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Enceladus (moon)

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: Enceladus

                        larger version
                          Discovery
   Discovered by           William Herschel
   Discovered on           August 28, 1789
                   Orbital characteristics
   Semimajor axis          237,948 km
   Eccentricity            0.0047
   Orbital period          1.370218 d
   Inclination             0.019° (to Saturn's equator)
   Satellite of            Saturn
                   Physical characteristics
   Mean diameter           504.2 km (513.2×502.8×496.6 km)

                           (0.0395 Earths)
   Mass                    1.08×10^20 kg

                           (1.8×10^-5 Earths)
   Mean density            1.61 g/cm^3
   Surface gravity         0.113 m/s^2 (0.0115 g)
   Escape velocity         0.241 km/s (866 km/h)
   Rotation period         synchronous
   Axial tilt              zero
   Albedo/Geometric albedo 0.99/1.41
   Surface temperature
                            min   mean  max
                           32.9 K 75 K 145 K
                 Atmospheric characteristics
   Pressure                trace, significant spatial

                           variability ^,
   Water vapour            91%
   Carbon dioxide          3.2%
   Nitrogen                4% ^,
   Methane                 1.7%

   Enceladus (en-sel'-ə-dəs, IPA: /ɛnˈsɛlədəs/), discovered in 1789 by
   William Herschel, is the sixth-largest moon of Saturn. 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 Enceladus is only 500 kilometers
   in diameter and reflects almost 100% 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.

   The Cassini spacecraft of the mid- to late 2000s acquired additional
   data on Enceladus, answering a number of the mysteries opened by the
   Voyager spacecraft and starting a few new ones. Cassini performed
   several close flybys of Enceladus in 2005, 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. The discovery
   of the plume has added further weight to the argument that material
   released from Enceladus is the source of the E-ring.

Name

   Enceladus is named after the Titan Enceladus of Greek mythology. It is
   also designated Saturn II or S II Enceladus. 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. He specifically chose these names because Saturn,
   known in Greek mythology as Chronos, was the leader of the Titans.

   Features on Enceladus are named after characters and places from the
   Arabian Nights by the International Astronomical Union (IAU). Impact
   craters are named after characters from Arabian Nights, 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. Examples of approved names
   include Samarkand Sulci, Aladdin crater, Daryabar Fossa, and Sarandib
   Planitia.

Exploration

   Figure 1: Enceladus as seen by Voyager 2, August 26, 1981
   Enlarge
   Figure 1: Enceladus as seen by Voyager 2, August 26, 1981

   Enceladus was discovered by Fredrick William Herschel on August 28,
   1789, during the first use of his new 1.2-meter telescope, then the
   largest in the world.^, Herschel first observed Enceladus in 1787, but
   in his smaller, 16.5- cm telescope, the moon was not recognized. Due to
   Enceladus' faint apparent magnitude (+11.8^m) and its proximity to much
   brighter Saturn and its rings, Enceladus is difficult to observe from
   Earth, requiring a telescope with a mirror of at least 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 ring crossing, when Earth is
   within the ring plane during Saturnian equinox. During these periods,
   Enceladus is easier to observe due to the reduction in glare from the
   rings.

   Prior to the Voyager program, the view of Enceladus improved little
   from the dot first observed by Herschel. Only its orbital
   characteristics, along with an estimation of its mass, density, and
   albedo, were known.
    Planned Cassini encounters with Enceladus
          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     23
   June 30, 2008      101,000

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

   Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981,
   allowing much higher resolution images of this satellite. These images
   revealed the youthful nature of much of its surface, as seen in Figure
   1. 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.

   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 at right. So far,
   three close flybys of Enceladus have been performed, yielding
   significant information concerning Enceladus' surface, as well as the
   discovery of water vapor venting from the geologically active South
   Polar Region. These discoveries have prompted the adjustment of
   Cassini's flight plan to allow closer flybys of Enceladus, including an
   encounter in March 2008 which will take the probe to within 23 km of
   the moon's surface.

