Moon of Saturn
Enceladus is the sixth-largest of the moons of Saturn. It was discovered
in 1789 by William Herschel. Enceladus seems to have liquid water under
its icy surface. Cryovolcanoes at the south pole shoot large jets of
water vapor, other volatiles and some solid particles (ice crystal, NaCl
etc) into space . Some of this water falls back onto the moon as
"snow", some of it adds to Saturn's rings, and some of it reaches
Saturn. The whole of Saturn's E ring is believed to have been made from
these ice particles. Because of the apparent water at or near the
surface, Enceladus may be one of the best places for humans to look for
extraterrestrial life. By contrast, the water thought to be on Jupiter's
moon Europa is locked under a very thick layer of surface ice.
Until the two Voyager spacecraft passed near it in the early 1980s
very little was known about this small moon besides the identification
of water ice on its surface. The Voyagers showed that the diameter of
Enceladus is only 310 mi), about a tenth of that of Saturn's largest
moon, Titan, and that it reflects almost all of the sunlight that
strikes it. Voyager 1 found that Enceladus orbited in the densest part
of Saturn's diffuse E ring, indicating a possible association between
the two, while Voyager 2 revealed that despite the moon's small size, it
had a wide range of terrains ranging from old, heavily cratered
surfaces to young, tectonically deformed terrain, with some regions with
surface ages as young as 100 million years old.
In 2005 the Cassini spacecraft performed several close flybys of
Enceladus, revealing the moon's surface and environment in greater
detail. In particular, the probe discovered a water-rich plume venting
from the moon's south polar region. This discovery, along with the
presence of escaping internal heat and very few (if any) impact craters
in the south polar region, shows that Enceladus is geologically active
today. Moons in the extensive satellite systems of gas giants often
become trapped in orbital resonances that lead to forced libration or
orbital eccentricity; proximity to Saturn can then lead to tidal heating
of Enceladus's interior, offering a possible explanation for the
activity.
Exploration
Size comparison of Earth and Enceladus,if you compare it to Saturn you wouldn't be able to see it,the dot would be so small
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 was not recognized.Its faint
apparent magnitude (+11.7m) and its proximity to much brighter Saturn
and its rings make Enceladus difficult to observe from Earth, requiring a
telescope with a mirror of 15–30 cm in diameter, depending on
atmospherical conditions and light pollution. Like many Saturnian
satellites discovered prior to the Space Age, Enceladus was first
observed during a Saturnian equinox, when Earth is within the ring
plane; at such times, the reduction in glare from the rings makes the
moons easier to observe.
(Dramatic plumes, both large and small, spray water ice out from many
locations along the famed "tiger stripes" near the south pole of
Saturn's moon Enceladus. From right to left, the four major stripes are
Damascus, Baghdad, Cairo and Alexandria sulci.)
Prior to the Voyager missions the view of Enceladus improved little from
the dot first observed by Herschel. Only its orbital characteristics
were known, with estimations of its mass, density and albedo.
The two Voyager spacecraft obtained the first close-up images of
Enceladus. Voyager 1 was the first to fly past Enceladus, at a distance
of 202,000 km on November 12, 1980. 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's surface.
Dramatic plumes, both large and small, spray water ice out from many
locations along the famed "tiger stripes" near the south pole of
Saturn's moon Enceladus. From right to left, the four major stripes are
Damascus, Baghdad, Cairo and Alexandria sulci.
Voyager 2 passed closer to Enceladus (87,010 km) on August 26, 1981,
allowing much higher-resolution images of this satellite.[25] These
images revealed the youthful nature of much of its surface, 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 had 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. The flybys have yielded significant information concerning
Enceladus's surface, as well as the discovery of water vapor and complex
hydrocarbons venting from the geologically active South Polar Region.
These discoveries prompted the adjustment of Cassini's flight plan to
allow closer flybys of Enceladus, including an encounter in March 2008
which took the probe to within 52 km of the moon's surface.The extended
mission for Cassini included seven close flybys of Enceladus between
July 2008 and July 2010, including two passes at only 50 km in the later
half of 2008.
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 was not 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.
The Titan Saturn System Mission (TSSM) is a joint NASA/ESA proposal
for exploration of Saturn's moons, including Enceladus. TSSM was
competing against the Europa Jupiter System Mission (EJSM) proposal for
funding. In February 2009 it was announced that ESA/NASA had given the
EJSM mission priority ahead of TSSM,although TSSM will continue to be
studied for a later launch date.
Size and shape
Enceladus is a relatively small moon, with a mean diameter of 314 mi), only one-seventh the diameter of Earth's own Moon.
Its mass and diameter make Enceladus the sixth most massive and
largest satellite of Saturn, after Titan (5150 km), Rhea (1530 km),
Iapetus (1440 km), Dione (1120 km) and Tethys (1050 km). It is also one
of the smallest of Saturn's spherical satellites, since all smaller
satellites except Mimas (390 km) have an irregular shape.
