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Mimas:
Mimas is a moon of Saturn which was discovered in
1789 by William Herschel. It is named after Mimas, a son of Gaia in Greek
mythology, and is also designated Saturn I.
Mimas is the smallest known astronomical body of the solar system which has a
near-spherical shape due to its self-gravitation.
Mimas' low density (1.17) indicates that it is composed mostly of water ice with
only a small amount of rock. Due to the tidal forces acting on it, the moon is
not perfectly spherical; its longest axis is about 10% longer than the shortest.
The somewhat ovoid shape of Mimas is especially noticeable in recent images from
the Cassini probe.
Mimas' most distinctive feature is a colossal impact crater 130 km across, named
Herschel after the moon's discoverer. Herschel's diameter is almost a third of
the moon's own diameter; its walls are approximately 5 km high, parts of its
floor measure 10 km deep, and its central peak rises 6 km above the crater
floor. If there were a crater of an equivalent scale on Earth it would be over
4,000 km in diameter, wider than Canada. The impact that made this crater must
have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas
that may have been created by shock waves from the impact travelling through the
moon's body.
The surface is saturated with smaller impact craters, but no others are anywhere
near the size of Herschel. Although Mimas is heavily cratered, the cratering is
not uniform. Most of the surface is covered with craters greater than 40 km in
diameter, but in the south polar region, craters greater than 20 km are
generally lacking. This suggests that some process removed the larger craters
from these areas.
Scientists officially recognise two types of geological features on Mimas:
craters and chasmata (chasms).
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Io:
Io is the innermost of the four Galilean moons of
Jupiter and, with a diameter of 3,642 kilometers, the fourth-largest moon in the
Solar System. It was named after Io, a priestess of Hera that became one of the
lovers of Zeus.
With over 400 active volcanoes, Io is the most geologically active object in the
Solar System. This extreme geologic activity is the result of tidal heating from
friction generated within Io's interior by Jupiter's varying pull. Several
volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500
km (310 mi). Io's surface is also dotted with more than 100 mountains that have
been uplifted by extensive compression at the base of the moon's silicate crust.
Some of these peaks are taller than Earth's Mount Everest.[6] Unlike most
satellites in the outer Solar System (which have a thick coating of ice), Io is
primarily composed of silicate rock surrounding a molten iron or iron sulfide
core. Most of Io's surface is characterized by extensive plains coated with
sulfur and sulfur dioxide frost.
Io's volcanism is responsible for many of that satellite's unique features. Its
volcanic plumes and lava flows produce large surface changes and paint the
surface in various shades of red, yellow, white, black, and green, largely due
to the sulfurous compounds. Numerous extensive lava flows, several longer than
500 kilometres (311 mi) in length, also mark the surface. These volcanic
processes have given rise to a comparison of the visual appearance of Io's
surface to a pizza. The materials produced by this volcanism provide material
for Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere.
Io played a significant role in the development of astronomy in the 17th and
18th centuries. It was discovered in 1610 by Galileo Galilei, along with the
other Galilean satellites. This discovery furthered the adoption of the
Copernican model of the Solar System, the development of Kepler's laws of
motion, and the first measurement of the speed of light. From Earth, Io remained
nothing more than a point of light until the late 19th and early 20th centuries,
when it became possible to resolve its large-scale surface features, such as the
dark red polar and bright equatorial regions. In 1979, the two Voyager
spacecraft revealed Io to be a geologically active world, with numerous volcanic
features, large mountains, and a young surface with no obvious impact craters.
The Galileo spacecraft performed several close flybys in the 1990s and early
2000s, obtaining data about Io's interior structure and surface composition.
These spacecraft also revealed the relationship between the satellite and
Jupiter's magnetosphere and the existence of a belt of radiation centered on
Io's orbit. The exploration of Io continued in the early months of 2007 with a
distant flyby by Pluto-bound New Horizons.
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Iapetus:
Iapetus is the third-largest moon of Saturn, and
eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671.
Iapetus is best known for its dramatic 'two-tone' coloration, but recent
discoveries by the Cassini mission have revealed several other unusual physical
characteristics, such as an equatorial ridge that runs about halfway around the
moon.
Iapetus was discovered by Giovanni Domenico Cassini in October 1671 on the
western side of Saturn. Then Cassini tried unsuccessfully to observe it on the
eastern side of the planet in early 1672. This pattern continued as Cassini
observed Iapetus in December 1672 and February 1673, each time tracking it for a
fortnight on the western side of Saturn, but he was unable to detect it during
the intervening period, when it should have been on the eastern side. Cassini
finally observed Iapetus on the eastern side in 1705 with an improved telescope,
finding it two magnitudes dimmer on that side.
