14, 2005 175
October 12, 2005 49,000
December 24, 2005 94,000
January 17, 2006 146,000
September 9, 2006 40,000
November 9, 2006 95,000
June 28, 2007 90,000
September 30, 2007 98,000
March 12, 2008 52
June 30, 2008 84,000
August 11, 2008 54
October 9, 2008 25
October 31, 2008 200
November 8, 2008 52,804
November 2, 2009 103
November 21, 2009 1,607
April 28, 2010 103
May 18, 2010 201
August 13, 2010 2554
November 30, 2010 48
December 21, 2010 50
January 30, 2011 60000
February 20, 2011 68000
September 13, 2011 42000
October 1, 2011 99
October 19, 2011 1231
November 5, 2011 496
November 23, 2011 35000
December 11, 2011 20000

Enceladus was discovered by Fredrick William Herschel on August 28, 1789, during the first use of his new 1.2 m telescope, then the largest in the world.[22][23] Herschel first observed Enceladus in 1787, but in his smaller, 16.5 cm telescope, the moon was not recognized.[24] Its faint apparent magnitude (+11.7m) and its proximity to much brighter Saturn and its rings make Enceladus difficult to observe from Earth, requiring a telescope with a mirror of 15–30 cm in diameter, depending on atmospherical conditions and light pollution. Like many Saturnian satellites discovered prior to the Space Age, Enceladus was first observed during a Saturnian equinox, when Earth is within the ring plane; at such times, the reduction in glare from the rings makes the moons easier to observe.

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.

The two Voyager spacecraft obtained the first close-up images of Enceladus. Voyager 1 was the first to fly past Enceladus, at a distance of 202,000 km on November 12, 1980.[25] Images acquired from this distance had very poor spatial resolution, but revealed a highly reflective surface devoid of impact craters, indicating a youthful surface.[26] 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.[26]

  Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. From right to left, the four major stripes are Damascus, Baghdad, Cairo and Alexandria sulci.

Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981, allowing much higher-resolution images of this satellite.[25] These images revealed the youthful nature of much of its surface, as seen in Figure 1.[27] 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. However, Voyager 2 failed to determine whether Enceladus was currently active or whether it was the source of the E-ring.

  False-color Cassini image of jets in the southern hemisphere of Enceladus

The answer to these and other mysteries would have to wait until the arrival of the Cassini spacecraft on July 1, 2004, when it went into orbit around Saturn. Given the results from the Voyager 2 images, Enceladus was considered a priority target by the Cassini mission planners, and several targeted flybys within 1,500 km of the surface were planned as well as numerous, "non-targeted" opportunities within 100,000 km of Enceladus. These encounters are listed on the left. The flybys have yielded significant information concerning Enceladus's surface, as well as the discovery of water vapor and complex hydrocarbons venting from the geologically active South Polar Region. These discoveries prompted the adjustment of Cassini's flight plan to allow closer flybys of Enceladus, including an encounter in March 2008 which took the probe to within 52 km of the moon's surface.[20] The extended mission for Cassini included seven close flybys of Enceladus between July 2008 and July 2010, including two passes at only 50 km in the later half of 2008.[28]

The discoveries Cassini has made at Enceladus have prompted several studies into follow-up missions. In 2007 NASA performed a concept study for a mission that would orbit Enceladus and would perform a detailed examination of the south polar plumes.[29] The concept was not selected for further study.[30] The European Space Agency also recently explored plans to send a probe to Enceladus in a mission to be combined with studies of Titan.[31]

The Titan Saturn System Mission (TSSM) is a joint NASA/ESA proposal for exploration of Saturn's moons, including Enceladus. TSSM was competing against the Europa Jupiter System Mission (EJSM) proposal for funding. In February 2009 it was announced that ESA/NASA had given the EJSM mission priority ahead of TSSM,[32] although TSSM will continue to be studied for a later launch date.

  Characteristics

  Orbit

  Figure 2: View of Enceladus's orbit (highlighted in red) from above Saturn's north pole

Enceladus is one of the major inner satellites of Saturn. It is the fourteenth satellite when ordered by distance from Saturn, and orbits within the densest part of the E Ring, the outermost of Saturn's rings, an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea.