Enceladus is a scalene ellipsoid in shape; its diameters, calculated
from pictures taken by Cassini's ISS (Imaging Science Subsystem)
instrument, are 513 km between the sub- and anti-Saturnian poles, 503 km
between the leading and trailing poles, and 497 km between the north
and south poles. This is the most stable orientation, with the moon's
rotation along the short axis, and the long axis aligned radially away
from Saturn.
Surface
A very young face for a very small moon.
Voyager 2, in August 1981, was the first spacecraft to observe the
surface in detail. Examination of the resulting highest-resolution
mosaic reveals at least five different types of terrain, including
several regions of cratered terrain, regions of smooth (young) terrain,
and lanes of ridged terrain often bordering the smooth areas. 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 probably the most reflective surface of any
body in the Solar System with a visual geometric albedo of 1.38. 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's surface features in much
greater detail than the Voyager 2 observations. For example, the smooth
plains observed by Voyager 2 resolved into relatively crater-free
regions filled with numerous small ridges and scarps. In addition,
numerous fractures were found within the older, cratered terrain,
suggesting that the surface has been subjected to extensive deformation
since the craters were formed.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
Impact cratering is a common occurrence on many Solar System bodies.
Much of Enceladus's surface is covered with craters at various densities
and levels of degradation. From Voyager 2 observations, three different
units of cratered topography were identified on the basis of their
crater densities, from ct1 and ct2, both containing numerous 10–20
km-wide craters though differing in the degree of deformation, to cp
consisting of lightly cratered plains.This subdivision of cratered
terrains on the basis of crater density (and thus surface age) suggests
that Enceladus has been resurfaced in multiple stages.
Recent Cassini observations have provided a much closer look at the
ct2 and cp cratered units. These high-resolution observations, reveal
that many of Enceladus's craters are heavily deformed through viscous
relaxation and fracturing. Viscous relaxation allows gravity, over
geologic time scales, to deform craters and other topographic features
formed in water ice, reducing the amount of topography over time. The
rate at which this occurs is dependent on the temperature of the ice:
warmer ice is easier to deform than colder, stiffer ice. Viscously
relaxed craters tend to have domed floors, or are recognized as craters
only by a raised, circular rim . Dunyazad, the large crater seen in
Figure 8 just left of top center, is a prime example of a viscously
relaxed crater on Enceladus, with a prominent domed floor. In addition,
many craters on Enceladus have been heavily modified by tectonic
fractures. The 10-km-wide crater right of bottom center in Figure 8 is a
prime example: thin fractures, several hundred meters to a kilometer
wide, have heavily altered the crater's rim and floor. Nearly all
craters on Enceladus thus far imaged by Cassini in the ct2 unit show
signs of tectonic deformation.
Tectonics
Voyager 2 found several types of tectonic features on Enceladus,
including troughs, scarps, and belts of grooves and ridges.Recent
results from Cassini suggest that tectonism is the dominant deformation
style on Enceladus. One of the more dramatic types of tectonic features
found on Enceladus are rifts. These canyons can be up to 200 km long,
5–10 km wide, and one km deep. Figure 7 Below shows a typical large
fracture on Enceladus cutting across older, tectonically deformed
terrain. Another example can be seen running along the bottom of the
frame in Figure 8. Such features appear relatively young, as they cut
across other tectonic features and have sharp topographic relief with
prominent outcrops along the cliff faces.
Another evidence of tectonism on Enceladus is grooved terrain,
consisting of lanes of curvilinear grooves and ridges. These bands,
first discovered by Voyager 2, often separate smooth plains from
cratered regions.An example of this terrain type can be seen in 10 (in
this case, a feature known as the Samarkand Sulci). Grooved terrains
such as the Samarkand Sulci are reminiscent of grooved terrain on
Ganymede. However, unlike those seen on Ganymede, grooved topography on
Enceladus is generally much more complex. Rather than parallel sets of
grooves, these lanes can often appear as bands of crudely aligned,
chevron-shaped features. In other areas, these bands appear to bow
upwards with fractures and ridges running the length of the feature.
Cassini observations of the Samarkand Sulci have revealed intriguing
dark spots (125 and 750 m wide), which appear to run parallel to narrow
fractures. Currently, these spots are interpreted as collapse pits
within these ridged plain belts.
Atmosphere
The first Cassini flybys of Enceladus revealed that it has a significant
atmosphere compared to the other moons of Saturn besides Titan. The
source of the atmosphere may be volcanism, geysers, or gasses escaping
from the surface or the interior.The atmosphere of Enceladus is composed
of 91% water vapor, 4% nitrogen, 3.2% carbon dioxide, and 1.7% methane.
Cryovolcanism
(Figure 13: Heat map (within white box) of the thermally active field of
fractures, measured at wavelengths between 12 and 16 micrometers,
superimposed on a visual-light image. One of the four fractures (right)
was only partially imaged.)
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 in the E ring, perhaps
through venting of water vapor from Enceladus's interior. However, the
Voyagers failed to provide conclusive evidence that Enceladus is active
today.