Cassini correctly surmised that Iapetus has a bright hemisphere and a dark
hemisphere, and that it is tidally locked, always keeping the same face towards
Saturn, so that the bright hemisphere is visible from Earth when Iapetus is on
the western side of Saturn, and the dark hemisphere on the other side. The dark
hemisphere was later named Cassini Regio in his honour.
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Tethys:
Tethys is a moon of Saturn that was discovered by
Giovanni Domenico Cassini in 1684.
Tethys is an icy body similar in nature to Dione and Rhea. The density of Tethys
is 0.97 g/cm³, indicating that it is composed almost entirely of water-ice. The
Tethyan surface is heavily cratered and contains numerous cracks caused by
faults in the ice. Its surface is one of the most reflective (at visual
wavelengths) in the solar system, with a visual albedo of 1.229. This very high
albedo is the result of the sandblasting of particles from Saturn's E-ring, a
faint ring composed of small, water-ice particles generated by Enceladus' south
polar geysers.
There are two different types of terrain found on Tethys, one composed of
densely cratered regions and the other consisting of a dark colored and lightly
cratered belt that extends across the moon. The light cratering of this second
region indicates that Tethys was once internally active, causing parts of the
older terrain to be resurfaced. The exact cause of the darkness of the belt is
unknown but a possible interpretation comes from recent Galileo orbiter images
of Jupiter's moons Ganymede and Callisto, both of which exhibit light polar caps
that are made from bright ice deposits on pole-facing slopes of craters. From a
distance the caps appear brighter due to the thousands of unresolved ice patches
in small craters present there. The Tethyan surface may have been formed in a
similar manner, consisting of hazy polar caps of unresolved bright ice patches
with a darker zone in between.
Odysseus is the huge, shallow crater on top, near the terminatorThe western
hemisphere of Tethys is dominated by a huge impact crater called Odysseus, whose
400 km diameter is nearly 2/5 of that of Tethys itself. The crater is now quite
flat (or more precisely, it conforms to Tethys' spherical shape), like the
craters on Callisto, without the high ring mountains and central peaks commonly
seen on the Moon and Mercury. This is most likely due to the slumping of the
weak Tethyan icy crust over geologic time.
The second major feature seen on Tethys is a huge valley called Ithaca Chasma,
100 km wide and 3 to 5 km deep. It runs 2000 km long, approximately 3/4 of the
way around Tethys' circumference. It is thought that Ithaca Chasma formed as
Tethys' internal liquid water solidified, causing the moon to expand and
cracking the surface to accommodate the extra volume within. The subsurface
ocean may have resulted from a 2:3 orbital resonance between Dione and Tethys
early in the solar system's history that led to orbital eccentricity and tidal
heating of Tethys' interior. The ocean would have frozen after the moons escaped
from the resonance. Earlier craters that formed before Tethys solidified were
probably all erased by geological activity before then. There is another theory
about the formation of Ithaca Chasma: when the impact that caused the great
crater Odysseus occurred, the shockwave traveled through Tethys and fractured
the icy, brittle surface on the other side. The Tethyan surface temperature is
-187°C.
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Callisto:
Callisto is a moon of the planet Jupiter,
discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar
System and the second largest in the Jovian system, after Ganymede. Callisto has
about 99% the diameter of the planet Mercury but only about a third of its mass.
It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of
about 1,880,000 kilometers. It does not form part of the orbital resonance that
affects three inner Galilean satellites—Io, Europa and Ganymede—and thus does
not experience appreciable tidal heating. Callisto rotates synchronously with
its orbital period, so the same face is always turned toward Jupiter. Callisto's
surface is less affected by Jupiter's magnetosphere than the other inner
satellites because it orbits further away.
Callisto is composed of approximately equal amounts of rock and ices, with a
mean density of about 1.83 g/cm3. Compounds detected spectrally on the surface
include water ice, carbon dioxide, silicates, and organics. Investigation by the
Galileo spacecraft revealed that Callisto may have a small silicate core and
possibly a subsurface ocean of liquid water at depths greater than 100
kilometers.
The surface of Callisto is heavily cratered and extremely old. It does not show
any signatures of subsurface processes such as plate tectonics, earthquakes or
volcanoes, and is thought to have evolved predominantly under the influence of
impacts. Prominent surface features include multi-ring structures, variously
shaped impact craters, and chains of craters (catenae) and associated scarps,
ridges and deposits. At a small scale, the surface is varied and consists of
small, bright frost deposits at the tops of elevations, surrounded by a
low-lying, smooth blanket of dark material. This is thought to result from the
sublimation-driven degradation of small landforms, which is supported by the
general deficit of small impact craters and the presence of numerous small
knobs, considered to be their remnants. The absolute ages of the landforms are
not known.
