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The Planets .... brought to you by QuasArt Web Designs.
S A T U R N (6th planet from the Sun)
In Roman mythology, Saturn is the god of agriculture. The associated Greek
god, Cronus, was the son of Uranus and Gaia and the father of Zeus
(Jupiter). Saturn is the root of the English word "Saturday".
CHAPTERS
I II III IV V VI VII VIII IX X XI
18 MOONS
orbit: 1,429,400,000 km (9.54 AU) from Sun
diameter: 120,536 km (equatorial)
mass: 5.68e26 kg
INTRODUCTION
I
Saturn (planet), sixth planet in order of distance
from the sun, and the second largest in the solar system. Saturn's
most distinctive feature is its ring system, which was first seen
in 1610 by Italian scientist Galileo,
using one of the first telescopes. He did 
not
understand that the rings were separate from the body of the planet,
so he described them as handles (ansae).The Dutch astronomer Christiaan
Huygens was the first to describe the rings correctly. In 1655, desiring
further time to verify his explanation without losing his claim to
priority, Huygens wrote a series of letters in code, which when properly
arranged formed a Latin sentence that read in translation, "It is
girdled by a thin flat ring, nowhere touching, inclined to the ecliptic."
The rings are named in order of their discovery, and from the planet
outward they are known as the D, C, B, A, F, G, and E rings. These
rings are now known to comprise more than 100,000 individual ringlets,
each of which circles the planet.
Saturn, (above) distinguished by its rings, ranks as the second largest
planet-(Jupiter is the largest) in the
solar system. Although the planet formed more than 4 billion years
ago, it continues to settle and contract, generating three times as
much heat as it receives from the sun. The Hubble Space Telescope
obtained this image of Saturn on August 26, 1990. Gamma Liaison
Saturn has been known since prehistoric times.
In 1610 the Italian physicist and astronomer Galileo
discovered Saturn. Galileo
noted its odd appearance but was confused by it. Early observations
of Saturn were complicated by the fact that the Earth passes through
the plane of Saturn's rings every few years as Saturn moves in its
orbit. A low resolution image of Saturn therefore changes drastically.
It was not until 1659 that Christiaan Huygens correctly inferred the
geometry of the rings. Saturn's rings remained unique in the known
solar system until 1977 when very faint rings were discovered around
Uranus (and shortly thereafter around Jupiter and Neptune).
Saturn is visibly flattened (oblate) when viewed through a small telescope;
its equatorial and polar diameters vary by almost 10% (120,536 km
vs. 108,728 km). This is the result of its rapid rotation and fluid
state. The other gas planets are also oblate, but not so much so.
Saturn's interior is similar to Jupiter's consisting of a rocky core,
a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces
of various ices are also present.
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EXPLORATION
OF THE SATURNIAN SYSTEM
II
As seen from earth, Saturn appears as a yellowish object-one
of the brightest in the night sky. Observed through a telescope, the
A and B rings are easily visible, whereas only under optimal conditions
can the D and E rings be seen. Sensitive earth-based telescopes have
detected nine satellites, and in the haze of Saturn's gaseous envelope,
pale belts and zones parallel to the equator can be distinguished.
Three United States spacecraft have enormously increased knowledge
of the Saturnian system. The Pioneer 11 (see Pioneer) probe flew by
in September 1979, followed by Voyager 1 in November 1980 and Voyager
2 (see Voyager) in August 1981. These spacecraft carried cameras and
instruments for analyzing the intensities and polarizations of radiation
in the visible, ultraviolet, infrared, and radio portions of the electromagnetic
spectrum (see Electromagnetic Radiation). The spacecraft were also
equipped with instruments for studying magnetic fields and for detecting
charged particles and interplanetary grains. The National Aeronautics
and Space Administration (NASA) plans to launch an orbiter called
the Cassini spacecraft toward Saturn in late 1997. It should reach
Saturn in 2004, when it will begin studying Saturn and its moons,
launching a probe (the Huygens Probe) into the atmosphere of Saturn's
moon Titan.
