question: What exactly are the rings around planets like Saturn? What are they made of? How were they formed?
comment: When a planet is massive enough, it exerts a gravitational pull which is strong enough to rip apart solid bodies as large as moons (or small planets!) which get close enough, even when they’re in orbit. This happens for a number of reasons, but the easiest to understand is the fact that the side of the moon closest to the planet is experiencing a much stronger pull than the side which is further away. This is called “gravitational tidal forces”. When the difference between the force of gravity on one side of the moon and the force on the other side is larger than the tensile strength of the material which makes up the moon, it will eventually be ripped apart. When this happens, the parts of the moon closer the surface will move faster ahead of the parts that are further away (the former far side of the moon), forming a ring. This is almost certainly how Saturn got its rings: it shredded a few moons. It’s worth noting that Saturn’s rings are not stable, and within a few millennia they will be gone, sucked into the planet. Some more fun facts about tidal forces: - The Moon experiences tidal forces from the Earth! This can cause some minor moonquakes, but more noticeably it creates the moon’s most obvious non-obvious feature: the same side always faces us. The pull of earth’s gravity causes the moon to rotate with the same period as its orbit, kind of like how a tether ball always faces the same side towards the one holding the line. - Several of Jupiter’s moons and (if I recall?) at least one of Saturn’s are within a radius where the tidal forces of their host body *should* rip them apart if they were made of the same stuff that Earth is made of. From this we can infer that they’re composed of much tougher materials with a higher tensile strength, to resist the forces tearing at them - Those forces don’t go away though! Tidal effects on moons (particularly Jupiter’s closer moons) causes the moon to bend and stretch and flex. All that energy gets trapped as heat, which is why some of Jupiter’s moons are vulcanic! - Eventually, the loss of angular momentum to tidal drag (which is the energy that is being indirectly converted into heat and causing vulcanic eruptions) will result in the moons in question drifting closer and closer to Jupiter, resulting in ever steeper tidal gradients which will eventually rip the moon apart, giving Jupiter a very pretty set of rings. - This won’t happen to our moon. The Earth is too small and the tidal effects are barely relevant. Our moon is in fact spiraling *away* from the Earth, though it’s happening so slowly that we’ll need to worry about the Sun long before we worry about the Moon.
passages: The "Cassini" UVIS team, led by Larry Esposito, used stellar occultation to discover 13 objects, ranging from 27 m to 10 km across, within the F ring. They are translucent, suggesting they are temporary aggregates of ice boulders a few m across. Esposito believes this to be the basic structure of the Saturnian rings, particles clumping together, then being blasted apart.
   In 1675, Giovanni Domenico Cassini determined that Saturn's ring was composed of multiple smaller rings with gaps between them; the largest of these gaps was later named the Cassini Division. This division is a 4,800-km-wide region between the A ring and B Ring.
  
  In 1787, Pierre-Simon Laplace proved that a uniform solid ring would be unstable and suggested that the rings were composed of a large number of solid ringlets.
   Section::::Subdivisions and structures within the rings.
   Other objects have also been found to possess rings. Haumea was the first dwarf planet and Trans-Neptunian object found to possess a ring system. Centaur 10199 Chariklo, with a diameter of about , is the smallest object with rings ever discovered consisting of two narrow and dense bands, 6–7 km (4 mi) and 2–4 km (2 mi) wide, separated by a gap of . The Saturnian moon Rhea may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk, the first predicted around a moon.
   Section::::Rings around exoplanets.
  
  Because all giant planets of the Solar System have rings, the existence of exoplanets with rings is plausible. Although particles of ice, the material that is predominant in the rings of Saturn, can only exist around planets beyond the frost line, within this line rings consisting of rocky material can be stable in the long term. Such ring systems can be detected for planets observed by the transit method by additional reduction of the light of the central star if their opacity is sufficient. As of January 2015, no such observations are known.
   Section::::History.
   Section::::See also.
  
  BULLET::::- Galileo Galilei – the first person to observe Saturn's rings, in 1610
  
  BULLET::::- Christiaan Huygens – the first to propose that there was a ring surrounding Saturn, in 1655
  
  BULLET::::- Giovanni Cassini – discovered the separation between the A and B rings (the Cassini Division), in 1675
  
  BULLET::::- Édouard Roche – French astronomer who described how a satellite that comes within the Roche limit of Saturn could break up and form the rings
  
  Section::::External links.
  
  BULLET::::- Planetary Rings Node: Saturn's Ring System
  
  BULLET::::- Saturn's Rings by NASA's Solar System Exploration
   Section::::Gossamer rings.
  