Characteristics

Orbit

   Figure 2: View of Enceladus' orbit (highlighted in red) from above
   Saturn's north pole
   Enlarge
   Figure 2: View of Enceladus' 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
   centre 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' orbital eccentricity (0.0047) and
   provides a heating source for Enceladus' geologic activity.

   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. This libration, like the resonance with Dione,
   could have provided Enceladus with an additional heat source.

Interaction with E Ring

   Figure 3: View of Enceladus' orbit from the side, showing Enceladus in
   relation to Saturn's E ring
   Enlarge
   Figure 3: View of Enceladus' orbit from the side, showing Enceladus in
   relation to Saturn's 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 proven
   by Cassini's flyby.

   There are actually two distinct mechanisms feeding the ring with
   particles. The first, and probably the most important, source of
   particles comes from the cryovolcanic plume in the South polar region.
   While a majority of particles fall back to the surface, some of them
   escape Enceladus' gravity and enter orbit around Saturn, since
   Enceladus' 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' size compared to the UK
   Enlarge
   Figure 4: Enceladus' size compared to the UK

   Enceladus is a relatively small satellite, with a mean diameter of
   505 km, making it only one-seventh as large as Earth's own Moon. Its
   dimensions would allow the satellite to be placed inside a state such
   as Arizona or Colorado, or the British Isles (see picture), although as
   a spherical object its surface area is much greater, just over 800,000
   square km, 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 (5550 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' exact dimensions, calculated from pictures taken by
   Cassini's ISS instrument, are of 513(a)×503(b)×497(c) km, with (a)
   corresponding to the diameter between the points on the surface facing
   toward and away from Saturn, (b) to the diameter between the points
   that face toward and away from the direction of Enceladus' orbital
   motion and (c) to the distance between the poles. Therefore, Enceladus
   has a shape of a flattened ellipsoid.

Surface

   Voyager 2, in August of 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. In
   addition, extensive linear cracks and scarps were observed. Given the
   relative lack of craters on the smooth plains, these regions are
   probably less than a few hundred million years old. Accordingly,
   Enceladus must have been recently active with " water volcanism" or
   other processes that renew the surface. The fresh, clean ice that
   dominates its surface gives Enceladus the most reflective surface of
   any body in the solar system with a visual geometric albedo of 1.41.
   Because it reflects so much sunlight, the mean surface temperature at
   noon only reaches -198 °C (somewhat colder than other Saturnian
   satellites).

   Observations during three flybys by Cassini on February 17, March 9,
   and July 14 of 2005 revealed Enceladus' 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
   after the craters were formed. 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.

Impact craters

   Figure 5: Degraded craters on Enceladus, imaged by Cassini, 17 February
   2005. Hamah Sulci can be seen running from left to right along the
   bottom quarter of the image. Craters from Enceladus' ct2 and cp
   cratered units are visible above Samarkand Sulci
   Enlarge
   Figure 5: Degraded craters on Enceladus, imaged by Cassini, 17 February
   2005. Hamah Sulci can be seen running from left to right along the
   bottom quarter of the image. Craters from Enceladus' ct[2] and cp
   cratered units are visible above Samarkand Sulci

   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 ct[1] and ct[2], both containing numerous
   10-20 km-wide craters though differing in the degree of deformation, to
   cp consisting of lightly cratered plains. 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
   ct[2] and cp cratered units. These high-resolution observations, like
   Figure 5, reveal that many of Enceladus' craters are heavily deformed
   through viscous relaxation and fracturing. Viscous relaxation causes
   craters and other topographic features formed in water ice to deform
   over geologic time scales due to the effects of gravity, 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 centre just below the terminator in Figure 5). Dunyazad, the
   large crater seen in Figure 7 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
   centre in Figure 7 is a prime example: thin fractures, several hundred
   metres to a kilometre 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. These two deformation
   styles—viscous relaxation and fracturing—demonstrate that, while
   cratered terrains are the oldest regions on Enceladus due to their high
   crater retention, nearly all craters on Enceladus are in some stage of
   degradation.

