Enceladus (moon)

Enceladus

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Discovery
Discovered by	William Herschel
Discovered in	August 28, 1789
Orbital characteristics
Semimajor axis	237,948 km
Eccentricity	0.0045 [1]
Orbital period	1.370218 d [2]
Inclination	0.019° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Mean diameter	504.2 km (513×503×497 km) [3]
Mass	1.08×1020 kg 1
Mean density	1.61 g/cm3
Surface gravity	0.113 m/s2
Escape velocity	0.241 km/s (866 km/h)
Rotation period	synchronous
Axial tilt	zero
Albedo	0.99±0.06 [4]
Surface temperature	min mean max 32.9 K 75 K 145 K [5]
Atmospheric characteristics
Pressure	trace, significant spatial variability [6]
Water vapour	65% [7]
Hydrogen	20% [8]
Other	CO2, CO, N2 [9]

Enceladus (en-sel'-ə-dəs, IPA /ɛnˈsɛlədəs/, Greek Ενκέλαδος) is a moon of Saturn discovered in 1789 by William Herschel [10]. 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 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.

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 ( [11]).

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

Physical characteristics

Interior

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' 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' geochemistry.

Mass estimates from the Voyager mission suggested that Enceladus was composed almost entirely of water ice (Rothery 1999). However, based on the effects of Enceladus' gravity on the Cassini Spacecraft, the Cassini Navigation team determined that its mass is much higher than previously thought, yielding a density of 1.61 g/cm³ (Turtle et al. 2005). 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' 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 so-called "short-lived radionuclides". 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' compliment of long-lived radionuclides would not have been enough to prevent rapid freezing of the interior (Castillo et al. 2006). Given Enceladus' 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, resulting perhaps from the 2:1 orbital resonance with Dione, would then have sustained these hot spots in the core until the present, and would power the current geological activity (Matson et al. 2006).

In addition to its mass and modeled geochemistry, researchers have also examined Enceladus' shape to test whether the satellite is differentiated or not. Thomas et al. 2006 used limb measurements to determine that Enceladus' shape, assuming it is in hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction with the geological and geochemical evidence that suggests otherwise. Further work on non-hydrostatic equilibrium models of the interior will be needed to reconcile this problem.

Surface

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 terrain (Rothery 1999). In addition, extensive linear cracks were observed crossing the smooth and cratered terrain. Given the relative lack of craters within 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 (Visual geometric albedo of 0.99). Because it reflects so much sunlight, the mean surface temperature at noon is only -198 °C.

Observations during three flybys by Cassini on February 17, March 9, and July 14, 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 since the craters 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 [12].

Impact Craters

Impact cratering is a common occurrence on many solar system bodies. Much of Enceladus' surface is covered with craters at various densities and levels of degradation. From Voyager 2 observations, three different types of cratered terrain 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)-unit containing fewer and smaller craters than the other two, cratered units (Rothery 1999). Though the high crater density of the ct1-unit makes it the oldest region on Enceladus, it is still less than the youngest regions on Saturn's other mid-sized icy satellites, like Rhea, again suggesting that even Enceladus' oldest terrains are younger than most surfaces in the rest of the Saturn system (except Titan) (Rothery 1999).

Recent Cassini observations have provided a much closer look at the last two cratered units, ct2 and cp. These high-resolution observations, like Figure 2, reveal that many of Enceladus' craters are heavily deformed, either through viscous relaxation, fracturing, or "softening" (Turtle et al. 2005). 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 up floors or to be recognized as craters only by a raised, circular rim (seen at center just below the terminator in Figure 2). Dunyazad, the large crater seen just left of top center in Figure 4, is another example of a crater on Enceladus with a domed-up 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 4 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 age, nearly all craters on Enceladus are in some stage of degradation.

Crater "softening" can be seen within craters in the cp and smooth plains units (Figure 6). Many of these craters have a smooth appearance, lacking many of the sharp relief features seen within many of Enceladus' tectonic features (though some apparently older fractures also exhibit this "softened" look, like some of those seen in figure at top and at higher resolution in Figure 6). It is not yet known what causes craters to degrade in this way— perhaps some process related to Enceladus' regolith (Turtle et al. 2005). Given the location of many of the "softened" craters, it is possible that the process that smooths out these craters is related to the formation process of many of Enceladus' younger terrains.

Tectonics

Voyager 2 found several types of tectonic features on Enceladus, including linear troughs and belts of curvilinear grooves. 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 two hundred kilometres long, 5-10 km wide, and up to a kilometre deep. Figure 3 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 4. Such features appear relatively young given their cross-cutting relationships with other tectonic features and their sharp, topographic relief with prominent blue outcrops along the cliff faces.

