The Visible Planets

Hubble Space Telescope Image of Saturn's rings edge on.

In 1610 Galileo turned his primitive telescope to the heavens and was astonished to observe a bright star flanked by two dimmer ones. What Galileo had discovered was a strange new world, a planet that was to become one of the wonders of the solar system, and to be named Saturn.

Saturn's beauty comes from its magnificent rings that Galileo described as "handles" or large moons on either side of the planet. He said, "I have observed the highest planet [Saturn] to be tripled-bodied. This is to say that to my very great amazement Saturn was seen to me to be not a single star, but three together, which almost touch each other". Two years later Galileo was puzzled to discovered the rings had disappeared. What Galileo did not know was that the Earth had passed through Saturn's ring plane and the rings were edge on and could not be resolved with his 20x telescope. Galileo's discovery of Saturn commenced the scientific process of unraveling the mysteries of Saturn's extensive ring system, but even after 400 years the mysteries have not been all solved.

In 1655 Christann Huyges proposed that Saturn was surrounded by a solid ring, "a thin, flat ring, nowhere touching, and inclined to the ecliptic." As time went on, more and more observations were made and theories proposed. It took until the mid 1800s before the notion that the ring was made out of small particles rather than being solid became the prominent theory.

In 1979 Pioneer 11 became the first spacecraft to fly past Saturn and take pictures of the planet and rings. Although the pictures are of low resolution by today's standards, they were better than any previous photos taken through telescopes. In 1980 and 1981 Voyager 1 and Voyager 2 flew past the planet and took numerous high-resolution images of Saturn and its rings. The next major milestone will be in 2004 when the Cassini spacecraft arrives at Saturn. Cassini will be the first spacecraft to go into orbit about the planet. Here it will operate studying Saturn, its moons, rings and environment for the next four years.

Cassini's Sketch of Saturn in 1676 Showing the Gap in the Rings.

Saturn's ring system is divided up into 7 major divisions with alphabetic designators in the order of discovery. From the innermost ring to the outermost ring the designators are D, C, B, A, F, G and E. Each major division is further subdivided into thousands of individual ringlets. The F and G rings are very thin and difficult to see while the A, B, and C rings are broad and quite visible. Between the A and B rings is a gap called the Cassini division named after Giovanni Cassini who discovered the Gap in 1676.

Much of the elaborate structure of the rings is due to gravitational effects of nearby satellites. This phenomenon is demonstrated by the relationship between the F-ring and two small moons Prometheus and Pandora that shepherd the ring material. The F-ring shows a complex structure made up of two narrow, braided, bright rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons.

In the mid 1980s Dr. Jeff Cuzzi noticed a wavy pattern in the ring material on both sides of the Encke gap. It was suggested that an unseen asteroid-sized moon in the gap caused the disturbance. Dr. Mark Showalter further analyzed the disturbance and used this "moonlet wake" pattern that resembled a motorboat wake, to determine the position and mass of the unseen body. The amplitude of the waves, he said suggested the mass of the unobserved object and the wavelength of the ripples revealed the moon's possible position. Using this mathematical model, Dr. Showalter was able to predict which Voyager images the moon would be in. In 1990 Dr. Showalter's work paid off and Pan was discovered within the Encke gap.

This picture shows a 35 minute sequence (top to bottom) of the change in spoke appearence. It also shows the swift formation of a new, radially aligned spoke
(see arrow in the bottom image).

One of the curious discoveries made by the Voyager spacecraft was the presence of dark radial features up
to 20,000 kilometers long that move in curious patterns
on the B ring. These features commonly known as spokes had scientists perplexed because they seemed to defy gravity.

Spokes are thought to be microscopic dust particles that have levitated away from the ring plane. They appear on both sides, near the densest part of the B ring. They rotate
at the same rate as Saturn's magnetic field indicating they
are affected by electromagnetic forces. They form about 104,000 kilometers from Saturn's center and extend out
to the Cassini division. They are shapped somewhat like
an hour glass with the narrow centers located near the synchronous orbit radius. Perhaps they are formed by meteorites that punch through the rings, by tiny moonlets as they plunge through the rings, or dust particles that have taken on a charge and levitated off the larger ring bodies.

Saturn's rings span a great distance with the inner D ring approximately 6,700 kilometers from Saturn's cloud tops to the fringes of the E ring, 480,000 kilometers out. Surprisingly, most of the rings are only a few tens of meters thick with a total mass equivalent to a medium sized moon. The rings are made out of particles ranging from microscopic dust to barnyard sized boulders with perhaps a few kilometer-sized objects as well.

This image of Saturn from the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), taken Jan. 4, 1998 shows the planet's reflected infrared light. This view provides detailed information on the clouds and hazes in Saturn's atmosphere and rings.

Near-infrared observations from Earth have shown that the rings are composed mostly of ice crystals with some impurities.Scientists once thought that the rings were formed at the same time, as the planets when they coalescing out of swirling clouds of interstellar gas 4.8 billion years ago. Under this model, remnants of material within the Roche limit could not condense and would become rings. However,in recent years this idea seems to be flawed. The rings appear to be young, perhaps only hundreds of millions of years old. One of the clues to this theory is that the rings are bright. As Saturn travels though space, the rings accumulate dust particles that have been darkened from solar radiation. If the rings were old, they should appear dark. Another theory suggests that perhaps a comet few too close to Saturn and tidal forces broke it into pieces similar to comet Shoemaker-Levy/9. Perhaps one of Saturn's moons was struck by an asteroid smashing it into the bits and pieces that form the rings. Hopefully the Cassini spacecraft can answer some of these questions.