Saturn's most remarkable feature is the system of thin, concentric rings lying in the plane of its equator. Although first observed by Galileo in 1610, it was not until 1656 that the rings were correctly interpreted by Christiaan Huygens, who did not reveal his findings about their phases and changes in shape until his treatise Systema Saturnium was published in 1659. Saturn's rings were believed to be unique until 1977, when very faint rings were found around Uranus; shortly thereafter faint rings were also detected around Jupiter and Neptune.
Although the main ring system is almost 167,770 mi (270,000 km) in diameter, it is only some 330 ft (100 m) thick. From earth, this system appears to consist mainly of two bright outer rings, denoted A and B (lettered from the outermost), separated by a dark rift—discovered by the Italian-French astronomer Gian Domenico Cassini—known as Cassini's division, plus a third, faint inner crepe ring (denoted C). The Encke Division, or Encke Gap, which splits the A ring, is named after the German astronomer Johann Franz Encke, who discovered it in 1837. In 1859 the Scottish physicist James Clerk Maxwell showed that the main rings must consist of countless tiny particles each orbiting the planet in accordance with the laws of gravitation. In the 1980s pictures from the Voyager probes showed four additional rings. The exceedingly faint D ring lies closest to the planet. The faint F Ring is a narrow feature just outside the A Ring. Beyond that are two far fainter rings named G and E. In 2009 an enormous but faint ring consisting of tiny dust particles was discovered extending from 3.7 to 7.4 million mi (6 to 12 million km) away from Saturn. Lying at a 27° angle to the main rings, this ring has a retrograde orbit and is believed to have originated in material ejected from the moon Phoebe by small impacts. When edgewise to the earth Saturn's main rings appear as a nearly imperceptible ribbon of light across the planet; this occurs twice during the 291⁄2-year period of revolution. Twice during each orbit the rings reach a maximum inclination to the line of sight, once when they are visible from above and once when visible from below.
The Voyager 1 (1980) and 2 (1981) space probes revealed incredible new detail as they passed within 78,000 mi (126,000 km) and 63,000 mi (101,000 km) of Saturn, respectively. They recorded hundreds of tiny rings that are grouped into the seven major rings. The three brightest rings (A, B, and C) dissolved into more than 1,000 narrow ringlets, 100 of which are in the Cassini division. The outer F ring was found to contain braids, knots, and strands, possibly caused by nearby moons that shepherd it, that is, limit the extent of a planetary ring through gravitational forces. The main rings are believed to have been formed mainly from larger satellites that were shattered by the impact of comets and meteoroids; geyserlike eruptions on Enceladus contribute material to the E ring. Cassini revealed that the rings consist mainly of water ice.
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