Enceladus orbits Saturn at a distance of 238,000 km from the planet's center and 180,000 km from its cloudtops, between the orbits of Mimas and Tethys, requiring 32.9 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Enceladus is currently in a 2:1 mean motion orbital resonance with Dione, completing two orbits of Saturn for every one orbit completed by Dione. This resonance helps maintain Enceladus's orbital eccentricity (0.0047) and provides a heating source for Enceladus's geologic activity.[2]

Like most of the larger satellites of Saturn, Enceladus rotates synchronously with its orbital period, keeping one face pointed toward Saturn. Unlike the Earth's Moon, Enceladus does not appear to librate about its spin axis (more than 1.5°). However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 forced secondary spin–orbit libration.[2] This libration, like the resonance with Dione, could have provided Enceladus with an additional heat source.

  Interaction with E Ring

The E Ring is the widest and outermost ring of Saturn. It is an extremely wide but very diffuse disk of microscopic icy or dusty material, beginning at the orbit of Mimas and ending somewhere around the orbit of Rhea, though some observations suggest that it extends beyond the orbit of Titan, making it 1,000,000 km wide. However, numerous mathematical models show that such a ring is unstable, with a lifespan between 10,000 and 1,000,000 years. Therefore, particles composing it must be constantly replenished. Enceladus is orbiting inside this ring, in a place where it is narrowest but present in its highest density. Therefore, several theories suspected Enceladus to be the main source of particles for the E Ring. This hypothesis was supported by Cassini's flyby.

  Figure 3: View of Enceladus's orbit from the side, showing Enceladus in relation to Saturn's E ring
  Enceladus orbiting within Saturn's E ring

There are actually two distinct mechanisms feeding the ring with particles.[33] The first, and probably the most important, source of particles comes from the cryovolcanic plume in the South polar region of Enceladus. While a majority of particles fall back to the surface, some of them escape Enceladus's gravity and enter orbit around Saturn, since Enceladus's escape velocity is only 866 km/h. The second mechanism comes from meteoric bombardment of Enceladus, raising dust particles from the surface. This mechanism is not unique to Enceladus, but is valid for all Saturn's moons orbiting inside the E Ring.

  Size and shape

  Figure 4: Enceladus (top left) transits Titan, as seen from Cassini on February 5, 2006. Enceladus was 4.1 million km away, and Titan a further 1.2 million km.
  Figure 5: NASA JPL to-scale size comparison of Enceladus.

Enceladus is a relatively small satellite, with a mean diameter of 505 kilometers (314 mi), only one-seventh the diameter of Earth's own Moon. In diameter Enceladus is small enough to fit within the length of the island of Great Britain. It could also fit comfortably within the states of Arizona or Colorado, although as a spherical object its surface area is much greater, just over 800,000 square kilometers (310,000 sq mi), almost the same as Mozambique, or 15% larger than Texas.

Its mass and diameter make Enceladus the sixth most massive and largest satellite of Saturn, after Titan (5150 km), Rhea (1530 km), Iapetus (1440 km), Dione (1120 km) and Tethys (1050 km). It is also one of the smallest of Saturn's spherical satellites, since all smaller satellites except Mimas (390 km) have an irregular shape.

Enceladus has the shape of an oblate spheroid; its dimensions, calculated from pictures taken by Cassini's ISS (Imaging Science Subsystem) instrument, are of 513(a)×503(b)×497(c) km,[2] with (a) corresponding to the diameter between sub- and anti-Saturnian poles, (b) to the diameter between the leading and trailing poles, and (c) to the distance between the north and south poles. This is the most stable orientation, with the moon's rotation along the short axis, and the long axis aligned radially away from Saturn.

  Surface

  Close-up of Enceladus's south polar terrain; individual boulders can be discerned amid the cracks and hills.
  Global basemap of Enceladus from Cassini images (2010)

Voyager 2, in August 1981, was the first spacecraft to observe the surface in detail. Examination of the resulting highest-resolution mosaic reveals at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain often bordering the smooth areas.[27] In addition, extensive linear cracks[34] and scarps were observed. Given the relative lack of craters on the smooth plains, these regions are probably less than a few hundred million years old. Accordingly, Enceladus must have been recently active with "water volcanism" or other processes that renew the surface. The fresh, clean ice that dominates its surface gives Enceladus probably the most reflective surface of any body in the Solar System with a visual geometric albedo of 1.38.[6] Because it reflects so much sunlight, the mean surface temperature at noon only reaches −198 °C (somewhat colder than other Saturnian satellites).[8]

Observations during three flybys by Cassini on February 17, March 9, and July 14 of 2005 revealed Enceladus's surface features in much greater detail than the Voyager 2 observations. For example, the smooth plains observed by Voyager 2 resolved into relatively crater-free regions filled with numerous small ridges and scarps. In addition, numerous fractures were found within the older, cratered terrain, suggesting that the surface has been subjected to extensive deformation since the craters were formed.[35] Finally, 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.[2]

  Impact craters

  Figure 6: Degraded craters on Enceladus, imaged by Cassini, February 17, 2005. The Hamah Sulci can be seen running from left to right along the bottom quarter of the image. Craters from Enceladus's ct2 and cp cratered units are visible above them.

