If he had sat in mission control alongside the scientists in charge of the Cassini spacecraft when it made its climactic entry into Saturn's orbit, Galileo Galilei would have been astonished. When he discovered the planet in 1610, looking through his primitive telescope, Galileo perceived Saturn's rings as two large moons, which he described as "handles," flanking the planet. He was baffled two years later when these moons seemed to disappear. (In fact, Saturn was then at a different tilt relative to Earth, and Galileo could not make out the thin edge of the rings.) Below, explore how Cassini is helping to answer fundamental questions about Saturn's rings, and marvel at close-up imagery that Galileo could never have imagined.—Susan K. Lewis
 | | How Old Are the Rings?
Saturn and the other planets of our solar system coalesced from swirling clouds of interstellar gas roughly 4.6 billion years ago, and scientists long assumed the rings dated back to this time. But images from Voyager in the 1980s and now from Cassini point to a younger age. It even appears that some rings are still being made. Scientists use dust contamination as a kind of clock to estimate the age of the icy rings. Saturn's main rings seem to be a mere few hundred million years old. Left: Cassini's ultraviolet spectrometer reveals varying levels of dust in the main rings. "Dirtier" rings are red; the blue rings farther out likely formed later. | 
| | How Did They Form?
It now seems that the rings were not all created from interstellar gas when the mother planet was born. So where do they come from? A medium-sized moon might have strayed so close to Saturn that it was pulled apart by the planet's gravity, or it might have been shattered by a meteor. Another theory is that a large meteor or comet ventured too near Saturn and was torn apart. And Cassini now provides yet another explanation for the E ring, which lies outside the main rings: geysers on the moon Enceladus are feeding the E ring's growth. Left: Saturn has more than 40 moons ranging from a few miles to 3,200 miles across. A medium-sized moon, if shattered, would account for the entire mass of the rings. | 
| | What Are They Made Of?
Countless billions of "particles" constitute Saturn's rings. Some are as fine as powdered sugar, but others are as big as barns, and a few are so large they may more aptly be called small moons. The Voyager mission found that the particles are nearly pure water-ice, with traces of dust and rocky matter that come from meteors. Temperature data from Cassini are slightly altering the picture, hinting that the particles of the main rings may be more like fluffy snowballs than ice cubes. Left: From Earth the rings look like solid disks, but if you could fly inside them, you would see a blizzard of icy particles. | 
| | How Many Rings Are There?
Saturn's main rings, visible from Earth through amateur telescopes, are part of a larger, intricately structured system. It includes seven major named rings, alphabetically labeled in the order in which they were discovered. Closest to Saturn is the faint D ring. Moving outward are the C, B, and A rings (the main rings). Just outside of the A ring lies the narrow F ring, and beyond it the thin G ring. The outermost ring, the E ring, is broad and diffuse, extending all the way to the moon Titan. Left: Within Saturn's main rings (from left to right C, B, and A), there are tens of thousands of narrower ringlets, most just a few miles wide. | 
| | How Vast Are They?
It all depends on what you mean by "vast." In terms of mass, the rings aren't terribly impressive; pack all the ring particles together, and you would make a medium-sized moon. But the rings cover an extraordinary expanse of space. The main rings are about 170,000 miles in diameter, yet less than half a mile thick. (Imagine a paper-thin ring that stretches two miles across.) The E ring's diameter is nearly 600,000 miles. Flying in a high-speed passenger jet, it would take you more than a month to cross this distance. Left: To put the size of the rings in perspective, consider that Saturn itself is over 750 times larger than Earth. | 
| | Why Do They Lie in One Plane?
When Saturn formed from a spinning disk of gas, the material left over continued to swirl around the planet's equator. Much of it coalesced into moons. Whether the rings date back to this process or are the remnants of a shattered moon, the ring particles would keep the angular momentum of the original spinning disk. Saturn's gravity is also stronger around its bulging middle, so particles tend to be drawn there. Lastly, any ring particles on an inclined orbit would soon collide with the billions of particles at the equator and be herded into the same plane. Left: As Saturn tilts on its axis, the majestic rings tilt with it. This montage of Hubble images, captured from 1996 to 2000, shows a change of seasons on the planet. | 
| | What Makes Gaps in the Rings?
Through their gravitational effects, Saturn's moons shape striking dark gaps in the rings. The largest gap in the main set of rings, nearly 3,000 miles wide, is the Cassini Division, named after Jean-Domenique Cassini, who discovered it in 1676. It separates the main rings B and A. While this "gap" actually contains faint ringlets and is not as empty as it appears from Earth, its inner boundary is kept sharp by the moon Mimas. Other, smaller moons help keep the Encke and Keeler Gaps of the A ring open. Left: In this contrast-enhanced image, the 200-mile-wide Encke Gap (left) and 26-mile wide Keeler Gap can be seen within the bright A ring. So-called shepherd moons maintain the boundaries of these gaps. | 
| | What Makes Bright Bands in the Rings?
The brighter bands of the main rings are essentially areas of greater particle density, which reflect more sunlight. (By contrast, when you look at the dark side of the rings, these bands block sunlight from shining through and appear darker than neighboring bands.) While scientists do not completely understand what determines the locations and boundaries of all the bright lanes, some at least are shaped by the gravity of Saturn's moons. Left: The bright band in this close-up view of Saturn's C ring has a particle density some five times greater than in the surrounding regions. The band is roughly 30 miles wide. | 
| | Are There Moons Within the Rings?
Yes. While most of Saturn's 40-plus known moons orbit outside the main ring system, the Voyager mission discovered a tiny moon orbiting within the A ring. Astrophysicists had predicted that such a moon might exist and be responsible for creating the 200-mile-wide Encke Gap. Now Cassini has confirmed suspicions that an even tinier moon is embedded in the A ring's Keeler Gap. The gravitational fields of these moons sweep the gaps clear of particles; if the diminutive moons didn't exist, the gaps wouldn't either. Left: In this view of the A ring, the moonlet Pan appears like a speck in the middle of the Encke Gap. Pan is just 16 miles across. The Keeler gap is at the upper right, but its four-mile-wide embedded moon is not pictured here. | 
| | Do the Rings Have an Atmosphere?
Cassini revealed that the rings have an atmosphere of sorts comprised largely of molecular oxygen. It is too thin and tenuous to be considered a true atmosphere, but it is nonetheless surprising given the ring's small total mass and weak gravity. Molecular oxygen is generally associated with biological processes, but in the rings it is made when ultraviolet light splits apart water molecules. Even if some oxygen gas escapes the ring's gravity, the thin atmosphere is continuously replenished with fresh oxygen. Left: The thin atmosphere of Saturn's rings is probably like that of Jupiter's icy moon Ganymede, the largest moon in our solar system. | 
| | How Did Cassini Pass Through the Rings?
The most nail-biting part of the spacecraft's seven-year voyage to Saturn was the 90 minutes when Cassini crossed the ring plane to enter orbit. Given that ring particles move faster than high-speed bullets, how did Cassini survive? Mission planners chose a 15,000-mile-wide gap in the rings, between the F and G rings. They also positioned the spacecraft's high-gain antenna as a shield. Cassini was indeed hit many times by fine grains of ice, but the crossing was a success, as the glorious imagery and invaluable data from the mission make clear. Left: An artist's vision of Cassini. After crossing the ring plane (to the left of the main rings shown here), the spacecraft had to fire its engines and reduce its velocity in order to be captured by Saturn's gravity and enter orbit. |
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