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Planetary Rings

All of the jovian planets in our solar system have rings. The rings of Saturn are the brightest and best known planetary ring systems. Saturn was a great puzzle to Galileo, the first person to point a telescope at the planet in the year 1610. In Galileo's relatively crude instrument, Saturn's rings looked like a pair of ears or bumps on either side of a larger disk. A few years later Galileo noted that these "bumps" had disappeared. As telescopes improved, they provided sharper views of the planets. In 1659, Christian Huygens announced that he was able to resolve the "bumps" observed by Galileo into a flat disk structure ringing the entire planet. The occasional disappearance of the rings was explained by the fact that every fifteen years or so the rings are tilted "edge on" toward the Earth, making it impossible to see them even in large telescopes, the most recent "ring plane crossing" occurred in August 1996. This in turn suggests that the rings of Saturn are very thin relative to their diameter; in fact, if you were to build a scale model of Saturn, making the rings only as thick as a piece of typing paper, about 0.1 millimeters, the diameter of the ring system would be about forty meters.

(Voyager 1 image of Saturn and its rings taken Nov. 16, 1980 four days after closest approach to Saturn, from a distance of 5,300, 000 km (3,300,000 miles))

The nineteenth-century physicist James Clerk Maxwell argued that the rings of Saturn are not solid, but consist of uncountable millions of smaller particles orbiting Saturn in much the same way as our Moon orbits the Earth. Maxwell's theory was confirmed in 1895, when spectroscopic analysis revealed that the inner edge of the Saturn ring system revolved faster than the outer edge, which would be impossible if the rings were a single, solid object. The American Pioneer and Voyager spacecraft have revealed the rings of Saturn to be composed of icy particles ranging in size from a fraction of a millimeter to twenty or thirty meters across.

Jupiter, Uranus, and Neptune also possess thin rings of icy and rocky material.  These ring systems are not nearly as extensive as the rings of Saturn and are practically invisible to ground-based telescopes.  The rings of Uranus were discovered through a phenomenon called occultation. An occultation occurs whenever one astronomical body passes in front of another.  Astronomers observing Uranus in 1976 noticed that a bright star passing behind Uranus "flickered" before it was blocked by the disk of the planet. The flickering pattern was repeated exactly as the star emerged on the other side of Uranus's disk, which ruled out the possibility that the flickering was nothing more than twinkling caused by turbulence in Earth's atmosphere.  The astronomers concluded that Uranus is encircled by several thin rings.

Planetary rings are probably created when a small moon collides with another moon, or ventures so close to its planet that gravitational tidal forces shred it apart.  The resulting fragments spread out into concentric orbits, breaking into ever smaller fragments through repeated collisions, eventually forming a set of rings.  Ring systems are thought to be a somewhat transient phenomena, lasting perhaps several hundred million years. Rings as magnificent as the rings of Saturn are probably rare.  It is truly a privilege to live in our solar system at a time when this extraordinary planetary spectacle is readily visible in backyard telescopes.