The Astronomer's Story
By Dick French

Saturn and its serenely floating ring system have fascinated people since they were first glimpsed in Galileo's primitive telescope more than 300 years ago.

The planet is about 75,000 miles across, large enough to hold 850 Earths. Being made mostly of hydrogen and helium gas, Saturn would float on water (if a large enough bathtub could be found). Deeper into the atmosphere, the visible clouds and gases merge gradually into hotter and denser gases, and nowhere would one find a solid surface to land upon. The huge storm system seen near the equator is a feature which comes and goes with Saturn's seasons; its structure is due to the jet stream winds (1000 miles per hour) which are found near the equator of Saturn - the strongest in the solar system.

The rings, only tens of meters in thickness, appear razor-thin when seen edge-on in telescopic images. They cover a distance more than two-thirds the distance from the Earth to the moon and are covered with filigreed structure. The two dark bands in this picture of the rings are the relatively wide Cassini Division near the center (which is not really empty but a kind of ring itself) and the relatively narrow Encke gap near the outer edge (which harbors a moon 12 miles across). The rings are made mostly of water ice, in the form of rubbly boulders and chunks, which constantly collide gently with each other as they orbit the planet at 35,000 miles per hour.

While it seems likely that constant interplay of gravity and collisions gives the rings their structure, the precise origin of the majority of the structure remains a mystery. Mixed in with the water ice are trace amounts of reddish material (perhaps organic molecules), and darker, carbon-rich material, which results from the hailstorm of cosmic debris, which populates interplanetary space.

These Hubble Space Telescope observations will help us determine the composition of the rings and how it varies in space and in time. Occasionally, a relatively large meteoroid strikes the rings and causes a "spoke" to form; a pair of such dark, shadowy smudges are seen faintly on the East (left) side of the rings, near the middle. Spokes actually look brighter than their surroundings when viewed from different directions, indicating they are regions rich in fine dust. Hubble results have also deepened our understanding of these features.

How these rings came to be, and how they survive the forces that are dimming their brightness and dragging them into the planet, are questions of great current interest. Their total mass is about the same as that of one of Saturn's small inner moons, Mimas, which about 240 miles across. Was a Mimas-sized local resident destroyed by a smaller intruder, leaving its rubble to encircle the planet? Or, was a large intruder itself torn apart by Saturn's tides, much as the smaller comet Shoemaker-Levy 9 was rent assunder by Jupiter in 1995? New results from the Hubble Space Telescope and the Cassini spacecraft, arriving at Saturn in 2004, will help us answer these questions.