Saturn

The planet Saturn is a delicate giant. Although it is the second largest planet in the solar system, it's the least dense — less dense than water. Chemical compounds in its upper atmosphere color its cloud bands in subtle shades of ivory, yellow, and tan. And broad, lacy rings encircle Saturn, making it the most beautiful planet in the solar system.

Saturn's rings

Saturn At a Glance

Discovery

Known since antiquity

Name

Roman god of agriculture

Average Distance from Sun

885,904,700 miles
1,426,725,400 km
9.5 Astronomical Units

Mass

95 times Earth's mass

Equatorial Diameter

74,898 miles
120,536 km

Length of Day

10 hours, 39 minutes

Length of Year

29.4 Earth years

Surface Gravity

0.74 that of Earth (If you weigh 100 pounds, you would weigh about 74 pounds on Saturn)

Known Moons

62 as of April 2009: Titan, Enceladus, Iapetus, and Mimas

Like its larger sibling, Jupiter, Saturn is a ball of hydrogen and helium gas wrapped around a dense, rocky core. Saturn spins so fast that it bulges outward at the equator, so the planet is much thicker at the equator than through the poles.

Saturn's rapid rotation and its layered structure produce a magnetic field. Observations by the Cassini spacecraft suggest that the field may be changing, which could mean that Saturn's interior is changing as well. Cassini monitored radio waves produced by the magnetic field as a way to measure Saturn's rotation rate. (Because Saturn has no solid surface, it's impossible to measure its rotation by tracking surface features like mountains or canyons.) But the craft found that the rotation rate appeared to have slowed by about six minutes since the Voyager missions two decades earlier. Scientists believe that Saturn is not actually slowing down. Instead, one possible explanation says that changes in the planet's core are creating changes in the magnetic field.

Saturn's clouds contain ammonia, methane, and other toxic compounds. They are buffeted by winds of up to 1,000 miles (1600 km) per hour, and they contain violent storm systems that produce lightning blasts a million times more powerful than those on Earth.

Saturn's most prominent feature, though, is its extensive ring system.

Galileo Galilei discovered the rings in the early 17th century. In his small, crude telescope, though, they looked like "bumps" on the side of the planet. Five decades later, Dutch astronomer Christaan Huygens, who had recently discovered Saturn's largest moon, Titan, detected a bit of space between Saturn and the bumps. He deduced that the bumps were really planet-circling rings.

Today, astronomers know that thousands of individual rings make up Saturn's ring system. Some rings are made of small bits of frozen water, others contain tiny grains of dust, and still others are a mixture of the two. In all, the rings are only a few hundred feet thick. Several small moons orbit inside or just outside the ring system. These "shepherd" satellites help keep the ring particles in place, but they also sculpt some rings into odd shapes, with twists and kinks.

Saturn's rings probably formed when a small moon or a comet passed close to Saturn and was pulled apart by the planet's gravity.

French mathematician Edward Roche first described this process around 1850. A planet's gravity pulls more strongly on the side of a moon that is facing it than on the side that is away from it. If a moon passes too close to its parent planet, this difference, called tidal gravity, pulls it apart, turning the moon into cosmic rubble. Over time, the rubble spreads out to form rings -- like those that encircle Saturn.

Although Saturn has no solid surface to stand on, humans may someday view its rings from close range. They may walk on some of its icy moons or even float above Saturn's clouds in big balloons. From such a lofty vantage point, the rings would form wide, sparkling bands across the sky. Sometimes, icy particles from the inner edge of Saturn's rings may fall into the planet's atmosphere, creating bright "shooting stars" as they streak through the sky of this delicate giant.

Viewing Guide

Shines brightest this year in May, as it moves through Libra.

Exploration

Cassini view of one of Saturn's moons, with the rings in the backgroundCassini view of a moon and ringsCassini entered orbit around Saturn in the summer of 2004, and has transmitted tens of thousands of images of the planet and its rings and moons. On January 14, 2005, a second part of the mission, the Huygens probe, parachuted to a soft landing on Titan. Its images showed a landscape carved by flowing liquid. Cassini's instruments have peered through Titan's atmospheric haze to discover possible pools of liquid and a possible volcano on Titan's surface. Cassini is scheduled to continue its reconnaissance of the Saturn system until 2017.

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