Saturn

Statistics:

 Early Exploration of Saturn
 The Rings and Moons of Saturn
 Saturn's Atmosphere
 The Cassini Mission to Saturn

 History of Exploration

Considered by many professional and amateur astronomers to be one of the most beautiful objects in the sky, Saturn and its wonderful system of rings are both easily viewed through even the smallest of optical telescopes.  In the year 1610, the famous astronomer Galileo discovered something previously unknown about the planet Saturn; with the aid of a telescope, Galileo noted that Saturn was not round in appearance like every other planet he'd observed.  Rather, he said that this uncanny world appeared to have "ears" sticking out from its sides.  These "ears" remained a mystery until 1656, when the Dutch astronomer Christian Huygens made further observations of Saturn and explained that they were actually a ring of material that surrounded the planet.

Little else was learned about this ringed world until the Pioneer 11 space probe encountered Saturn in 1979.  Shortly thereafter, Voyager 1 approached Saturn in 1980 followed by Voyager 2 in 1981.  A great deal more information was returned by the Voyager probes than by Pioneer 11 due to improved technology.

Saturn's Wondrous Rings & Moons

Saturn's Rings -- The most spectacular feature of this gas giant planet is its ring system.  The rings measure about 280,000 km from edge to edge, and they are only a mere 20 meters or so in thickness; this means that (relatively speaking) the rings of Saturn are many times thinner than an old, vinyl phonograph record!  Despite this fact, however, the Saturnian rings are amazingly stable.  The rings are not solid objects because different parts of the rings have been observed to move at different rotational speeds; this indicates that the rings must be made of many smaller pieces of matter, with each having its own individual orbit around the planet.  In 1973, experiments were performed where radar waves were reflected off of the rings, and the results of this experiment indicated that the particles in the ring ranged anywhere from the size of dust to many meters across.  In addition, spectrographic analysis in the infrared wavelengths showed that many of these particles are water-ice covered.

The dynamics associated with the Rings of Saturn are very complex.  Due to the fact that the particles that make up the rings are rather closely packed together, then they can have a substantial gravitational influence upon one another; collisions between ring particles can take place that can lead to wave-like motion across the rings.  Many thousands of rings make up the Rings of Saturn, and these are sometimes referred to as "ringlets".  Large gaps can appear in the rings, such as the famous Cassini Division, discovered by Giovanni Cassini in 1675.  The following image (taken by the Voyager 2 probe) shows the intricate and complex nature of the rings when viewed close up...

 
False Color Image of Saturn's Rings
 
A common question is "How could such a structure form around a planet and be stable?"  There are two separate theories that attempt to answer this question:
  1. the Rings of Saturn are made of the remains of a satellite of the planet that was torn apart by gravitational tides.  Gravitational tides are an effect produced upon an object when one end of the object is being pulled upon by a world's gravity more than the other side; eventually, if the tidal forces are strong enough, the satellite will be torn apart.  In 1849, E. A. Roche was able to mathematically determine the sphere surrounding a planet inside of which collections of matter cannot be held together; he named this boundary the Roche limit.  Roche limits for most planetary bodies tend to be about 2.5 times the radius of the planet.
  2. the second idea is that the rings are material left over from the formation of Saturn.  Instead of collecting into the main body of the planet or into that of a moon, this material remained left over and eventually stabilized into a ring about the planet.
The Saturnian Moons -- Much like the Jupiter system, Saturn sort of represents a miniature solar system.  This is, of course, due to the extensive number of moons that orbit Saturn.  Of all of the planets in our solar system that have moons, Saturn has the most with a whopping 18 natural satellites!  However, for the purposes of this analysis, we will be focusing only on the largest of Saturn's satellites...

Titan is one of the largest (second only to Ganymede in size) moons in the entire planetary system; it is even larger than the innermost planet, Mercury.  However, the most interesting feature of Titan is its substantial atmosphere.  In fact, the smog-like atmosphere of Titan is so thick that no surface features can be seen in the optical wavelengths of light!
 

 
Two views of Titan from the Voyager Probes -- the rightmost image is Titan seen from behind.
 
Spectrographic analysis shows that the obscuring atmosphere of Titan is composed mostly of nitrogen (N2), with methane (CH4) making up only about 1% of the atmosphere.  Infrared measurements of this moon have also revealed the possible existence of hydrocarbon compounds on Titan; this combination of organic molecules has peaked the interest of many planetary scientists as it could give some clues as to the development of life.  Indeed, some wonder if there are life forms on Titan!  Titan's surface temperature was measured by the Voyager probes to be only about -180 oC; a point of interest to many scientists is that this temperature rests near what is called the triple point of methane, which means that methane could exist on Titan in solid, liquid, and gaseous form depending upon the local conditions at the time.  Many scientists think that methane could play the same role on Titan as water does here on Earth; that is, there could exist on Titan oceans and rivers of liquid methane or ethane (C2H6).  This strange mixture of temperatures and chemicals has led some planetary scientists to think of Titan as a sort of cold, primordial soup.

Though the smoggy atmosphere of Titan has blocked any optical views of its surface, the Hubble Space Telescope recently made observations of this moon by examining it in infrared (IR) light, much in the same way that radar observations of Venus were conducted.  The result is the map shown below.
 

Note that this is a false-color image of the surface of Titan, and that light areas represent high elevations whereas darker areas correspond to lower elevations.

Saturn's Atmosphere

Much like Jupiter, Saturn is a gas giant world.  And like Jupiter, Saturn has many of the same features associated with a gas giant...
 

  1. Saturn's mean density is only about 0.7 grams per cubic centimeter, which attests to the fact that Saturn must be composed mostly of lighter elements.  An interesting joke among astronomers and space scientists is that it one could find a large enough bath tub, then Saturn would float in it since its density is less than that of liquid water!
  2. Saturn' atmosphere is known to be made up of roughly 92.4% hydrogen and 7.4% helium with methane and ammonia making up the rest.  Like Jupiter, it is thought that as one goes deeper into the atmosphere, eventually hydrogen will be compressed into a liquid state.
Saturn's weather is not a violent as that of Jupiter, and some scientists attribute this to a lack of internal heat energy inside of Saturn.  However, large storms are observed to occur on Saturn, as shown by the following photograph...
 
Saturnian Storm in 1990 as seen by the Hubble Telescope

The Cassini Mission

Many more interesting discoveries about Saturn, its moons, and its rings await us.  These are the focus of a newly launched robotic probe named the Cassini/Huygens probe.  The Cassini mission started when the probe was launched from the Earth in late 1997, and the craft is expected to reach Saturn in 2004.  One of the top priorities of the mission is to study Titan in greater detail by dropping an atmospheric probe (Huygens) into the clouds of Titan.
 

The Huygens Probe descending into the atmosphere of Titan

For more information on this current expedition to Saturn, check out Purdue's special homepage on the Cassini mission, or look at NASA's Cassini mission website.

 

Back to Jupiter Move on to Uranus
 
Return to Main Tour Page
 
M. Lowry -- 1-8-98