Callisto is surrounded by an extremely thin atmosphere composed of carbon
dioxide and probably molecular oxygen, as well as by a rather intense
ionosphere. Callisto is thought to have formed by slow accretion from the disk
of the gas and dust that surrounded Jupiter after its formation. Its slowness
and the lack of tidal heating prevented rapid differentiation. The slow
convection in the interior of Callisto, which commenced soon after formation,
led to partial differentiation and possibly to the formation of a subsurface
ocean at a depth of 100–150 kilometers and a small, rocky core.
The likely presence of an ocean within Callisto indicates that it can or could
harbor life. However, this is less likely than on nearby Europa. Various space
probes from Pioneers 10 and 11 to Galileo and Cassini have studied the moon.
Callisto has long been considered the most suitable place for a human base for
future exploration of the system of Jupiter.
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Ganymede:
Ganymede is a natural satellite of Jupiter and
the largest natural satellite in the Solar System. Completing an orbit in a
little more than seven days, it is the seventh satellite and third Galilean
satellite from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with
the satellites Europa and Io, respectively. It is larger in diameter than the
planet Mercury but has only about half its mass.
Ganymede is composed primarily of silicate rock and water ice. It is a fully
differentiated body with an iron-rich, liquid core. A saltwater ocean is
believed to exist nearly 200 km below Ganymede's surface, sandwiched between
layers of ice. Its surface comprises two main types of terrain. Dark regions,
saturated with impact craters and dated to four billion years ago, cover about a
third of the satellite. Lighter regions, crosscut by extensive grooves and
ridges and only slightly less ancient, cover the remainder. The cause of the
light terrain's disrupted geology is not fully known, but was likely the result
of tectonic activity brought about by tidal heating.
Ganymede is the only satellite in the Solar System known to possess a
magnetosphere, likely created through convection within the liquid iron core.
The meager magnetosphere is buried within Jupiter's much larger magnetic field
and connected to it through open field lines. The satellite has a thin oxygen
atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a
minor atmospheric constituent. Whether the satellite has an ionosphere to
correspond to its atmosphere is unresolved.
Ganymede's discovery is credited to Galileo Galilei, who observed it in 1610.
The satellite's name was soon suggested by astronomer Simon Marius, for the
mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved. Beginning
with Pioneer 10, spacecraft have been able to examine Ganymede closely. The
Voyager probes refined measurements of its size, while the Galileo craft
discovered its underground ocean and magnetic field. The Jupiter Icy Moons
Orbiter was meant to orbit Ganymede, but the NASA project was cancelled in 2005.
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Titan:
Titan or Saturn VI is the largest moon of Saturn,
the only moon known to have a dense atmosphere, and the only object other than
Earth for which clear evidence of stable bodies of surface liquid has been
found.
Titan is the twentieth most distant moon of Saturn and sixth farthest among
those large enough to assume a spheroid shape. Frequently described as a
satellite with planet-like characteristics, Titan has a diameter roughly 50%
larger than Earth's moon and is 80% more massive. It is the second-largest moon
in the Solar System, after Jupiter's moon Ganymede, and it is larger by diameter
than the smallest planet, Mercury (although only half as massive). Titan was the
first known moon of Saturn, discovered in 1655 by the Dutch astronomer
Christiaan Huygens.
Titan is primarily composed of water ice and rocky material. The dense
atmosphere prevented understanding of Titan's surface until new information
accumulated with the arrival of the Cassini–Huygens mission in 2004, including
the discovery of liquid hydrocarbon lakes in the satellite's polar regions.
These are the only large, stable bodies of surface liquid known to exist
anywhere other than Earth. The surface is geologically young; although mountains
and several possible cryovolcanoes have been discovered, it is relatively smooth
and few impact craters have been discovered.
The atmosphere of Titan is largely composed of nitrogen and its climate includes
methane and ethane clouds. The climate—including wind and rain—creates surface
features that are similar to those on Earth, such as sand dunes and shorelines,
and, like Earth, is dominated by seasonal weather patterns. With its liquids
(both surface and subsurface) and robust nitrogen atmosphere, Titan is viewed as
analogous to the early Earth, although at a much lower temperature. The
satellite has thus been cited as a possible host for microbial extraterrestrial
life or, at least, as a prebiotic environment rich in complex organic chemistry.
Researchers have suggested a possible underground liquid ocean might serve as a
biotic environment.
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