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Enceladus

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Enceladus

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Discovery
Discovered by William Herschel
Discovered on August 28, 1789
Orbital characteristics
Semimajor axis 237,948 km
Eccentricity 0.0045 [1]
Orbital period 1.370218 d [1]
Inclination 0.019° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Mean diameter 504.2 km (513×503×497 km) [2]
Mass 1.08×1020 kg [2]
Mean density 1.61 g/cm3 [2]
Surface gravity 0.113 m/s2 (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 [3]
Surface temperature
min mean max
32.9 K 75 K 145 K
[4]
Atmospheric characteristics
Pressure trace, significant spatial

variability [5], [6]

Water vapour 91% [7]
Carbon dioxide 3.2% [7]
Nitrogen 4% [7], [6]
Methane 1.7% [7]

Enceladus (en-sel'-ə-dəs, IPA /ɛnˈsɛlədəs/, Greek Εγκέλαδος) is the sixth-largest moon of Saturn, discovered in 1789 by William Herschel.[8] Despite its small size, Enceladus has a wide range of surface types ranging from old, heavily cratered surfaces to young, tectonically-deformed terrain. Outgassing near the south pole, the youthful age of the surface, and the presence of escaping internal heat indicate that Enceladus, and the south polar region in particular, is geologically active today. 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.[9]

It is the most shiny place in the Solar System.[1][2]

Name

Enceladus is named after the mythological Enceladus, one of the Giants of Greek mythology. It is also designated Saturn II.

The name Enceladus, and the names of all seven satellites of Saturn then known, were suggested by Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.[10]

The geological features on Enceladus are named after people and places from The Arabian Nights.

Orbital characteristics

File:Enceladus orbit top.jpg
Figure 1: View of Enceladus' orbit from above Saturn's north pole

Enceladus orbits Saturn at a distance of 238,000 km, between the orbits of Mimas and Tethys, requiring 32.9 hours to revolve once. Enceladus is currently in a 2:1 mean motion orbital resonance with Dione. This resonance would maintain Enceladus' orbital eccentricity (0.0047) and provide 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 it at some point was in a 1:4 forced secondary spin-orbit libration.[2] This libration, like the resonance with Dione, could have provided Enceladus an additional heat source.


Interior

Figure 2: False-color view of Enceladus showing the various terrain types.

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.[11] However, based on the effects of Enceladus's gravity on Cassini, the navigation team determined that its mass is 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 radiogenic heating.

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.[12] These radionuclides, like aluminium-26 and iron-60, have comparatively 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' relatively high rock-mass fraction, given Enceladus' small size (505 km in diameter, see Figure 3).[13] 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.[14]

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

Surface

File:PIA07724.jpg
Figure 3: Picture comparing Enceladus's size to the UK

Voyager 2, in August of 1981, was the first spacecraft to take topographically meaningful observations of Enceladus. The title image above shows the highest resolution image taken of Enceladus by Voyager 2. Examination of this mosaic revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth (young) terrain, and lanes of ridged terrain that often border the smooth areas.[11] In addition, extensive linear cracks were observed crossing both the smooth and cratered terrains. Given the relative lack of craters on the smooth plains, these regions are probably less than 100 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 highest albedo of any body in the solar system with a visual geometric albedo of 1.41.[3] Because it reflects so much sunlight, the mean surface temperature at noon is -198 °C (somewhat colder than other Saturnian satellites).[4]

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 cratered terrain, suggesting extensive deformation in the time since crater formation.[15] 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 4: Degraded craters on Enceladus, imaged by Cassini, 17 February 2005.