Another example of tectonism on Enceladus is grooved terrain. These lanes of curvilinear grooves and ridges were first discovered by Voyager 2. These bands often separate regions of smooth plains and more heavily cratered regions (Rothery 1999). The lower portion of Figure 2 and the middle of Figure 6 show an example of this terrain type (in this case, a feature known as Samarkand Sulci), as seen at higher resolution by Cassini. 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 down the length of the feature. Cassini also found intriguing dark, 125- to 750- metre-wide spots, which appear to run parallel to narrow fractures. Currently, these spots are interpreted as collapse pits within these ridged plain belts (Turtle et al. 2005).

In addition to deep fractures and grooved lanes, Enceladus has several other tectonic deformation styles. Figure 5 shows narrow, several-hundred-metre-wide fractures 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 metres of weakened ground formed during the formation of Enceladus' craters. Another example of tectonic feature 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 6 (lower left), and Figure 1, running from north-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 6, 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 been in the lower left corner of Figure 3. These ridges are relatively limited in extent and are one kilometre tall. One-kilometre tall domes have also been observed (Turtle et al. 2005). Finally, several regions on Enceladus have a background of various styles of tectonic deformation. This tortured terrain, best seen in Figure 3, sometimes appears similar to the ridged plains of Europa (given rise to the suggestion that Enceladus might have a liquid water sub-surface ocean, as is assumed with Europa), while other areas, like those seen near the top of Figure 3, 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

The final terrain type noted in Voyager 2 images are smooth plains. Smooth Plains generally have low relief and few craters, again indicating an age of perhaps only a few hundred million years old (Rothery 1999). 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 1 (left side) and Figure 6 (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 (Turtle et al. 2005).

South Polar Region

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 [13]. 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 (Johnson 2005). 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 feature in this region and are surrounded by blue-colored, coarse-grained water ice, seen elsewhere on the surface within outcrops and fracture walls [14]. Here the "blue" ice is on flat surface, indicating that the region is young enough not to have been coated by fine-particulates from the E-ring. VIMS results suggest that the blue-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) [15]. VIMS also detected simple organic compounds in the "tiger stripes", chemistry not found anywhere else on the satellite thus far [16]

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 metres across [17].

The boundary of the south polar region is marked by a pattern of Y- or 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 Enceladus' rotation rate and orbital distance from Saturn over time [18] are a likely cause. However, similar features have not been observed in the north polar region, as would be expected from this theory (Rathbun et al. 2005). In fact, the north polar region is one of the most heavily cratered regions on Enceladus (Rothery 1999).

Cryovolcanism

Following the Voyager encounters with Enceladus in the early 1980s, scientists postulated that the moon maybe cryovolcanically active based on the relatively youthful surface, a higher albedo than the other mid-sized icy satellites of Saturn, and its 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.

Data from a number of instruments on the Cassini spacecraft aided in confirming this hypothesis. First, data from the Magnetometer instrument onboard Cassini during the February 17, 2005 encounter with Enceladus found evidence for an atmosphere on Enceladus. 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 vapour (Burton 2005). 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' "atmosphere" are concentrated over the south polar region, with atmospheric density away from the pole being much lower. The Ultraviolet Imaging Spectrograph (UVIS) instrument confirmed this result during 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 [19]. 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 and Carbon dioxide [20]. 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], [22]. 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" [23].

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 [24]. 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 were confirmed [25]. 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 of Enceladus [26].

What type of mechanism could be responsible for this outgassing? One possibility is that the water vapor (and dust, as seen by CDA) emanates from sub-surface, pressurized chambers, similar to geysers on Earth [27]. 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. Pure water would require more energy, either from tidal or radiogenic sources, to melt, than an ammonia-water mixture. Another method for generating a plume is sublimation of a warmed 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 kelvins, 15 kelvins warmer than expected from solar heating alone. In addition, colour temperatures of several features in the region indicate small areas at greater than 110 kelvins, too warm to be explained by sunlight alone [28]. Small areas within the "tiger stripes" were found to have colour temperatures as warm as 140 kelvins [29], though small, warmer regions maybe possible. This ice 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.

Named surface features

Features on Enceladus are named after characters and places from the Arabian Nights. Scientists officially recognise the following types of geological feature on Enceladus.

• Craters
• Fossae (long, narrow depressions)
• Planitia ( plains)
• Sulci (long parallel grooves)

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
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	94,000
June 28, 2007	91,000
September 30, 2007	83,000
March 12, 2008	100
June 30, 2008	90,000

The first spacecraft images of Enceladus were taken by the two Voyager spacecraft. 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.

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 kilometres 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