A Surprising Turn

Saturn, which can be seen in the night sky with the unaided eye, was the farthest planet from Earth known to ancient astronomers. But studying this intriguing planet is not easy. When we look at Saturn through telescopes or even from spacecraft, we see only its dense cloud-covered atmosphere, partially obscured by a brightly lit high-altitude haze. The Italian astronomer and physicist Galileo (1564-1642) discovered Saturn's rings in the early 1600's. However, he thought he was seeing three satellites—and was annoyed to discover, when he looked at Saturn again a few months later, that two of the “satellites” had disappeared. (In fact, the angle of the rings relative to Earth had changed.) In the 1650's, Dutch astronomer Christiaan Huygens (1629-1695) discovered Titan and also concluded that what astronomers called “Saturn's arms” was actually a ring. Italian-born French astronomer Giovanni Domenico Cassini in 1675 discovered that Saturn had more than one ring.

Saturn is one of the oddest planets in the solar system. For example, although Saturn is the second largest planet (after Jupiter), it has the lowest density (amount of matter per unit volume) of all the planets. Saturn is the only planet that is, on average, less dense than water. In fact, if you could find a bathtub big enough to hold Saturn, the planet would float. This low density results from Saturn's composition, which is mostly hydrogen and helium, the two lightest elements. The planet gets its white and golden hues from the small amounts of ammonia and other, less common chemicals in its atmosphere.

Some of Cassini's findings have strengthened Saturn's reputation for strangeness. For example, Saturn is the only planet whose exact rotation rate continues to be a mystery to scientists. As the two Voyager spacecraft flew by Saturn in 1980 and 1981, they measured intense radio waves coming from an area of the planet near the equator. Because the most energetic of these radio waves had wavelengths about 1 kilometer (0.6 mile) in length, they were called Saturn Kilometric Radiation (SKR). (Wavelengths measure the distance between one peak or crest of a wave and the next.) Scientists believe the SKR are caused by ions (electrically charged particles) interacting with a magnetic field that originates deep inside Saturn. In the 1980's, NASA scientists used the Voyager measurements of the SKR to calculate that Saturn took 10 hours 39 minutes and 24 seconds to spin on its axis (compared with 23 hours 56 minutes and 4 seconds for Earth).

Cassini's measurements of the planet's SKR, taken by the probe's Radio and Plasma Wave Science (RPWS) instrument, however, suggested the amazing possibility that Saturn was spinning more slowly than it had in the 1980's. According to Cassini's data, Saturn was spinning on its axis once every 10 hours 45 minutes 45 seconds, about six minutes slower than in 1981. Scientists were astounded. If all Saturn's rings and all its moons had crashed into the planet, the collisions would not have decreased Saturn's spin by that much. And at any rate, astronomers on Earth certainly would have noticed any event monumental enough to affect this huge planet to such an extent.

Rather than conclude that Saturn has slowed down, Cassini scientists settled on another possibility. They speculated that Saturn's magnetic field might not rotate at the same rate, the way all of Earth's or Jupiter's does. Instead, the field's—and, thus, the planet's—rotation rate might vary with latitude, like the sun's. That is, regions nearer the poles spin at a slower rate than regions nearer the equator. Support for that idea came from RPWS data indicating that the source area for the SKR had moved to a region above Saturn's southern pole. As the mission progresses, scientists will examine additional data to try to solve Saturn's rotation-rate puzzle.

Cassini also found that Saturn's storms seem to have changed dramatically since the Voyager visits. Storms on Saturn develop along the edges of high-altitude bands of wind that encircle the planet at various latitudes. Some of these bands blow eastward; some blow in the opposite direction. The Voyager probes clocked the winds in the bands at about 1,600 kilometers (1,000 miles per hour), almost 10 times as fast as Earth's high-altitude winds. Along the boundaries between bands, the conflicting winds can churn into hurricanelike storms. The storms detected by the Voyager probes appeared almost exclusively near Saturn's equator and lasted for many months.

Cassini searched for storms using its RPWS instrument, which can detect a type of radio wave generated by lightning. On Earth, these radio waves cause the crackle and pop that you hear when listening to AM radio during a thunderstorm. The RPWS instrument found that the storms were widely distributed over mid- to high-latitude areas of Saturn's Southern Hemisphere.

Scientists think that the differences in storm patterns may be linked to the location of the shadow cast by Saturn's rings. As Saturn orbits the sun, the degree to which its axis tilts toward or away from the sun varies from 0 degrees to 26 degrees. The rings circle Saturn around its equator, and so they tilt with respect to the sun at the same angle as the planet.

In the 1980's, Saturn had almost no tilt with respect to the sun, and, thus, the rings cast a narrow, deep shadow near the equator. The result was turbulence between the cold, shadowed region near the equator and the warmer sunlit regions elsewhere. (In general, winds are fueled by differences in regional atmospheric temperatures based on the amount of energy received from the sun. The greater the temperature difference, the more violent the winds.) In 2004 and 2005, however, Saturn was tilting at its maximum angle with respect to the sun, and so the ring shadow covered most of its Northern Hemisphere. As a result, storms occurred over a broader region. In addition, because the temperature differences between regions were less, the storms lasted for shorter periods.

Storms on Saturn can blow for weeks, months, or even years. Storms on Earth rarely last more than one week, fading as they lose energy. On Saturn, storms tend to merge and continue their sweep across the planet. These combined storms also fade. But because Saturn does not have a solid surface to create friction, the storms fade more slowly than they do on Earth. On March 20, 2004, Cassini photographed the merger of two hurricanelike storms, each 370 kilometers (600 miles) in diameter. Both lasted for about a month before they approached each other, spun counterclockwise around each other, and then combined into an even larger storm.