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 types of cratered terrain (CT) were discovered: the CT1-unit consisting of numerous, viscously relaxed craters, the CT2-unit consisting of slightly fewer, less-deformed craters, and the cratered plains (CP), containing fewer and smaller craters than the other two.[11] Though the high crater density of the CT1-unit makes it the oldest region on Enceladus, it is still younger than the youngest regions on Saturn's other mid-sized icy satellites, like Rhea, again suggesting that even Enceladus's oldest terrains are younger than most surfaces in the rest of the Saturn system (such as those on Titan).[11]

Recent Cassini observations have provided a much closer look at the last two cratered units (CT2 and CP). These high-resolution observations, like Figure 4, reveal that many of Enceladus's craters are heavily deformed, either through viscous relaxation, fracturing, or "softening".[16] Viscous relaxation causes craters formed in water ice to deform over geologic time scales. The rate at which this occurs is dependent on the temperature of the ice: warmer ice is less viscous and thus easier to deform. 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 4). Dunyazad, the large crater seen just left of top center in Figure 6, is another example of a crater on Enceladus with a domed floor. In addition, many craters on Enceladus have been heavily modified by tectonic fractures. The 10-kilometre-wide crater right of bottom center in Figure 6 is a prime example: thin fractures, several hundred metres to a kilometre wide, have heavily deformed the rim and floor of the crater. 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.

Crater "softening" can be seen within craters in the CP and smooth plain units (Figure 8). Many of these craters have a smooth appearance, lacking many of the sharp relief features observed in many of Enceladus's tectonic features. Some apparently older fractures also exhibit this softened look, like some seen in the figure at top, and at higher resolution in Figure 8. It is not yet known what causes craters to degrade in this way—perhaps some process related to Enceladus's regolith.[16] Given the location of many of the softened craters, it is possible the process that smooths out these craters is related to the formation process of many of Enceladus's younger terrains.

Tectonics

File:EN003 Enceladus Mosaic.jpg
Figure 5: Enceladus' Europa-like surface, imaged by Cassini, 17 February 2005.

Voyager 2 found several types of tectonic features on Enceladus, including linear troughs and belts of curvilinear grooves.[11] 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 that can run up to 200 kilometres, 5-10 km wide, and up to one kilometre deep. Figure 5 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 6. Such features appear relatively young, given their crosscutting relationships with other tectonic features, and their sharp topographic relief with prominent outcrops along the cliff faces.

Figure 6: False-color 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 regions of smooth plains and more heavily cratered regions.[11] An example of this terrain type can be seen in Figures 4 and 8 (in this case, a feature known as Samarkand Sulci). Grooved terrain such as Samarkand Sulci is reminiscent of grooved terrain on Ganymede. However, unlike the terrain on Ganymede, the grooved lanes on Enceladus are 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 have a convex cross-section with fractures or ridges running the length of the feature. Cassini also found 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.[16]

Figure 7: 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 tectonic deformation styles. Figure 7 shows narrow fractures (still several hundred meters wide) that were first discovered by the Cassini spacecraft. The fractures tend to form subparallel groupings and are found largely within cratered terrain. These fractures demonstrate focusing within craters, suggesting that the propagation of these fractures is heavily influenced by the upper few hundred meters of weakened ground generated during the formation of Enceladus's craters.[16] 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, Figure 8 (lower left), and Figure 2, running from north to south from top center before turning to the southwest. These linear grooves can be seen cross-cutting other terrain types, like the curvilinear groove lanes. Like the deep fractures, they appear to be among the youngest features on Enceladus. However, some linear grooves, like those seen in the image at top and in Figure 8, appear to be softened like the craters nearby. 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 5. These ridges are relatively limited in extent and are up to one kilometer tall. One-kilometer high domes have also been observed.[16] Finally, several regions on Enceladus have a background of various styles of tectonic deformation. This tortured terrain, best seen in Figure 5, sometimes appears similar to the ridged plains of Europa (giving rise to the suggestion that Enceladus might have a sub-surface ocean of liquid water, as is theorized with Europa), while other areas, like those seen near the top of Figure 5, appear like nothing else in the solar system. 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 8: Samarkand Sulci on Enceladus. Taken by Cassini on 17 February 2005.

