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The Ringed Planet: Saturn
Image of Saturn
Saturn is the most distant of the five planets known to ancient stargazers. In 1610, Italian Galileo Galilei was the first astronomer to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet, which he later drew as "cup handles" attached to the planet on each side. In 1659, Dutch astronomer Christiaan Huygens announced that this was a ring encircling the planet. In 1675, Italian-born astronomer Jean Dominique Cassini discovered a gap between what are now called the A and B rings.

Image to right: Voyager 2 was able to capture Saturn's true color during its' mission in the early 1980s. Credit: NASA

Like Jupiter, Uranus, and Neptune, Saturn is a gas giant. It is made mostly of hydrogen and helium. Its volume is 755 times greater than Earth's. Winds in the upper atmosphere reach 500 meters per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters per second.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in its atmosphere.

Saturn's ring system is the most extensive and complex in our solar system; it extends hundreds of thousands of kilometers from the planet. In fact, Saturn and its rings would just fit in the distance between Earth and the Moon. In the early 1980s, NASA's two Voyager spacecraft revealed that Saturn's rings are made mostly of water ice, and they found "braided" rings, ringlets and "spokes" - dark features in the rings that seem to circle the planet at a different rate from that of the surrounding ring material. Some of the small moons orbit within the ring system as well. Material in the rings ranges in size from a few micrometers to several tens of meters.

Saturn has 31 known natural satellites (moons). The largest, Titan, is a bit bigger than the planet Mercury. Titan is shrouded in a thick nitrogen-rich atmosphere that might be similar to what Earth's was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth as well.

In addition to Titan, Saturn has many smaller "icy" satellites. From Enceladus, which shows evidence of surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn's satellites is unique.

Saturn, the rings, and many of the satellites lie totally within Saturn's enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. Recent images by NASA's Hubble Space Telescope show that Saturn's polar regions have aurorae similar to Earth's Northern and Southern Lights. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines.

The next chapter in our knowledge of Saturn is already under way, as the Cassini-Huygens spacecraft began its journey to Saturn in October 1997 and will arrive on July 1, 2004. The Huygens probe will descend through Titan's atmosphere in late November 2004 to collect data on the atmosphere and surface of the moon. Cassini will orbit Saturn more than 70 times during a four-year study of the planet, its moons, rings and magnetosphere. Cassini-Huygens is a joint NASA/European Space Agency mission.

Published by the Solar System Exploration Web site