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THE
INTERIOR OF SATURN
III
The mean density of Saturn is
eight times less than that of Earth because the planet consists mainly
of hydrogen. The enormous weight of Saturn's atmosphere causes the
atmospheric pressure to increase rapidly toward the interior, where
the hydrogen gas condenses into a liquid. Closer to the center of
the planet, the liquid hydrogen is compressed into metallic hydrogen,
which is an electrical conductor. Electrical currents in this metallic
hydrogen are responsible for the planet's magnetic field. At the center
of Saturn, heavy elements have probably settled into a small rocky
core with a temperature close to 15,000° C (27,000° F). Both Jupiter
and Saturn are still settling gravitationally, following their original
accretion from the gas and dust nebula from which the solar system
was formed more than 4.7 billion years ago. This contraction generates
heat, causing Saturn to radiate into space three times as much heat
as it receives from the sun.
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THE
ATMOSPHERE OF SATURN
IV
Saturn's atmospheric constituents are, in order by
mass, hydrogen (88 percent) and helium (11 percent); and traces of
methane, ammonia, ammonia crystals, and such other gases as ethane,
acetylene, and phosphine comprise the remainder. Voyager images showed
whirls and eddies of clouds occurring deep in a haze that is much
thicker than that of Jupiter because of Saturn's lower temperature.
The temperatures of Saturn's cloud tops are close to -176° C (-285°
F), about 27° C (49° F) lower than such locations on Jupiter. Based
on the movements of Saturnian storm clouds, the period of rotation
of the atmosphere near the equator is about 10 hr 11 min. Radio emissions
that have been detected coming from the body of the planet indicate
that the body of Saturn and its magnetosphere rotate with a period
of 10 hr 39 min 25 sec. The approximately 28.5-min difference between
these two times indicates that Saturnian equatorial winds have velocities
close to 1700 km/h (1060 mph). In 1988, from studies of Voyager photos,
scientists determined an odd atmospheric feature around Saturn's north
pole. What may be a standing-wave pattern (see Wave Motion), repeated
six times around the planet, makes cloud bands some distance from
the pole appear to form a huge, permanent hexagon.
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THE
MAGNETOSPHERE
V
Saturn's magnetic field is substantially weaker than
that of Jupiter, and Saturn's magnetosphere is about one-third the
size of Jupiter's. Saturn's magnetosphere consists of a set of doughnut-shaped
radiation belts in which electrons and atomic nuclei are trapped.
The belts extend to more than 2 million km (1.3 million mi) from the
center of Saturn and even farther in the direction away from the sun,
although the size of the magnetosphere fluctuates, depending on the
intensity of the solar wind (the flow of charged particles from the
sun). The solar wind and Saturn's rings and satellites supply the
particles that are trapped in the radiation belts. The rotation period
of 10 hr 39 min 25 sec for Saturn's interior was measured by Voyager
1 while passing through the magnetosphere, which rotates in synchrony
with the interior of Saturn. The magnetosphere interacts with the
ionosphere, the topmost layer of Saturn's atmosphere, causing auroral
emissions of ultraviolet radiation. Surrounding the Saturnian satellite
Titan and Titan's orbit, and extending to the orbit of Saturn's moon
Rhea, is an enormous doughnut-shaped cloud of neutral hydrogen atoms.
A disk of plasma, composed of hydrogen and possibly oxygen ions, extends
from outside the orbit of the moon Tethys almost to the orbit of Titan.
The plasma rotates in nearly perfect synchrony with Saturn's magnetic
field.
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THE
RING SYSTEM
VI
The visible rings stretch out to a distance of 136,200
km (84,650 mi) from Saturn's center, but in many regions they may
be only 5 m (16.4 ft) thick. Saturn's
rings are thought to consist of aggregates of rock, frozen gases,
and water ice ranging in size from less than 0.0005 cm (0.0002 in)
in diameter to about 10 m (33 ft) in diameter-from dust to boulder
size.
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Saturn's
rings (left) are extremely wide, but very flat, bands created
by orbiting fragments of rock, gas, and ice. Although there are
more than 100,000 separate ringlets, they are generally referred
to as part of the broad A, B, and C rings (starting from the outside).
Cassini's Division, the distinct gap between the A and B rings,
is approximately 4800 km wide. Voyager 2 captured this enhanced
view from 8.9 million km (5.5 million mi) away as it flew by the
planet in 1981. Robert Harding Picture Library |
An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system. The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring. The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick. They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter-from dust to boulder size. An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system. The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring.
