A ring system is a disc or torus orbiting an astronomical object that is composed of solid material such as gas, dust, meteoroids, planetoids or moonlets and stellar objects.
Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn, but it also orbits synchronously, like many of Saturn's other major moons; that is, it rotates at the same speed it revolves (orbits), so one hemisphere is always held facing inwards to the planet.
Helene is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory, and was designated S/1980 S 6. In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus (Saturn) in Greek mythology. Helene is also designated Saturn XII (12), which it was given in 1982, and Dione B, because it is co-orbital with Dione and located in its leading Lagrangian point (L4). It is one of four known trojan moons.
Oberon, also designated Uranus IV, is the outermost and second-largest major moon of the planet Uranus. It is the second-most massive of the Uranian moons, and the tenth-most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.
Tethys, or Saturn III, is the fifth-largest moon of Saturn, measuring about 1,060 km (660 mi) across. It was discovered by Giovanni Domenico Cassini in 1684, and is named after the titan Tethys of Greek mythology.
Iapetus is the outermost of Saturn's large moons. With an estimated diameter of 1,469 km (913 mi), it is the third-largest moon of Saturn and the eleventh-largest in the Solar System. Named after the Titan Iapetus, the moon was discovered in 1671 by Giovanni Domenico Cassini.
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.
Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe and was provisionally designated S/1980 S 26. In late 1985, it was officially named after Pandora from Greek mythology. It is also designated as Saturn XVII.
Dione, also designated Saturn IV, is the fourth-largest moon of Saturn. With a mean diameter of 1,123 km and a density of about 1.48 g/cm3, Dione is composed of an icy mantle and crust overlying a silicate rocky core, with rock and water ice roughly equal in mass. Its trailing hemisphere is marked by large cliffs and scarps called chasmata; the trailing hemisphere is also significantly darker compared to the leading hemisphere.
Enceladus is the sixth-largest moon of Saturn and the 19th-largest in the Solar System. It is about 500 kilometers in diameter, about a tenth of that of Saturn's largest moon, Titan. It is mostly covered by fresh, clean ice, making it one of the most reflective bodies of the Solar System. Consequently, its surface temperature at noon reaches only −198 °C, far colder than a light-absorbing body would be. Despite its small size, Enceladus has a wide variety of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrain.
Mimas, also designated Saturn I, is the seventh-largest natural satellite of Saturn. With a mean diameter of 396.4 kilometres or 246.3 miles, Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock, and study of Mimas's motion suggests that it may have a liquid ocean beneath its surface ice. The surface of Mimas is heavily cratered and shows little signs of recent geological activity. A notable feature of Mimas's surface is Herschel, one of the largest craters relative to the size of the parent body in the Solar System. Herschel measures 139 kilometres across, about one-third of Mimas's mean diameter, and formed from an extremely energetic impact event. The crater's name is derived from the discoverer of Mimas, William Herschel, in 1789. The moon's presence has created one of the largest 'gaps' in Saturn's ring, named the Cassini Division, due to orbital resonance destabilizing the particles' orbit there.
Prometheus is an inner satellite of Saturn. It was discovered on 24 October 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 27.
The moons of Saturn are numerous and diverse, ranging from tiny moonlets only tens of meters across to the enormous Titan, which is larger than the planet Mercury. There are 146 moons with confirmed orbits, the most of any planet in the solar system. This number does not include the many thousands of moonlets embedded within Saturn's dense rings, nor hundreds of possible kilometer-sized distant moons that were seen through telescopes but not recaptured. Seven Saturnian moons are large enough to have collapsed into a relaxed, ellipsoidal shape, though only one or two of those, Titan and possibly Rhea, are currently in hydrostatic equilibrium. Three moons are particularly notable. Titan is the second-largest moon in the Solar System, with a nitrogen-rich Earth-like atmosphere and a landscape featuring river networks and hydrocarbon lakes. Enceladus emits jets of ice from its south-polar region and is covered in a deep layer of snow. Iapetus has contrasting black and white hemispheres as well as an extensive ridge of equatorial mountains among the tallest in the solar system.
Odysseus is the largest crater on Saturn's moon Tethys. It is 445 km across, more than 2/5 of the moon's diameter, and is one of the largest craters in the Solar System. It is situated in the western part of the leading hemisphere of the moon—the latitude and longitude of its center are 32.8°N and 128.9°W, respectively. It is named after the Greek hero Odysseus from Homer's the Iliad and the Odyssey. Odysseus was discovered by the Voyager 2 spacecraft on 1 September 1981 during its flyby of Saturn.
Ithaca Chasma is a valley (graben) of Saturn's moon Tethys, named after the island of Ithaca, in Greece. It is up to 100 km (62 mi) wide, 3–5 km (1.9–3.1 mi) deep and 2,000 km (1,200 mi) long, running approximately three-quarters of the way around Tethys' circumference, making it one of the longer valleys in the Solar System. Ithaca Chasma is approximately concentric with Odysseus crater.
Rhea, the second-largest moon of Saturn, may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon. The potential discovery was announced in the journal Science on March 6, 2008.
Gertrude is the largest known crater on Uranus's moon Titania. A peak-ring impact basin, it is roughly 326–400 kilometers across, 1/5 to 1/4 of Titania's diameter. Gertrude was first observed by the Voyager 2 spacecraft on its January 1986 flyby of the Uranian system. It is named after Gertrude, the mother of Prince Hamlet in William Shakespeare's play Hamlet. The name Gertrude was officially adopted by the International Astronomical Union (IAU) in 1988. Features on Titania are named after female Shakespearean characters.
Yangoor is the largest known crater on the surface of the Uranian moon Ariel. A central-peak impact crater, it is about 80 km in diameter and is located approximately 450 km from Ariel's south pole. The northwestern edge of the crater was erased by formation of ridged terrain. The crater lacks bright ejecta deposits and was imaged for the first time by the Voyager 2 spacecraft in January 1986. The crater is named after a spirit that brings day in Australian Aboriginal mythology. The name Yangoor was officially approved by the International Astronomical Union in 1988.
Inktomi, informally nicknamed The Splat, is a prominent rayed impact crater 47.2 kilometres (29.3 mi) in diameter located in the southern hemisphere of Saturn's moon Rhea. The crater is named for the Lakota spider-god Iktomi and is located at 14.1 S, 112.1 W. Inktomi is thought to be the youngest major surface feature on Rhea, with estimates ranging from 8 to 280 million years.
Pharos is the largest known impact crater on Neptune's moon Proteus. It is named after the island of Pharos, making it the only named surface feature on Proteus as of 2024. It measures 10–15 km deep and has a diameter of around 250 km (160 mi), making it more than half the diameter of Proteus itself. Debris ejected from the impact that created Pharos may have formed Hippocamp, a small moon orbiting unusually close to Proteus.
