Download Program 8: Saturn

THE COMPLETE COSMOS
Program 8: Saturn
The many rings and moons of this exotic gas giant. Preview of a landing on Titan, a
moon like primitive Earth.
After Jupiter, Saturn is the largest planet in the Solar System. Nearly ten times farther from the
Sun than Earth, Saturn has a ten-and-quarter-hour day and its orbit of the Sun takes almost 30
years.
Although 750 times larger than Earth, Saturn is so buoyant that it would float in water. An
explanation of Saturn's composition, atmospheric features, internal structure, magnetic field and
aurorae. Saturn's rings - although spanning the distance between the Earth and Moon - are less
than a kilometer thick. Viewed edge-on from Earth, they all but disappear. Comprising billions of
moonlets, from specks of dust to icy boulders the size of trucks, the rings may well be pulverized
space rocks. Voyager space probes reveal the complexity of the rings - ringlets, divisions,
mysterious spokes and shepherd moons.
A tour of some of Saturn's 18 known moons. Most exotic is the largest Saturnian satellite, Titan.
Discovered in 1655 by the Dutch astronomer Huygens, Titan is like a frozen infant Earth, its
surface entirely shrouded by smog. In 2004, the Cassini spacecraft will reach Saturn, deploying a
lander to investigate Titan's atmosphere and surface. Will there be oceans of liquid hydrocarbons?
Is Titan a laboratory for life?
Program Segments
Ringed Planet
• Lift-off for Cassini, a one-billion-dollar space mission to explore the ringed planet.
• Saturn's position in the Solar System - an animated journey from Earth, past Mars and
Jupiter.
• Saturn's orbit, daily rotation, and size comparison with Jupiter.
• A buoyant gas giant that would float in water.
• Saturn swallowing Earth nearly 750 times.
Atmosphere & Magnetic Field
• Saturn's composition - 94 per cent hydrogen, the rest mainly helium.
• Belts of weather, obscured by haze. Winds roaring at 1,500 kilometers per hour.
• Saturn's atmosphere, layer-by-layer - and internal structure, down to the rocky core.
• A magnetic field - like Jupiter's, generated in a layer of metallic hydrogen.
• Auroral haloes at the poles, created by the solar wind.
Rings in Close-Up
• Fly-through of the rings from afar. In close-up, they're revealed as billions of rocks of
widely different sizes.
• Saturn and its system of rings that would fit between Earth and the orbit of the Moon.
• From Earth, our view of the rings changes as Saturn orbits the Sun - because of the
planet's axial tilt. Sometimes the rings appear wide open. At other times, they're edge-on
and virtually disappear because they are so thin.
• The enormous complexity of the rings as revealed by the Voyager spacecraft.
• The Cassini Division.
Small Icy Moons
• Shepherd moons marshalling the ring particles - and a theory of ring formation.
• Voyager timelapse of Saturn and its moons. A selection of the most diverse:
• Tiny Mimas, made of ice and rock, scarred by a huge impact.
• Enceladus, nearly twice the size of Mimas - icy fountains may spurt as Enceladus is
flexed by the pull off its neighbors.
Larger Moons
• Tethys, with a great canyon scouring its icy surface.
• Dione, one hemisphere brighter than the other - a world of craters and troughs.
• Rhea, larger still, and pocked by numerous impacts.
• Iapetus, one hemisphere snow white, the other coal black.
• Tiny Hyperion, shaped like a hamburger.
Hail Cassini!
• Titan, Saturn's biggest moon, larger than the planet Mercury. Carbon-based compounds
may be present, but Titan is hidden beneath an impenetrable veil of smog.
• The Cassini craft due to arrive in 2004 - deploying a lander named Huygens that
descends through the Titan atmosphere.
• Touchdown of Huygens - maybe a splashdown if there are oceans of hydrocarbons there.
• Data is flashed back to Earth via Cassini's orbiter.
Background
Saturn's Weather and Winds
In many ways, Saturn looks Jupiter. There are alternating bands of dark and light clouds
- called belts and zones, respectively. These bands lack the colorful contrast of Jupiter's. After
computer enhancement, however, images acquired by the Voyager spacecraft revealed features in
the clouds such as storm systems. Voyager also spotted ovals of white, brown and red. White
clouds are composed of ammonia particles but scientists are baffled why other clouds are brown,
blue and red. They may be colored by other chemicals, organic compounds, or complex reactions
in the atmosphere.
The different appearances of Jupiter and Saturn are linked to their different masses.
Saturn is less than one-third as massive as Jupiter. Jupiter's powerful "surface" gravity - the result
of the planet's much greater mass - compresses its main cloud layers to a depth of just 75
kilometers. Saturn's relatively weaker gravity causes less compression, so its main cloud layers
increase to a depth of nearly 300 kilometers. Consequently, the colors of Saturn’s cloud belts are
less dramatic than those on Jupiter because they are partly obscured by the deep haze layers lying
above.
By following various features in the cloud belts of Saturn, scientists have analyzed the
planet's winds. Like Jupiter, Saturn has counterflowing westward-moving and eastward moving
currents - causing strongly zonal weather systems. At Saturn's equator, the cloud top winds reach
500 meters per second (1,800 kilometers per hour), which makes them, at this level,
approximately two-thirds the speed of sound! Saturn, like Jupiter, has an internal heat source.