Thanks to data from a number of instruments on the Cassini spacecraft
in 2005, cryovolcanism, where water and other volatiles are the
materials erupted instead of silicate rock, has been discovered on
Enceladus. The first Cassini sighting of a plume of icy particles above
Enceladus's south pole came from the Imaging Science Subsystem (ISS)
images taken in January and February 2005,though the possibility of the
plume being a camera artifact stalled an official announcement. Data
from the magnetometer instrument during the February 17, 2005 encounter
provided a hint that the feature might be real when it found evidence
for an atmosphere at Enceladus. The magnetometer observed an increase in
the power of ion cyclotron waves near Enceladus. These waves are
produced by the interaction of ionized particles and magnetic fields,
and the frequency of the waves can be used to identify the composition,
in this case ionized water vapor.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 analyzer (CDA) to directly
sample the plume. INMS measured the composition of the gas cloud,
detecting mostly water vapor, as well as minor components like molecular
nitrogen, methane, and carbon dioxide. CDA "detected a large increase
in the number of particles near Enceladus", confirming Enceladus 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
observed from a distance with its magnetometer and UVIS, was actually a
water-rich cryovolcanic plume, originating from vents near the south
pole.
Visual confirmation of venting came in November 2005, when ISS
(Imaging Science Subsystem) imaged geyser-like jets of icy particles
rising from the moon's south polar region.(As stated above, the plume
was imaged before, in January and February 2005, but additional studies
of the camera's response at high phase angles, when the Sun is almost
behind Enceladus, and comparison with equivalent high-phase-angle images
taken of other Saturnian satellites, were required before this could be
confirmed.The images taken in November 2005 showed the plume's fine
structure, revealing numerous jets (perhaps issuing from numerous
distinct vents) within a larger, faint component extending out nearly
500 km from the surface, thus making Enceladus the fourth body in the
Solar System to have confirmed volcanic activity, along with Earth,
Neptune's Triton, and Jupiter's Io.Cassini's UVIS later observed gas
jets coinciding with the dust jets seen by ISS during a non-targeted
encounter with Enceladus in October 2007.
Additional observations were acquired during a flyby on March 12,
2008. Data on this flyby revealed additional chemicals in the plume,
including simple and complex hydrocarbons such as propane, ethane, and
acetylene.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.
The intensity of the eruption of the south polar jets varies
significantly as a function of the position of Enceladus in its orbit.
The plumes are about four times brighter when Enceladus is at apoapsis
(the point in its orbit most distant from Saturn) than when it is at
periapsis. Geophysical modeling of tidal stresses indicates that at
apoapsis the Tiger Stripes region is in a state of maximum tension,
which would tend to open the fissures, while compression is maximal at
periapsis.
Ammonia discovery
In July 2009 it was announced that ammonia had been discovered during
flybys in July and October 2008. This acts like anti-freeze this will
allow the water under the surface freezing temp to be increased
Internal structure
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/cm³. 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 subnebula, 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's comparatively high rock–mass
fraction, given Enceladus's 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 it 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.However, the current shape also
supports the possibility that Enceladus is not in hydrostatic
equilibrium, and may have rotated faster at some point in the recent
past (with a differentiated interior).
Possible water ocean
In late 2008, scientists observed water vapor spewing from Enceladus's
surface, and it was later discovered that this vapor trails into Saturn.
This could indicate the presence of liquid water, which might also make
it possible for Enceladus to support life. Candice Hansen, a scientist
with NASA's Jet Propulsion Lab, headed up a research team on the plumes
after they were found to be moving at1,360 miles per hour). Since that
speed is difficult to attain unless liquids are involved, they decided
to investigate the compositions of the plumes.
Eventually it was discovered that in the E-ring about 6% of particles
contain 0.5–2% of sodium salts by mass, which is a significant amount.
In the parts of the plume close to Enceladus the fraction of "salty"
particles increases to 70% by number and >99% by mass. Such particles
presumably are frozen spray from the salty underground ocean. On the
other hand, the small salt-poor particles form by homogenous nucleation
directly from the gas phase. The sources of salty particles are
uniformly distributed along the tiger stripes, whereas sources of
"fresh" particles are closely related to the high-speed gas jets. The
"salty" particles move slowly and mostly fall back onto the surface,
while the fast "fresh" particles escape to the E-ring, explaining its
salt-poor composition.
The "salty" composition of the plume strongly suggests that its
source is a subsurface salty ocean or subsurface caverns filled with
salty water. Alternatives such as the clathrate sublimation hypothesis
can not explain how "salty" particles form.Additionally, Cassini found
traces of organic compounds in some dust grains. Enceladus is therefore a
candidate for harboring extraterrestrial life.
The presence of liquid water under the crust implies that there is an
internal heat source. It is now thought to be a combination of
radioactive decay and tidal heating, as tidal heating alone is not
sufficient to explain the heat.
I wish we could send a probe into every one of these moon with water
under their surface, But we must be careful not to place life there from
the Earth. This why when Cassini is done. Its orbit would be aim at
Saturn this away it wouldn't land on this or Titan . The orbiter wasn't
clean .well it wasn't clean to get rid of any thing a life on its
surface. Before the probe if its going to be send into this moon
sea,would be clean up extra careful. Did you know that a bug on a landed
we send to the moon was still alive when they took the leg back to
Earth( Surveyor 3)
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