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. From Voyager 2 observations, three different units of cratered topography were identified on the basis of their crater densities, from ct1 and ct2, both containing numerous 10–20 km-wide craters though differing in the degree of deformation, to cp consisting of lightly cratered plains.[36] This subdivision of cratered terrains on the basis of crater density (and thus surface age) suggests that Enceladus has been resurfaced in multiple stages.

Recent Cassini observations have provided a much closer look at the ct2 and cp cratered units. These high-resolution observations, like Figure 6, reveal that many of Enceladus's craters are heavily deformed through viscous relaxation and fracturing.[37] 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 (seen at center just below the terminator in Figure 6). Dunyazad, the large crater seen in Figure 8 just left of top center, is a prime example of a viscously relaxed crater on Enceladus, with a prominent domed floor. In addition, many craters on Enceladus have been heavily modified by tectonic fractures. The 10-km-wide crater right of bottom center in Figure 8 is a prime example: thin fractures, several hundred meters to a kilometer wide, have heavily altered the crater's rim and floor. Nearly all craters on Enceladus thus far imaged by Cassini in the ct2 unit show signs of tectonic deformation.

  Tectonics

Voyager 2 found several types of tectonic features on Enceladus, including troughs, scarps, and belts of grooves and ridges.[27] Recent results from Cassini suggest that tectonism is the dominant deformation style on Enceladus. One of the more dramatic types of tectonic features found on Enceladus are rifts. These canyons can be up to 200 km long, 5–10 km wide, and one km deep. Figure 7 shows a typical large fracture on Enceladus cutting across older, tectonically deformed terrain. Another example can be seen running along the bottom of the frame in Figure 8. Such features appear relatively young, as they cut across other tectonic features and have sharp topographic relief with prominent outcrops along the cliff faces.

  Figure 7: View of Enceladus's Europa-like surface with the Labtayt Sulci fractures at center and the Ebony (left) and Cufa dorsa at lower left; imaged by Cassini on 17 February 2005
  Figure 8: Cassini false-color view of Enceladus's surface, showing several tectonic and crater degradation styles. The crater Dunyazad is at top; the Misr Sulci run along the bottom.

Another evidence of tectonism on Enceladus is grooved terrain, consisting of lanes of curvilinear grooves and ridges. These bands, first discovered by Voyager 2, often separate smooth plains from cratered regions.[27] An example of this terrain type can be seen in Figures 6 and 10 (in this case, a feature known as the Samarkand Sulci). Grooved terrains such as the Samarkand Sulci are reminiscent of grooved terrain on Ganymede. However, unlike those seen on Ganymede, grooved topography on Enceladus is generally much more complex. Rather than parallel sets of grooves, these lanes can often appear as bands of crudely aligned, chevron-shaped features. In other areas, these bands appear to bow upwards with fractures and ridges running the length of the feature. Cassini observations of the Samarkand Sulci have revealed intriguing dark spots (125 and 750 m wide), which appear to run parallel to narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts.[37]

  Figure 9: High-resolution mosaic of Enceladus's surface, showing several tectonic and crater degradation styles. The Misr (upper) and Al-Yaman sulci run horizontally at upper right.

In addition to deep fractures and grooved lanes, Enceladus has several other types of tectonic terrain. Figure 9 shows sets of narrow fractures (still several hundred meters wide) that were first discovered by the Cassini spacecraft. Many of these fractures are found in bands cutting across cratered terrain. These fractures appear to propagate down only a few hundred meters into the crust. Many appear to have been influenced during their formation by the weakened regolith produced by impact craters, often changing the strike of the propagating fracture.[37][38] 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. Examples of linear grooves can be found in the lower left of the figure at top and Figure 10 (lower left), running from north to south from top center before turning to the southwest. These linear grooves can be seen cutting across other terrain types, like the groove and ridge belts. Like the deep rifts, they appear to be among the youngest features on Enceladus. However, some linear grooves appear to be softened like the craters nearby, suggesting an older age. Ridges have also been observed on Enceladus, though not nearly to the extent as those seen on Europa. Several examples can be seen in the lower left corner of Figure 7. These ridges are relatively limited in extent and are up to one km tall. One-kilometer high domes have also been observed.[37] Given the level of tectonic resurfacing found on Enceladus, it is clear that tectonism has been an important driver of geology on this small moon for much of its history.