Tectonics

   Figure 6: Enceladus' Europa-like surface near the fracture Labtayt
   Sulci, imaged by Cassini, 17 February 2005
   Enlarge
   Figure 6: Enceladus' Europa-like surface near the fracture Labtayt
   Sulci, imaged by Cassini, 17 February 2005

   Voyager 2 found several types of tectonic features on Enceladus,
   including troughs, scarps, and belts of grooves and ridges. 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 6 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 7. 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: False-color view of Enceladus' surface, showing several
   tectonic and crater degradation styles. Taken by Cassini on 9 March
   2005
   Enlarge
   Figure 7: False-colour view of Enceladus' surface, showing several
   tectonic and crater degradation styles. Taken by Cassini on 9 March
   2005

   Another example 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. An example of this terrain type can be seen in
   Figures 5 and 9 (in this case, a feature known as Samarkand Sulci).
   Grooved terrain such as 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 Samarkand Sulci have revealed
   intriguing dark spots (125 and 750 meters wide), which appear to run
   parallel to narrow fractures. Currently, these spots are interpreted as
   collapse pits within these ridged plain belts.
   Figure 8: High-resolution mosaic of Enceladus' surface, showing several
   tectonic and crater degradation styles. Taken by Cassini on 9 March
   2005.
   Enlarge
   Figure 8: High-resolution mosaic of Enceladus' surface, showing several
   tectonic and crater degradation styles. Taken by Cassini on 9 March
   2005.

   In addition to deep fractures and grooved lanes, Enceladus has several
   other types of tectonic terrain. Figure 8 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.^, 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 9 (lower left),
   running from north to south from top centre 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 6. These ridges
   are relatively limited in extent and are up to one kilometer tall.
   One-kilometer high domes have also been observed. Given the level of
   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 9: Samarkand Sulci on Enceladus. Taken by Cassini on 17 February
   2005. The northwest portion of Sarandib Planitia can be seen at right
   Enlarge
   Figure 9: Samarkand Sulci on Enceladus. Taken by Cassini on 17 February
   2005. The northwest portion of Sarandib Planitia can be seen at right

   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. 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. 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.^,

   The expanded surface coverage provided by Cassini has allowed for the
   identification of additional regions of smooth plains, particularly on
   Enceladus' 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.

South polar region

   Figure 10: False-color mosaic of Enceladus taken by the Cassini-Huygens
   probe July 14, 2005. Shows the south polar region, as demarcated by the
   circumpolar set of ridges and troughs in the bottom half of the mosaic
   Enlarge
   Figure 10: False-colour mosaic of Enceladus taken by the
   Cassini-Huygens probe July 14, 2005. 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' south
   pole. This area, reaching as far north as 60° south latitude, is
   covered in tectonic fractures and ridges. The area has few sizable
   impact craters, suggesting that it is the youngest surface on Enceladus
   and on any of the mid-sized icy satellites; modeling of the cratering
   rate suggests that the region is less than 10-100 million years old.
   Near the centre 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 colour, UV-Green-near IR images),
   coarse-grained water ice, seen elsewhere on the surface within outcrops
   and fracture walls. Here the "blue" ice is on a flat surface,
   indicating that the region is young enough not to have been coated by
   fine-grained water ice from E ring. Results from the Visual and
   Infrared Spectrometer (VIMS) instrument suggest that the green-colored
   material surrounding the tiger stripes is chemically distinct from the
   rest of the surface of Enceladus. VIMS detected crystalline water ice
   in the stripes, suggesting that they are quite young (likely less than
   1,000 years old) or the surface ice has been thermally altered in the
   recent past. VIMS also detected simple organic compounds in the tiger
   stripes, chemistry not found anywhere else on the satellite thus far.

   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 meters across.

   The boundary of the South Polar Region is marked by a pattern of
   parallel, Y- and V-shaped ridges and valleys. The shape, orientation,
   and location of these features 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, leading to an increase in
   Enceladus' rotation rate. Such a shift would have lead to a flattening
   of Enceladus' rotation axis. Another theory suggests that a rising mass
   of warm, low density material in Enceladus' interior led to a shift in
   the position of the current south polar terrain from Enceladus'
   southern mid-latitudes to its south pole. 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
   region should have the similar tectonic deformation histories. However,
   the north polar region is densely cratered, and has a much older
   surface age than the south pole. Thickness variations in Enceladus'
   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 they lead into are correlated with younger
   terrain with presumably thinner lithospheres. The V-shaped cusps are
   adjacent to older, more heavily cratered terrains.

