The Far Planets - Saturn

Saturn is the sixth planet from the Sun, and the second largest planet in our solar system. It takes 29.5 Earth years to orbit around the Sun. Saturn's atmosphere is mostly hydrogen, with a little helium and methane. It is a hazy yellow color. The planet is very windy, with wind speeds reaching 1,000 miles per hour.

Saturn is very flat at the poles. The planet is surrounded by many thin rings of orbiting material that circle near its equator. Saturn has dozens of moons. They are named after various characters from Greek and Roman mythology (mainly Saturn's siblings, the titans) and after giants from Gallic, Inuit, and Norse legends.

Galileo Sees Saturn's Handles

In 1610, when Galileo first saw Saturn through his telescope, its rings appeared to him to be two dim stars on either side of the planet. He described these stars as "handles." In 1612 Galileo reported that he could no longer see the dim stars. Much to his amazement, they had disappeared.

In the following years other astronomers saw the strange shapes around Saturn. They were variously described as ears or arms extending from the planet's surface. It would take an improvement in telescopic power before their true nature was revealed.

Huygens Finds a Moon and a Ring

Christiaan Huygens (1629–95) was a Dutch astronomer who became famous for his observations of Saturn. He and his brother Constantyn built new and more powerful telescopes that were greatly admired by astronomers of the time.

On March 25, 1655, Christiaan Huygens discovered a satellite around Saturn. This turned out to be the planet's largest moon. In 1656 Huygens wrote about his discovery in De Saturni Luna Observatio Nova (The Discovery of a Moon of Saturn). Huygens referred to his discovery as simply Saturn's Moon. Later, it would be called Titan.

Huygens also figured out that the mysterious shapes near Saturn were not stars, arms, or ears, but a ring of material around the planet. Huygens mistakenly thought the ring was one solid object. In 1659 he published his observations in Systema Saturnium (The Saturn System).

Ring Plane Crossings

Huygens explained that the ring around Saturn was difficult to view, because it is very thin. Every fourteen to fifteen years the Earth moved into the same plane as the ring. If you tried to observe Saturn from Earth at this time, Huygens said, you would be viewing the outer edge of the ring head-on, making it virtually invisible. This explained why Galileo was unable to see the handles around Saturn in 1612. It was a year in which Earth passed through Saturn's ring plane.

There are several planetary alignments that cause Saturn's rings to be invisible to Earth observers. One of these is when Earth passes into the Saturn ring plane. A similar effect occurs when the Sun passes through Saturn's ring plane and when the Sun and Earth are on opposite sides of the ring plane. The next Saturn ring plane passage will occur in August and September of 2009. Throughout history ring crossings have been the best times to discover new moons around Saturn.

More Moons of Saturn

Giovanni Cassini (1625–1712) was born in Italy but lived in France. He was the first director of the Royal Observatory in Paris. During the late 1600s he discovered four more of Saturn's moons. The first two he observed during the ring crossing of 1671–72. The second pair he discovered just prior to the ring crossing of 1685.

Over the next three centuries many more Saturn moons were discovered. In December 2004 astronomers at the Mauna Kea observatory in Hawaii found twelve previously unknown moons around Saturn. The latest discovery occurred in May 2005 when NASA reported that the Cassini spacecraft orbiting Saturn had captured an image of a "new" moon.

Cassini and Saturn's Rings

During the 1600s Giovanni Cassini discovered a major gap in the ring around Saturn. This proved that the structure was not one solid object as Huygens had thought. The gap would later be called the Cassini Division. Cassini believed that Saturn's rings were composed of millions of small particles. This view was shared by French astronomer Jean Chapelain (1595–1674). However, it was not generally accepted until the eighteenth century.

FIGURE 8.2 Saturn ring composition Linda J. Spilker, editor, "Saturn's Flying Snowballs," in Passage to a Ringed World: The Cassini-Huygens Mission to Saturn and Titan, National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, October 1997, http://saturn.jpl.nasa.gov/multimedia/products/pdfs/ptarw.pdf (accessed December 28, 2005)

Modern astronomers believe that the rings are composed of chunks of ice. These chunks range in size from tiny particles to icebergs as large as automobiles. (See Figure 8.2.) Saturn's ring system is actually many ringlets of different sizes nestled within each other with gaps between ring systems. Scientists use letters to designate distinct ring systems around the planet, as shown in Table 8.1.