  Smooth plains

  Figure 10: The Samarkand Sulci (vertical, right of center), with the northwest portion of Sarandib Planitia to their right; Daryabar Fossa extends up and right from the image bottom

Two units of smooth plains were also observed by Voyager 2. These plains generally have low relief and have far fewer craters than in the cratered terrains and plains, indicating a relatively young surface age.[36] In one of the smooth plain regions, Sarandib Planitia, no impact craters were visible down to the limit of resolution. Another region of smooth plains to the southwest of Sarandib, is criss-crossed by several troughs and scarps. Cassini has since viewed these smooth plains regions, like Sarandib Planitia and Diyar Planitia at much higher resolution. Cassini images show smooth plain regions to be filled with low-relief ridges and fractures. These features are currently interpreted as being caused by shear deformation.[37] The high-resolution images of Sarandib Planitia have revealed a number of small impact craters, which allow for an estimate of the surface age, either 170 million years or 3.7 billion years, depending on assumed impactor population.[2][c]

The expanded surface coverage provided by Cassini has allowed for the identification of additional regions of smooth plains, particularly on Enceladus's leading hemisphere (the side of Enceladus that faces the direction of motion as the moon orbits Saturn). Rather than being covered in low relief ridges, this region is covered in numerous criss-crossing sets of troughs and ridges, similar to the deformation seen in the south polar region. This area is on the opposite side of the satellite from Sarandib and Diyar Planitiae, suggesting that the placement of these regions is influenced by Saturn's tides on Enceladus.[39]

  South polar region

  Figure 11: False-color mosaic of Enceladus taken by the Cassini–Huygens probe on July 14, 2005. It shows the south polar region, as demarcated by the circumpolar set of ridges and troughs in the bottom half of the mosaic

Images taken by Cassini during the flyby on July 14, 2005 revealed a distinctive, tectonically deformed region surrounding Enceladus's south pole. This area, reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges.[2][40] The area has few sizable impact craters, suggesting that it is the youngest surface on Enceladus and on any of the mid-sized icy satellites; modeling of the cratering rate suggests that some regions of the south polar terrain (SPT) are possibly as young as 500,000 years, or younger.[2] Near the center of this terrain are four fractures bounded on either side by ridges, unofficially called "tiger stripes". These fractures appear to be the youngest features in this region and are surrounded by mint-green-colored (in false color, UV–green–near IR images), coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls.[40] Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine-grained water ice from the E ring. Results from the visual and infrared spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is chemically distinct from the rest of the surface of Enceladus. VIMS detected crystalline water ice in the stripes, suggesting that they are quite young (likely less than 1000 years old) or the surface ice has been thermally altered in the recent past.[41] VIMS also detected simple organic compounds in the tiger stripes, chemistry not found anywhere else on the satellite thus far.[42]

One of these areas of "blue" ice in the south polar region was observed at very high resolution during the July 14 flyby, revealing an area of extreme tectonic deformation and blocky terrain, with some areas covered in boulders 10–100 m across.[43]

  Composite map of the south polar region (to 65 deg. S. latitude) of Enceladus (2007)

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 indicate that they are caused by changes in the overall shape of Enceladus. Currently, there are two theories for what could cause such a shift in shape. First, the orbit of Enceladus may have migrated inward (from the article: "the lack of any plausible mechanism for increased flattening"), leading to an increase in Enceladus's rotation rate. Such a shift would have led to a flattening of Enceladus's rotation axis.[2] Another theory suggests that a rising mass of warm, low density material in Enceladus's interior led to a shift in the position of the current south polar terrain from Enceladus's southern mid-latitudes to its south pole.[39] Consequently, the ellipsoid shape of Enceladus would have adjusted to match the new orientation. One consequence of the axial flattening theory is that both polar regions should have similar tectonic deformation histories.[2] However, the north polar region is densely cratered, and has a much older surface age than the south pole.[36] 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.[2]

  Cryovolcanism

  Figure 12: Plumes above the limb of Enceladus feeding the E ring. These appear to emanate from the "tiger stripes" near the south pole. (View from Cassini spacecraft)
  Figure 13: Heat map (within white box) of the thermally active field of fractures, measured at wavelengths between 12 and 16 micrometers, superimposed on a visual-light image. One of the four fractures (right) was only partially imaged.
  Cassini views the backlit south polar plumes of Enceladus from a distance of 190,000 km.

Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon may be geologically active based on its young, reflective surface and location near the core of the E ring.[27] Based on the connection between Enceladus and the E ring, it was thought that Enceladus was the source of material in the E ring, perhaps through venting of water vapor from Enceladus's interior. However, the Voyagers failed to provide conclusive evidence that Enceladus is active today.

Thanks to data from a number of instruments on the Cassini spacecraft in 2005, cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, has been discovered on Enceladus. The first Cassini sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005,[2] though the possibility of the plume being a camera artifact stalled an official announcement. Data from the magnetometer instrument during the February 17, 2005 encounter provided a hint that the feature might be real when it found evidence for an atmosphere at Enceladus. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced by the interaction of ionized particles and magnetic fields, and the frequency of the waves can be used to identify the composition, in this case ionized water vapor.[9] During the next two encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower.[9] The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked for evidence for an atmosphere over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.[10]

Fortuitously, Cassini flew through this gas cloud during the July 14 encounter, allowing instruments like the ion and neutral mass spectrometer (INMS) and the cosmic dust analyzer (CDA) to directly sample the plume. INMS measured the composition of the gas cloud, detecting mostly water vapor, as well as minor components like molecular nitrogen, methane, and carbon dioxide.[11] CDA "detected a large increase in the number of particles near Enceladus", confirming the satellite as the primary source for the E ring.[33] 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.[44]

Visual confirmation of venting came in November 2005, when ISS (Imaging Science Subsystem) imaged geyser-like jets of icy particles rising from the moon's south polar region.[2] (As stated above, the plume was imaged before, in January and February 2005, but additional studies of the camera's response at high phase angles, when the Sun is almost behind Enceladus, and comparison with equivalent high-phase-angle images taken of other Saturnian satellites, were required before this could be confirmed.[45]) The images taken in November 2005 showed the plume's fine structure, revealing numerous jets (perhaps issuing from numerous distinct vents) within a larger, faint component extending out nearly 500 km from the surface, thus making Enceladus the fourth body in the Solar System to have confirmed volcanic activity, along with Earth, Neptune's Triton, and Jupiter's Io.[44] Cassini's UVIS later observed gas jets coinciding with the dust jets seen by ISS during a non-targeted encounter with Enceladus in October 2007.

Additional observations were acquired during a flyby on March 12, 2008. Data on this flyby revealed additional chemicals in the plume, including simple and complex hydrocarbons such as propane, ethane, and acetylene.[46] This finding further raises the potential for life beneath the surface of Enceladus.[47] The composition of Enceladus's plume as measured by the INMS instrument on Cassini is similar to that seen at most comets.[46]

Mimas, another satellite of Saturn, is a geologically dead body, even though it experiences stronger tidal forces than Enceladus. This apparent paradox can be explained by models of tidal heating that predict a low-energy thermal state for Mimas but an excited, high-energy state for Enceladus despite Enceladus being further from Saturn.[48]

  Prior to the ammonia discovery

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanates from pressurized sub-surface chambers, similar to geysers on Earth.[2] Because no ammonia was found in the vented material by INMS or UVIS, which could act as an anti-freeze, such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least 270 K (−3 °C), as illustrated in Figure 14. Pure water would require more energy to melt, either from tidal or radiogenic sources, than an ammonia-water mixture. Another possible method for generating a plume is sublimation of warm surface ice. During the July 14, 2005 flyby, the Composite Infrared Spectrometer (CIRS) found a warm region near the South Pole. Temperatures found in this region range from 85–90 K, to small areas with temperatures as high as 157 K (−116 °C), much too warm to be explained by solar heating, indicating that parts of the south polar region are heated from the interior of Enceladus.[8] Ice at these temperatures is warm enough to sublimate at a much faster rate than the background surface, thus generating a plume. This hypothesis is attractive since the sub-surface layer heating the surface water ice could be an ammonia-water slurry at temperatures as low as 170 K (−103 °C), and thus not as much energy is required to produce the plume activity. However, the abundance of particles in the south polar plume favors the "cold geyser" model, as opposed to an ice sublimation model.[2]

Alternatively, Kieffer et al. (2006) suggest that Enceladus's geysers originate from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the active, tiger stripe fractures.[49] This hypothesis would not require the amount of heat needed to melt water ice as required by the "cold geyser" model, and would explain the lack of ammonia.

