Everything about Enceladus Moon totally explained
Enceladus (
en-SEL-ə-dəs, or as Greek
Εγκέλαδος), 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
km 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
Giant 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 chose these names because
Saturn, known in Greek mythology as
Cronus, was the leader of the Titans. The adjectival form of the name is either
Enceladean or
Enceladan (both are used with roughly equal frequency).
Features on Enceladus are named by the
International Astronomical Union (IAU) after characters and places from the
Arabian Nights.
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. Examples of approved names include
Samarkand Sulci,
Aladdin crater,
Daryabar Fossa, and
Sarandib Planitia.
Exploration
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. Herschel first observed Enceladus in 1787, but in his smaller, 16.5-
cm telescope, the moon wasn't recognized. Due to Enceladus' faint
apparent magnitude (+11.7
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 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 |
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 |
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. 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. 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, four close flybys of Enceladus have been performed, yielding significant information concerning Enceladus' surface, as well as the discovery of
water vapor and complex
hydrocarbons 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 took the probe to within 50 km of the moon's surface.
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. The concept wasn't selected for further study. The European Space Agency also recently explored plans to send a probe to Enceladus in a mission to be combined with studies of Titan.
Characteristics
Orbit
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' orbital eccentricity (0.0047) and provides a heating source for Enceladus' geologic activity. 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' gravity and enter orbit around Saturn, since Enceladus'
escape velocity is only . The second mechanism comes from meteoric bombardment of Enceladus, raising dust particles from the surface. This mechanism isn't unique to Enceladus, but is valid for all Saturn's moons orbiting inside the E Ring.
Size and shape
Enceladus is a relatively small satellite, with a mean diameter of 505 km, only one-seventh the diameter of 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 km², 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,
Rhea,
Iapetus,
Dione and
Tethys . It is also one of the smallest of Saturn's spherical satellites, since all smaller satellites except
Mimas have an irregular shape.
Enceladus has a shape of a flattened
ellipsoid; its dimensions, calculated from pictures taken by Cassini's ISS instrument, are of, 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. 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. This subdivision of cratered terrains on the basis of crater density (and thus surface age), believes 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 6, 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 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 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
Voyager 2 found several types of tectonic features on Enceladus, including
troughs, scarps, and
belts of
grooves and
ridges. 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-kilometre high domes have also been observed.
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
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's 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. 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
m 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're 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' rotation rate. Such a shift would have led to a flattening of Enceladus' rotation axis.
Visual confirmation of venting came in November 2005, when ISS imaged fountain-like
jets of icy particles rising from the moon's south polar region.) 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. This finding further raises the potential for life beneath the surface of Enceladus. The composition of Enceladus's plume as measured by the INMS instrument on Cassini is similar to that seen at most comets. This hypothesis wouldn't require the amount of heat needed to melt water ice as required by the "Cold Geyser" model, and would explain the lack of ammonia.
Internal structure
[[Image:Enceladus Roll.jpg|thumb|250px|right|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. 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 wouldn't 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's in
hydrostatic equilibrium, is consistent with an undifferentiated interior, in contradiction to the geological and geochemical evidence.. 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 can't be seen from the far side of the satellite.
Saturn's rings would be seen from an angle of only 0.019°, and would appear as a very narrow, bright line crossing the disk of Saturn, but their shadow on Saturn's disk would be clearly distinguishable. Like our own Moon from Earth, Saturn itself would show regular
phases, cycling from "new" to "full" in about 16 hours. 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. Its apparent size would be at most 26 minutes of arc, about the same size as the Moon seen from Earth.
Pallene and
Methone would appear nearly star-like.
Tethys would reach a maximum apparent size just above one degree of arc, about twice the Moon as seen from the Earth, but is visible only from Enceladus' anti-Saturnian side when it's at closest approach.
Enceladus in popular culture
» See Saturn's moons in fiction.
Notes and references
Further Information
Get more info on 'Enceladus Moon'.
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