Both planets radiating more energy than they receive from the Sun.
The Rings of Saturn
Saturn is perhaps best known for its magnificent system of rings. Astronomers have discovered
that all four gas giants (Jupiter, Saturn, Uranus and Neptune) have ring systems. Saturn's are by
far the brightest and best known.
The bright rings are 273,000 kilometers in diameter - rather less than the distance between Earth
and the Moon. They rings, however, are remarkably thin - probably only a few hundred meters
deep and, in some places, no more than 30 meters. How did they form?
Either by the break-up of one of Saturn's moons which strayed too close to the planet and
was torn apart by tidal forces. Or by the failure of small fragments of icy material to
accrete into a moon-sized body.
Three major rings are visible from Earth. The outer A ring is separated from the brighter B ring
by the 4,000-kilometre wide Cassini Division. The division, surprisingly, is not empty but simply
has a lower density of fragments. Another division is in the outer half of the A ring. Named the
Encke Division, it is only about 270 kilometers wide. Within the B ring is the faint, semitransparent C ring, also known as the Crepe Ring.
When the Voyager spacecraft flew past Saturn, they showed that the three major rings were
subdivided into thousands of ringlets - with many, many gaps. The Voyagers also discovered two
more rings - G and E - much farther from Saturn. They were quite tenuous and extended towards
the planet's moon system. Pioneer 11 detected the very narrow F ring just beyond the outer edge
of the A ring.
The rings are not solid, rigid sheets of matter. They consist of myriads of tiny moonlets, ranging
in size from tiny dust grains to icy boulders tens of meters across, all moving along their own
individual orbits around Saturn. Observations have shown that frozen water is definitely present
in the ring particles. At temperatures of between -180 and -200 degrees Celsius, this water ice is
in no danger of evaporating.
Shepherd Moons
The Voyagers showed that the particles in Saturn's F ring are confined to a well defined, narrow
band, no more than 100 kilometers in width. Voyager cameras also revealed that two tiny
satellites, each measuring about 50 kilometers across, orbit Saturn on either side of the F ring. It
is the gravitational influence of these two moons which keeps the ring particles in position. The
inner satellite, Prometheus, moves around Saturn at a slightly faster speed than icy particles at the
inner edge of F ring. As this satellite overtakes the ring particles, its gravitational pull tends to
speed them up, nudging them into orbits a little farther from Saturn, and back into the narrow
central region of the F ring.
The outer satellite, Pandora, moves around Saturn at a somewhat slower speed than particles at
the outer edge of the F ring, so as they pass by the satellite they experience a tiny gravitational tug
that tends to slow them down. This causes them to move into orbits a little closer to Saturn, and
back into the main part of the F ring. The combined effect of the two moons confines the F ring
particles into the narrow, well-defined band that we observe. For obvious reasons, the two tiny
satellites, Prometheus and Pandora, are called "shepherd moons".
Another shepherd satellite, Atlas, orbits Saturn just beyond the A ring's outer edge. It is the
influence of this moon that is responsible for the sharp border to the ring. Towards the outer edge
of ring A, particles overtake the slower moving Atlas. In doing so, they experience a gravitational
tug that slows them down very slightly, preventing them from wandering into orbits farther from
Saturn.
Links for Further Information:
Cassini Home Page. Information on Saturn, its rings and moons, the spacecraft, the
mission, facts and images and a downloadable teachers' guide.
http://www.jpl.nasa.gov/cassini/
Information on Saturn, internal and external features, atmosphere and magnetosphere,
moons and rings, facts and myths, missions, news and images.
http://www.windows.umich.edu/cgi-bin/ tour.cgi?link=/saturn/saturn.html
Animation images of Saturn, its features, moons and rings, plus text. Cassini
information and images.
http://www.jpl.nasa.gov/cassini/Images/slides/slidetop.html
Exploring the planets - Saturn. View from Earth, atmosphere, magnetosphere, rings,
moons, future exploration, past missions, images and links.
http://ceps.nasm.edu:2020/ETP/SATURN/etpsaturn.html
Saturn's ring-plane crossing of 1995 -1996. Images of Saturn's rings as viewed from
Earth. Links to Saturn's rings, moons, science background and images.
http://newproducts.jpl.nasa.gov/saturn/
Questions and Activities for the Curious:
1. If Earth were the size of tennis ball, how big would Saturn be on the same scale?
What would be the diameter of its rings? How big would the Sun be, and how would Saturn
be from the Sun on this scale?
2. If there were an ocean big enough, Saturn would float - whereas Jupiter would sink.
Explain why this is so.
3. Why do features in Saturn's atmosphere appear to be much fainter and "washed out"
compared to the features in Jupiter's atmosphere?
4. Discuss the various layers that make up Saturn's atmosphere and its internal
structure. Where is its magnetic field produced?
5. Imagine you are in a spacecraft skimming across Saturn's rings and heading towards
the planet. Describe what you would see.
6. During the almost 30 years that it takes Saturn to orbit the Sun, the appearance of
its rings as seen from Earth changes. Describe these changes and why they happen.
7. Research and report on the similarities and differences between the known moons of
Saturn. Explain how "shepherd moons" work.
8. Describe the appearance of Saturn's largest moon Titan, the composition of its
atmosphere, and the probable conditions on its surface.