Cryovolcanism

   Figure 11: Plumes above the limb of Enceladus feeding the E ring. These
   appear to emanate from the "tiger stripes" near the south pole
   Enlarge
   Figure 11: Plumes above the limb of Enceladus feeding the E ring. These
   appear to emanate from the "tiger stripes" near the south pole

   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.
   Based on the connection between Enceladus and the E ring, it was
   thought that Enceladus was the source of material from the E ring,
   perhaps through venting of water vapor from Enceladus' 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
   during three encounters with Enceladus in 2005, cryovolcanism, where
   water and other volatiles are the materials erupted instead of silicate
   rock, has been discovered on Enceladus. Data from the magnetometer
   instrument during the February 17, 2005 encounter provided the first
   hints 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.
   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.
   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.

   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 Analyser (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. CDA "detected a large
   increase in the number of particles near Enceladus," confirming the
   satellite as the primary source for the E ring. Analysis of the CDA and
   INMS data suggest that the gas cloud Cassini flew through during the
   July encounter, and was observed from a distance by the magnetometer
   and UVIS, was actually a water-rich cryovolcanic plume, originating
   from vents near the south pole.
   Figure 12: South polar brightness temperatures as measured by CIRS,
   overlain on a false-color image of the tiger stripes
   Enlarge
   Figure 12: South polar brightness temperatures as measured by CIRS,
   overlain on a false-colour image of the tiger stripes

   Visual confirmation of venting came in November 2005, when Cassini
   imaged fountain-like plumes of icy particles rising from the moon's
   south polar region. The plume was imaged before, in January and
   February 2005, but additional studies on the camera's response at high
   phase angles, when the sun is almost behind Enceladus, were required
   before they could be confirmed. The images taken in November 2005
   showed the plume's fine structure, revealing numerous jets (perhaps due
   to 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.
   Figure 13: Diagram of Enceladus's cryovolcanism.
   Enlarge
   Figure 13: Diagram of Enceladus's cryovolcanism.

   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. 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, as illustrated in Figure 13. 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, much too warm to be explained by
   solar heating, indicating that parts of the south polar region are
   heated from the interior of Enceladus. 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, 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.

Internal structure

   Figure 14: 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.
   Enlarge
   Figure 14: 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.

   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. 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/cm^3. 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. 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' comparatively high rock-mass
   fraction, given Enceladus' small size. Given Enceladus's relatively
   high rock-mass fraction, the proposed enhancement in ^26Al and ^60Fe
   would result in a differentiated body, with an icy mantle and a rocky
   core. Subsequent radioactive and tidal heating would raise the
   temperature of the core to 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.

   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. Further work
   on non-hydrostatic equilibrium models of the interior is needed to
   reconcile this problem.

Sky from Enceladus

   Figure 15: An artist's view of Enceladus' sky.
   Enlarge
   Figure 15: An artist's view of Enceladus' sky.

   Seen from Enceladus, Saturn would have a visible diameter of almost
   30°, sixty times more than the Moon visible from Earth. Moreover, since
   Enceladus rotates synchronously with its orbital period and therefore
   keeps one face pointed toward Saturn, the planet never moves in
   Enceladus' sky (albeit with slight variations coming from the orbit's
   eccentricity), and cannot be seen from the far side of the satellite.

   Saturn's rings would be seen from an angle of only 0.019° and would be
   almost invisible, but their shadow on Saturn's disk would be clearly
   distinguishable. Like our own Moon from Earth, Saturn itself would show
   regular phases. From Enceladus, the Sun would have a diameter of only
   3.5 minutes of arc, nine times smaller than that of the Moon as seen
   from Earth.

   An observer located on Enceladus could also observe Mimas (the biggest
   satellite located inside Enceladus' orbit) transit in front of Saturn
   every 72 hours on average.
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