Saturn (planet)

Saturn (planet), sixth planet in order of distance from the sun, and the second largest in the solar system. Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes. He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae).

The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.” The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.

Courtesy of E. Karkoschka (University of Arizona), and NASA/Material created with support to AURA/ST Sci from NASA contract NAS5-26555.

This infrared photo of the planet Saturn has been color coded to indicate the cloud level in Saturn's atmosphere. Violet and blue represent areas in which Saturn's atmopshere is clear down to the main cloud layer. Green and yellow show layers of haze above the main cloud layer (yellow represents thicker haze). Red and orange indicate the highest level of clouds, thicker than the haze. White areas are areas of the atmosphere with high levels of water vapor. The bright dots at the upper left and lower right of the picture are Saturn's satellites Tethys and Dione, respectively. The Hubble Space Telescope took this image in 1998.

The Ring System

The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick. They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter—from dust to boulder size. An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system.

The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring.

Saturn's rings are extremely wide, but very flat, bands created by orbiting fragments of rock, gas, and ice. Although there are more than 100,000 separate ringlets, they are generally referred to as part of the broad A, B, and C rings (starting from the outside). Cassini's Division, the distinct gap between the A and B rings, is approximately 4800 km wide. Voyager 2 captured this enhanced view from 8.9 million km (5.5 million mi) away as it flew by the planet in 1981. © Microsoft Corporation. All Rights Reserved.

Contributed By: Tom Gehrels, Ph.D. Professor, Lunar and Planetary Laboratory, University of Arizona. Recipient of NASA Medal for Exceptional Scientific Achievement. Editor, Planets, Stars, and Nebulae Studied with Photopolarimetry. Author of On the Glassy Sea: An Astronomer's Journey.

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