Quest to the lord of the Rings

With its mesmerizing rings and icy moons, Saturn is an icon for the word planet. The last spacecraft to visit Saturn was Voyager 2 in 1981. Now, after a long, seven year journey, on July 1, the first spacecraft specifically designed to explore Saturn and its vicinity will reach the ringed planet. Seventeen nations contributed to Cassini in this quest to the lord of the rings.

Cassini has trekked across more than two billion miles of space to reach its goal. It will orbit Saturn for at least four years. In January 2005, Cassini will release its piggybacked Huygens probe which will descend through the thick atmosphere of Titan, Saturn's largest moon. The spacecraft are loaded with instruments and cameras, far more sophisticated than those of Voyager. Cassini will be studying Saturn's atmosphere, the planet's inner composition and the ring system.

Saturn is a curious world. With a diameter of almost 75,000 miles (120,536 km) it is the second largest planet in the solar system. Like Jupiter, Saturn is composed of mostly hydrogen and helium gases. But Saturn's mass is only 30 percent that of Jupiter and it has the lowest density of all the planets, less than water. In fact, if you had a giant cosmic bathtub - Saturn could easily float in it!

Distances between the outer planets are much greater than those of the inner worlds. Saturn is about twice as far from Earth as Jupiter. If you were on Saturn, the Sun would look about ten times smaller than we see it from Earth. An area on Earth receives ninety times the amount of sunlight as the same area on Saturn, which makes it a very cold world.

The farther out from the Sun, the slower the planets move in their orbits. Saturn takes thirty of our years to go around the Sun. But as slow as Saturn is going around the Sun, it makes up for it in the speed at which the planet turns on its axis. A Saturnian day lasts only ten hours and fifteen minutes! This fast rotation, along with Saturn's gaseous composition, causes the planet to flatten at the poles and bulge at the equators. This effect can be viewed in even small telescopes.

Winds in Saturn's upper atmosphere reach terrific speeds of 1,118 mph (1,800 kmh.) near the equator. The strongest hurricane on Earth has winds which reach a maximum of 246 mph (396 kmh.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in Saturn's atmosphere.

Hubble has been used to study weather patterns on Saturn. The space telescope has tracked storms and auroral activity over the planet. Cassini will study these storm systems close up, and try to determine just what drives weather on Saturn.

We think Saturn's interior is similar to Jupiter. Deep in the interior we think Saturn has a rocky core about the size of Earth surrounded by a liquid metallic hydrogen layer and then a molecular hydrogen layer. We have also detected traces of various ices. Also like Jupiter, Saturn radiates more energy into space than it receives from the Sun, indicating a hot interior. Saturn's core probably has temperatures of about 12,000 Kelvin (21,150 F.)

Without a doubt, the most noticeable thing about Saturn is the rings. There are actually hundreds of rings, composed of billions of ice and rock particles. Sizes likely range from tiny grains to house sized chunks. We think the rings are made up of pieces of comets, asteroids and moons, drawn in by Saturn's massive gravity and shattered before ever reaching the planet. Each ring rotates at a different speed around the planet.

The rings are thin, just over a half mile thick (1 km.) but they spread out to about 175,000 miles (282,000 km.) about three quarters the distance from the Earth to the Moon. There are also divisions in the rings, the largest gap, called the Cassini division (named after the astronomer who first discovered it) can be seen through a small telescope. It is hoped that Cassini will gather information about the composition of the rings, and why they are there in the first place.

Perhaps the most intriguing thing about the Cassini mission is the Huygens probe which will descend into Titan's atmosphere. Titan is a very exciting world. It is the only moon in the solar system that has an atmosphere. Discovered by Christiaan Huygens, a Dutch astronomer in 1655, Titan is the largest of the thirty-one known moons of Saturn. Titan is a mysterious world surrounded by smoggy clouds and, we think, covered by an ocean of liquid petrochemicals with waves seven times higher than those on Earth!

Titan is the second largest moon in the solar system with a diameter of 3,200 miles (5,150 km.) larger even than the planet Mercury. It is also a very cold world with surface temperatures of -178 C (-289 F.) Titan orbits Saturn at a distance of 745,000 miles (1.2 million km.) and takes almost 16 days to complete one orbit.

Scientists are so interested in Titan because it has such a thick planet-like atmosphere. The atmospheric pressure on Titan is about sixty percent greater than Earth's, about the same pressure at the bottom of a swimming pool. We also think Titan may have oceans. In 1994, the Hubble space telescope returned images of Titan that suggested there was a huge continent facing forward in orbit. This doesn't prove there are oceans, but we think there are, but we will have to wait for Huygens' dive through Titan's atmosphere to hopefully know for sure.

Titan's atmosphere is not an atmosphere as we think of it. Temperatures that far from the Sun are so low that the structure of Titan's chemical atmosphere is in a constant state of deep freeze. This chemical composition is so interesting to us is because it might consist of compounds similar to the atmosphere of a primordial Earth. Titan's atmosphere contains mostly nitrogen, like Earth's. It also seems to contain much higher percentages of "smog" like methane and ethane. It is likely the smog is so thick that Titan has rains consisting of gasoline like liquids. That sounds pretty nasty, but in fact, the organic nature of some of the chemicals in the atmosphere indicate the possibility for some form of life could exist.

There are other moons which hold our interest. The Cassini division in Saturn's rings, mentioned earlier is actually caused by a moon, Mimas. Mimas is a small world that is heavily cratered. Voyager images showed that one face is dominated by a large crater nearly 80 miles (130 km) in diameter, with possibly the highest crater walls in the solar system. Little Mimas has crater walls higher than Mount Everest! Mimas orbits outside the large A ring portion and sweeps the division clear by displacing particles with its gravity that would otherwise orbit there. But we don't know yet why little Mimas, only 218 miles diameter (351 km) causes such a large division of 2600 miles (4183 km) in the rings. Again, it is hoped Cassini will provide some answers.

Iapetus is another curious Saturnian moon. It is another small icy world about 894 miles (1440 km) in diameter. It is a curious world however, one hemisphere is about six times brighter than the other. We aren't sure why this is and will look to Cassini for answers.

Cassini is already returning valuable information. On June 12, Cassini flew by Saturn's moon Phoebe. Images showed an old world that was very battered. The moon's surface was very old, and raises many questions about Phoebe's past. There was also a large variation in surface brightness. The great number of craters, large and small show that Phoebe has been hit many times. Phoebe is the largest of the outer moons and the large craters on the surface may actually explain the source of some of the smaller moons in the vicinity. It will take more time to decipher all the data received.

For more information on the Cassini mission, and for the latest images and press releases, go to: http://saturn.jpl.nasa.gov/

Quest to the lord of the Rings

Constellations

The Solar System