  Figure 14: One possible scheme for Enceladus's cryovolcanism

  Ammonia discovery

In July 2009 it was announced that ammonia had been discovered during flybys in July and October 2008.[50]

  Internal structure

  Figure 15: Model of the interior of Enceladus based on recent Cassini findings. The inner, silicate core is represented in brown, while the outer, water-ice-rich mantle is represented in white. The yellow and red colors in the mantle and core respectively represent a proposed diapir under the south pole.[39]

Prior to the Cassini mission, relatively little was known about the interior of Enceladus. However, results from recent flybys of Enceladus by the Cassini spacecraft have provided much needed information for models of Enceladus's interior. These include a better determination of the mass and tri-axial ellipsoid shape, high-resolution observations of the surface, and new insights on Enceladus's geochemistry.

Mass estimates from the Voyager program missions suggested that Enceladus was composed almost entirely of water ice.[27] 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.[2] This density is higher than Saturn's other mid-sized icy satellites, indicating that Enceladus contains a greater percentage of silicates and iron. With additional material besides water ice, Enceladus's interior may have experienced comparatively more heating from the decay of radioactive elements.

Castillo et al. 2005 suggested that Iapetus, and the other icy satellites of Saturn, formed relatively quickly after the formation of the Saturnian sub-nebula, and thus were rich in short-lived radionuclides.[51] These radionuclides, like aluminium-26 and iron-60, have short half-lives and would produce interior heating relatively quickly. Without the short-lived variety, Enceladus's complement of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior, even with Enceladus's comparatively high rock–mass fraction, given Enceladus's small size.[52] 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.[53] Subsequent radioactive and tidal heating would raise the temperature of the core to 1000 K, enough to melt the inner mantle. However, for Enceladus to still be active, part of the core must have melted too, forming magma chambers that would flex under the strain of Saturn's tides. Tidal heating, such as from the resonance with Dione or from libration, would then have sustained these hot spots in the core until the present, and would power the current geological activity.[54]

In addition to its mass and modeled geochemistry, researchers have also examined Enceladus's shape to test whether the satellite is differentiated or not. Porco et al. 2006 used limb measurements to determine that Enceladus's shape, assuming it is in hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.[2] 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).[53]

  Possible water ocean

In late 2008, scientists observed water vapor spewing from Enceladus's surface. This could indicate the presence of liquid water, which might also make it possible for Enceladus to support life.[55] Candice Hansen,[56] a scientist with NASA's Jet Propulsion Lab, headed up a research team on the plumes after they were found to be moving at ~2,189 kilometers per hour (1,360 miles per hour). Since that speed is difficult to attain unless liquids are involved, they decided to investigate the compositions of the plumes.[57]

Eventually it was discovered that in the E-ring about 6% of particles contain 0.5–2% of sodium salts by mass, which is a significant amount. In the parts of the plume close to Enceladus the fraction of "salty" particles increases to 70% by number and >99% by mass. Such particles presumably are frozen spray from the salty underground ocean. On the other hand, the small salt-poor particles form by homogenous nucleation directly from the gas phase. 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 move slowly and mostly fall back onto the surface, while the fast "fresh" particles escape to the E-ring, explaining its salt-poor composition.[58]

The "salty" composition of the plume strongly suggests that its source is a subsurface salty ocean or subsurface caverns filled with salty water.[59] Alternatives such as the clathrate sublimation hypothesis can not explain how "salty" particles form.[58] Additionally, Cassini found traces of organic compounds in some dust grains.[58] Enceladus is therefore a candidate for harboring extraterrestrial life.[60]

The presence of liquid water under the crust implies that there is an internal heat source. It is now thought to be a combination of radioactive decay and tidal heating,[61][62] as tidal heating alone is not sufficient to explain the heat. Mimas, another of Saturn's moons, is closer to the planet and has a much more eccentric orbit, meaning it should be exposed to far greater tidal forces than Enceladus, and yet its old and scarred surface implies that it is geologically dead.[63]

  See also

  Notes and references

Explanatory notes

  1. ^ In US dictionary transcription, US dict: ĕn·sĕl′·ə·dəs, or as Greek Εγκέλαδος
  2. ^ Both are used with roughly equal frequency.
  3. ^ 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.

Citations

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