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MOONS AND SATELLITES
INTRODUCTION
VII
Saturn has 18 confirmed moons and as many as 14 proposed new, unconfirmed moons. In the past many proposed new moons have turned out to be just dense spots in Saturn's rings, but the Cassini spacecraft should be able to definitively catalog Saturn's moons. The diameters of Saturn's satellites range from 20 to 5150 km (12 to 3200 mi). They consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the solar system was formed and where radiation from the distant sun could not evaporate the frozen gases. The five larger inner satellites-Mimas, Enceladus, Tethys, Dione, and Rhea-are roughly spherical in shape and composed mostly of water ice. Rocky material may constitute up to 40 percent of Dione's mass. The surfaces of the five are heavily cratered by meteorite impacts. Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old. (Possibly Enceladus is still undergoing tectonic activity; see Plate Tectonics.) Astronomers suspect that Enceladus supplies particles to the E ring, which neighbors Enceladus's orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself. Tethys also bears a large crater and a valley 100 km (62 mi) in width that stretches more than 2000 km (1200 mi) across the surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces. Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from the interior. Several small satellites have been discovered immediately outside the A ring and close to the F and G rings. Possibly four so-called Trojan satellites of Tethys and one of Dione have also been discovered. Trojan satellites occur in regions of stability that lead or follow a body in its orbit around a massive central body, in this case, Saturn. See Solar System. The outer satellites Hyperion and Iapetus also consist mainly of water ice. Iapetus has a very dark region in contrast to most of its surface, which is bright. This dark region and the rotation of the satellite are the cause of the variations of brightness that were noticed by Cassini in 1671. Phoebe, the farthest satellite, moves in a retrograde orbit (in the opposite direction of the orbits of the other satellites) that is at a sharp angle to Saturn's equator. Phoebe is probably a cometary body captured by Saturn's gravitational field. Between the inner and outer satellites orbits Titan, Saturn's largest moon. Its diameter is 5150 km (3200 mi), larger even than the planet Mercury. The diameter of Titan is not known, however, because a dense orange haze hides the surface. The thickness of Titan's atmosphere is probably about 300 km (about 186 mi). Titan has a nitrogen atmosphere with traces of methane, ethane, acetylene, ethylene, hydrogen cyanide, and carbon monoxide and dioxide. On the surface, the temperature is about -182° C (-296° F), and methane or ethane may be present in the forms of rain, snow, ice, and vapor. The interior of Titan probably consists of equal amounts of rock and water ice. No magnetic fields have been detected. The southern hemisphere is slightly brighter, and the only detail visible is a dark ring in the northern polar region. Much more will be known about Titan after the visit of the
Cassini spacecraft and the Huygens Probe in 2004.
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MOONS AND SATELLITES
VIII
The following 2 Moons are within Saturn's
ring system.
PAN
Pan was the god of woods, fields, and flocks, having a human torso
and head with a goat's legs, horns, and ears.
Pan is the innermost of Saturn's known satellites, it is with the Ring System:
orbit: 133,583 km from Saturn
diameter: 20 km
mass: ?
Discovered by Mark R. Showalter in 1990 from Voyager photos. Pan is within the Encke Division in Saturn's A ring. Small moons near the rings produce wave patterns in the rings. Prior to the discovery of Pan, an analysis of the patterns in the edge of Saturn's A ring predicted the size and location of a small moon. Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot. It is possible that there are more moons within Saturn's rings yet to be discovered.
ATLAS
Atlas was a Titan condemned by Zeus to support the heavens
upon his shoulders; son of Iapetus and the nymph Clymene; brother
of Prometheus and Epimetheus. Atlas is within Saturn's
ring system.
Atlas is the second of Saturn's known satellites:
orbit: 137,670 km from Saturn
diameter: 30 km (40 x 20)
mass: ?
Discovered by R. Terrile in 1980 from Voyager photos.
Atlas seems to be a shepherd satellite of the A ring.
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MOONS AND SATELLITES
XV
MISSIONS AND PROBES
IX
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An
artist's conception shows how the Cassini spacecraft might look
in November 2004 after it releases the Huygens probe, which will
descend through the atmosphere of Titan, Saturn's largest moon.
In August 1999 scientists reported evidence of liquid seas on
Titan. Courtesy of JPL/Caltech/NASA |
WEB LINKS
XI
Saturn' s Rings
MORE COMING SOON
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