Satellites of Saturn

Satellites of Saturn

By Daniel Zimmermann

Shepherd Satellites of Saturn

Rings of Saturn

Several planets have a system of rings, but Saturn is the ringed planet par excellence.. In fact, until the era of space exploration, people thought that Saturn was the only planet with rings.

Saturn’s rings have gradually been revealing their secrets to the prying eyes of modern science. They consist of many small bodies, most of which revolve around Saturn in a counterclockwise direction. The only exception is the so-called Phoebe ring, which exhibits a retrograde motion, just like the satellite Phoebe.

Frozen water is frequently a constituent of these revolving bodies, but silicates and other materials also occur. Some are hefty rocks ten meters in diameter, while others are too small to measure with an elementary school ruler. Oxygen is present in the rings, but you would not find enough to breathe.

Astronomers have applied easily remembered nomenclature to Saturn’s rings. The D Ring is closest to Saturn. Farther out is the C Ring, then the B Ring. Beyond the B Ring comes the Cassini Division, discovered by the illustrious astronomer after whom it is named. The A Ring lies beyond the Cassini Division, followed by the Roche Division and the F Ring.

Other rings lie beyond, somewhat separated from this more compact group. They are the Janus/Epimetheus Ring, the G Ring, the Methone Ring Arc, the Anthe Ring Arc, the Pallene Ring, and the E Ring. Finally, far beyond the other rings, the Phoebe Ring forms a large angle with the plane of Saturn’s equator. These outer rings seem to be composed of material detached from various moons of Saturn. For example, the Phoebe Ring occurs inside the orbit of Phoebe and may be particles detached from Phoebe.

This skeleton outline of Saturn ring geography is a convenient simplification. The complete story is more complex. For example, gaps occur in the major rings, such as the Encke Gap, which lies within the A Ring. The rings are divided into many ringlets; and to further confuse the issue, the Cassini Division contains various ringlets. They are just harder to see than the material in the neighboring A Ring and B Ring.

Christiaan Huygens discovered the rings of Saturn in 1655. Galileo Galilei had viewed them nearly a half century earlier, but he had no idea what they were, probably because his telescope was not as good as that of Huygens. Galileo made the telescope that he used not long after optical instruments had been invented in the Netherlands. Like most new technologies, these first telescopes left considerable room for improvement.

Pan

Of all the satellites of Saturn that have received official names, Pan is closest to the planet Saturn. However, some small moonlets are closer to Saturn than Pan. [See update at end the of this e-book.]

Pan revolves around Saturn within the Encke Gap. In fact, it is believed that the satellite Pan keeps the Encke Gap open. However, ringlets do occur in the Encke Gap including the Pandean Ringlet, which shares the same orbit as Pan.

As might be expected, the gravitational attraction of Pan causes disturbances in ring material. As Pan revolves around Saturn, it creates waves. In spite of these disturbances, astronomers believe that Pan stabilizes the ring. Pan is therefore called a shepherd satellite.

Pan looks like a flying saucer, according to the NASA website. Its equator protrudes considerably.

The average distance between Pan and the center of Saturn is 133,583 km. Daphnis, the satellite that is next closest to Saturn, is 136,500 km from the center of the mother planet at an average. (All figures for mean distances from the center of Saturn are taken from a table prepared by the European Space Agency.)

The discovery of Pan was one of the fruits of the Voyager II space mission. Voyager II photographed it in 1981. Cuzzi and Scargle noticed waves at the edges of the Encke Gap and theorized that an undiscovered satellite might be causing them. They accurately predicted the location of the satellite. The actual discovery was made in 1990, when Mark Showalter and others analyzed the Voyager II photographs.

Most of Saturn’s moons are named after