<P> Four robotic spacecraft have observed Saturn's rings from the vicinity of the planet . Pioneer 11's closest approach to Saturn occurred in September 1979 at a distance of 20,900 km . Pioneer 11 was responsible for the discovery of the F ring . Voyager 1's closest approach occurred in November 1980 at a distance of 64,200 km . A failed photopolarimeter prevented Voyager 1 from observing Saturn's rings at the planned resolution; nevertheless, images from the spacecraft provided unprecedented detail of the ring system and revealed the existence of the G ring . Voyager 2's closest approach occurred in August 1981 at a distance of 41,000 km . Voyager 2's working photopolarimeter allowed it to observe the ring system at higher resolution than Voyager 1, and to thereby discover many previously unseen ringlets . Cassini spacecraft entered into orbit around Saturn in July 2004 . Cassini's images of the rings are the most detailed to - date, and are responsible for the discovery of yet more ringlets . </P> <P> The rings are named alphabetically in the order they were discovered . The main rings are, working outward from the planet, C, B and A, with the Cassini Division, the largest gap, separating Rings B and A. Several fainter rings were discovered more recently . The D Ring is exceedingly faint and closest to the planet . The narrow F Ring is just outside the A Ring . Beyond that are two far fainter rings named G and E. The rings show a tremendous amount of structure on all scales, some related to perturbations by Saturn's moons, but much unexplained . </P> <P> The dense main rings extend from 7,000 km (4,300 mi) to 80,000 km (50,000 mi) away from Saturn's equator, whose radius is 60,300 km (37,500 mi) (see Major subdivisions). With an estimated local thickness of as little as 10 m and as much as 1 km, they are composed of 99.9% pure water ice with a smattering of impurities that may include tholins or silicates . The main rings are primarily composed of particles ranging in size from 1 cm to 10 m . </P> <P> Based on Voyager observations, the total mass of the rings was estimated to be about 3 × 10 kg . This is a small fraction of the total mass of Saturn (about 50 ppb) and is just a little less than the moon Mimas . More recent observations and computer modeling based on Cassini observations show that this may be an underestimate due to clumping in the rings and the mass may be three times this figure . Although the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, both Voyager spacecraft discovered that the rings have an intricate structure of thousands of thin gaps and ringlets . This structure is thought to arise, in several different ways, from the gravitational pull of Saturn's many moons . Some gaps are cleared out by the passage of tiny moonlets such as Pan, many more of which may yet be discovered, and some ringlets seem to be maintained by the gravitational effects of small shepherd satellites (similar to Prometheus and Pandora's maintenance of the F ring). Other gaps arise from resonances between the orbital period of particles in the gap and that of a more massive moon further out; Mimas maintains the Cassini Division in this manner . Still more structure in the rings consists of spiral waves raised by the inner moons' periodic gravitational perturbations at less disruptive resonances . Data from the Cassini space probe indicate that the rings of Saturn possess their own atmosphere, independent of that of the planet itself . The atmosphere is composed of molecular oxygen gas (O) produced when ultraviolet light from the Sun interacts with water ice in the rings . Chemical reactions between water molecule fragments and further ultraviolet stimulation create and eject, among other things, O. According to models of this atmosphere, H is also present . The O and H atmospheres are so sparse that if the entire atmosphere were somehow condensed onto the rings, it would be about one atom thick . The rings also have a similarly sparse OH (hydroxide) atmosphere . Like the O, this atmosphere is produced by the disintegration of water molecules, though in this case the disintegration is done by energetic ions that bombard water molecules ejected by Saturn's moon Enceladus . This atmosphere, despite being extremely sparse, was detected from Earth by the Hubble Space Telescope . Saturn shows complex patterns in its brightness . Most of the variability is due to the changing aspect of the rings, and this goes through two cycles every orbit . However, superimposed on this is variability due to the eccentricity of the planet's orbit that causes the planet to display brighter oppositions in the northern hemisphere than it does in the southern . </P>

How thick are saturn's rings from top to bottom
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