the Earth's entire Antarctic ice shelf, spread across a surface area 80 times larger than that of Earth. The estimate is close to the value of 0.40 Mimas masses derived from Cassini observations of density waves in the A, B and C rings. It is a small fraction of the total mass of Saturn (about 0.25 ppb). Earlier Voyager observations of density waves in the A and B rings and an optical depth profile had yielded a mass of about 0.75 Mimas masses, with later observations and computer modeling suggesting that was an underestimate. Although the largest gaps in the rings, such as the Cassini Division and Encke Gap, can be seen from Earth, the 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 (O2) 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, O2. According to models of this atmosphere, H2 is also present. The O2 and H2 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 O2, 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