The final terrain type noted in Voyager 2 images are smooth plains. Smooth Plains generally have low relief and few craters, again indicating a relatively young surface age.[11] Cassini has since viewed two of the most prominent regions of smooth plains, Sarandib Planitia and Diyar Planitia at much higher resolution, examples of which can be seen in Figure 2 (left side) and Figure 8 (upper right). Cassini images show smooth plain regions to be filled with low-relief ridges and fractures. These features have currently been interpreted as being caused by shear deformation.[16] Crater counts using Cassini images have suggested ages for Sarandib Planitia between 170 million years assuming a constant impactor flux and 3.7 billion years assuming a lunar-like flux.[2]

South polar region

See also: Tiger_Stripes (Enceladus)
Figure 9: False-color image of Enceladus taken by the Cassini-Huygens probe July 14, 2005.

Images taken by Cassini during the flyby on July 14, 2005 revealed a new type of "smooth" plain. This region, surrounding Enceladus' south pole and reaching as far north as 60° south latitude, is covered in tectonic fractures and ridges.[17] 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.[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.[17] Here the "blue" ice is on a flat surface, indicating that the region is young enough not to have been coated by fine particulates from the E ring. Results from the Visual and Infrared Spectrometer (VIMS) instrument suggest that the green-colored material surrounding the tiger stripes is spectrally distinct from the rest of the surface of Enceladus. VIMS detected crystalline ice in the stripes, suggesting that they are quite young (likely less than 1000 years old).[18] VIMS also detected simple organic compounds in the tiger stripes, chemistry not found anywhere else on the satellite thus far.[18]

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.[19]

The boundary of the South Polar Region is marked by a pattern of Y- and V-shaped regions of parallel ridges and valleys. The shape, orientation, and location of these features indicate that they are caused by changes in the shape of Enceladus. Changes in the rotation rate of Enceladus leading to a flattening of the rotation axis[2] or true polar wander caused by a water or silicate diapir in Enceladus' interior[20] are two possible mechanisms for this shape change. Similar features have not been observed in the north polar region, as would be expected from the axial flattening theory.[2] In fact, the north polar region is one of the most heavily cratered regions on Enceladus.[11] 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 crater terrains.[2]

Cryovolcanism

Figure 10: 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 cryovolcanically active based on its relatively youthful surface, location near the core of the E ring, and high albedo—significantly higher than that of any other mid-sized icy Saturnian satellite.[11] 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.

Data from a number of instruments on the Cassini spacecraft during three encounters with Enceladus in 2005 confirmed this hypothesis. First, the magnetometer instrument during the February 17, 2005 encounter found evidence of an atmosphere. The magnetometer observed an increase in the power of ion cyclotron waves near Enceladus. These waves are produced through the ionization of particles within a magnetosphere and the frequency of the waves can be used to identify the composition, in this case ionized water vapor.[5] Thanks to the low altitude of the July 14 flyby and improved modeling results of data from the previous two flybys, 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.[5] The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. UVIS failed to detect an atmosphere above Enceladus during the February encounter, but did detect water vapor during an occultation over the south polar region during the July encounter.[6] The Ion and Neutral Mass Spectrometer (INMS) instrument, during the July 14 encounter when Cassini flew through the gas cloud, also detected a concentration of water vapor, as well as molecular nitrogen, methane, and carbon dioxide.[7] Finally, the Cosmic Dust Analyzer (CDA) instrument "detected a large increase in the number of particles near Enceladus," confirming the satellite as the primary source for the E ring.[21] Analysis of the CDA and INMS data suggest that the material Cassini flew through during the July encounter was being vented from near the Tiger stripes. [22]

Figure 11: South polar brightness temperatures as measured by CIRS, overlain on a false-color 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.[2] The plume was imaged before, in January and February 2005, but additional studies on the camera's response at high phase angles were required before they could be confirmed.[23] The images taken in November 2005 show numerous jets (perhaps due to several 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.[22]

File:PIA07799.jpg
Figure 12: Diagram of Enceladus's cryovolcanism.

The combined analysis of imaging, mass spectrometry, and magnetospheric data suggests that the observed south polar plume emanate from pressurized sub-surface chambers, similar to geysers on Earth.[2] Because no ammonia was found in the vented material by INMS or UVIS, such a heated, pressurized chamber would consist of nearly pure liquid water with a temperature of at least 270 K, as illustrated in Figure 12. 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 Spectromer (CIRS) instrument found a warm region near the South Pole. This region was found to have a brightness temperature of 85-90 K, 15 kelvins warmer than expected from solar heating alone. In addition, color temperatures of several features in the region indicate small areas at greater than 110 K, some as high as 157 K, too warm to be explained by even the most generous of solid-state greenhouse models, indicating that parts of the south polar region are warmed by internal heat.[4] 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.[2]

Named surface features

Main article: List of geological features on Enceladus

Features on Enceladus are named after characters and places from the Arabian Nights. Official names have been given to features in the following terrain types on Enceladus:

All names and officially recognized feature types were defined in 1982, shortly after the Voyager flybys. Features discovered by the Cassini mission have not yet received names.

Exploration of Enceladus

Planned Cassini encounters with Enceladus [24]
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
Figure 13: Cassini shows Enceladus against a backdrop of Saturn and its rings

The two Voyager spacecraft obtained the first spacecraft images of Enceladus. Whereas Voyager 1 only got a distant look at Enceladus in December 1980, Voyager 2 in August 1981 was able to take much higher resolution images of this satellite, revealing the youthful nature of much of its surface.[11]

Detailed reconnaissance would have to wait until the arrival of the Cassini spacecraft on June 30, 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 kilometers 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 have been performed of Enceladus, yielding significant results on Enceladus' surface as well as the discovery of water vapor venting from the geologically active South Polar Region.

Notes

  1. ^ a b Celestia Solar System Definition File. Retrieved March 22, 2006.
  2. ^ a b c d e f g h i j k l m n o C. C. Porco et al., Science 311, 1393 (2006).
  3. ^ a b A. Verbiscer et al., Icarus 173, 66 (2005).
  4. ^ a b c J. R. Spencer et al., Science 311, 1401 (2006).
  5. ^ a b c M. K. Dougherty et al., Science 311, 1406 (2006).
  6. ^ a b c C. J. Hansen et al., Science 311, 1422 (2006).
  7. ^ a b c d e J. H. Waite et al., Science 311, 1419 (2006).
  8. ^ Planetary Body Names and Discoverers. Retrieved March 22, 2006.
  9. ^ Cassini Images Of Enceladus Suggest Geysers Erupt Liquid Water At The Moon’s South Pole. Retrieved March 22, 2006.
  10. ^ W. Lassell, MNRAS 8, 42 (1847).
  11. ^ a b c d e f g h i j D. A. Rothery, Satellites of the Outer Planets (Oxford University Press, New York, 1999), p. 178-184
  12. ^ J. C. Castillo et al., Eos Trans. AGU 82, Abstract P32A-01 (2005).
  13. ^ J. C. Castillo et al., Lunar Planet. Sci. Conf. XXXVII, Abstract 2200 (2006).
  14. ^ D. L. Matson et al., Lunar Planet. Sci. Conf. XXXVII, Abstract 2219 (2006).
  15. ^ J. A. Rathbun et al., Eos Trans. AGU 82, Abstract P32A-03 (2005).
  16. ^ a b c d e f E. P. Turtle et al. Cassini CHARM Telecon. 28 April 2005
  17. ^ a b Enceladus in False Color. Retrieved March 22, 2006.
  18. ^ a b R. H. Brown et al., Science 311, 1425 (2006).
  19. ^ Boulder-Strewn Surface. Retrieved March 22, 2006.
  20. ^ R. T. Pappalardo et al., Lunar Planet. Sci. Conf. XXXVII, Abstract 2113 (2006).
  21. ^ F. Spahn et al., Science 311, 1416 (2006).
  22. ^ a b NASA's Cassini Images Reveal Spectacular Evidence of an Active Moon. 6 December 2005. Retrieved March 22, 2006.
  23. ^ Spray Above Enceladus. Retrieved March 22, 2005
  24. ^ Planetary Society. Cassini's Tour of the Saturn System. Retrieved March 31, 2006.

References

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