  Section::::Gossamer rings.:Amalthea gossamer ring.
   Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years. Studies of Saturn's rings however indicate that they may date to the early days of the Solar System. Current theories suggest that some ring systems may form in repeating cycles, accreting into natural satellites that break up as soon as they reach the Roche limit. This theory has been used to explain the longevity of Saturn's rings as well the moons of Mars.
  
  Section::::Features and interactions.:Gravitational interactions.
  
  Section::::Features and interactions.:Gravitational interactions.:Orbital configurations.
   Saturn's rings are the most extensive ring system of any planet in the Solar System, and thus have been known to exist for quite some time. Galileo Galilei first observed them in 1610, but they were not accurately described as a disk around Saturn until Christiaan Huygens did so in 1655. With help from the NASA/ESA/ASI Cassini mission, a further understanding of the ring formation and active movement was understood. The rings are not a series of tiny ringlets as many think, but are more of a disk with varying density. They consist mostly of water ice and trace amounts of rock, and the particles range in size from micrometers to meters.
   Section::::Ring systems of planets.:Neptune.
  
  The system around Neptune consists of five principal rings that, at their densest, are comparable to the low-density regions of Saturn's rings. However, they are faint and dusty, much more similar in structure to those of Jupiter. The very dark material that makes up the rings is likely organics processed by radiation, like in the rings of Uranus. 20 to 70 percent of the rings are dust, a relatively high proportion. Hints of the rings were seen for decades prior to their conclusive discovery by "Voyager 2" in 1989.
  
  Section::::Rings systems of minor planets and moons.
   Section::::Halo ring.
  
  Section::::Halo ring.:Appearance and structure.
   In the skies of Saturn's inner moons, Saturn is an enormous object. For instance, Saturn seen from Pan has an apparent diameter of ~50°, 104 times larger than our Moon and occupying 11% of Pan's sky. Because Pan orbits along the Encke division within Saturn's rings, they are visible from anywhere on Pan, even on its side facing away from Saturn.
  
  Section::::Saturn.:The skies of Saturn's moons.:The rings from Saturn's moons.
   The rings of Neptune, like those of Uranus, are thought to be relatively young; their age is probably significantly less than that of the Solar System. Also, like those of Uranus, Neptune's rings probably resulted from the collisional fragmentation of onetime inner moons. Such events create moonlet belts, which act as the sources of dust for the rings. In this respect the rings of Neptune are similar to faint dusty bands observed by "Voyager 2" between the main rings of Uranus.
  
  Section::::Inner rings.
   Section::::Natural satellites.:Planetary rings.
  
  Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from outward from Saturn's equator and average approximately in thickness. They are composed predominantly of water ice with trace amounts of tholin impurities, and a peppered coating of approximately 7% amorphous carbon. The particles that make up the rings range in size from specks of dust up to 10 m. While the other gas giants also have ring systems, Saturn's is the largest and most visible.
   In 1980, "Voyager 1" made a fly-by of Saturn that showed the F ring to be composed of three narrow rings that appeared to be braided in a complex structure; it is now known that the outer two rings consist of knobs, kinks and lumps that give the illusion of braiding, with the less bright third ring lying inside them.
   Most giant planets (except Neptune) have retinues of moons, rings, ring shepherds, and moon Trojans analogous to mini-solar systems. These systems are postulated to have accreted from analogous gas clouds, and possibly with analogous migrations during their formation periods. The Cassini mission was defended on the grounds that Saturn system dynamics would contribute to studies of Solar System dynamics and formation.
   Section::::Dusty rings.:Other dust bands.
   Well beyond the main rings is the Phoebe ring, which is presumed to originate from Phoebe and thus to share its retrograde orbital motion. It is aligned with the plane of Saturn's orbit. Saturn has an axial tilt of 27 degrees, so this ring is tilted at an angle of 27 degrees to the more visible rings orbiting above Saturn's equator. 
  
  Section::::History.
  
  Section::::History.:Galileo's work.
   Section::::Himalia ring.
   The dense main rings extend from to away from Saturn's equator, whose radius is (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.
   Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets.
  
  Section::::Attributes.:Dynamic characteristics.
  
  Section::::Attributes.:Dynamic characteristics.:Orbit.
   Section::::Origin.
  
  The origin of the rings is unknown, but both are likely to be remnants of a debris disk, which could have formed via an impact on Chariklo, a collision with or between one or more pre-existing moons, tidal disruption of a former retrograde moon, or from material released from the surface by cometary activity or rotational disruption. If the rings formed through an impact event with Chariklo, the object must have impacted at a low velocity to prevent ring particles from being ejected beyond Chariklo's Hill sphere.
   In 1657 Christopher Wren became Professor of Astronomy at Gresham College, London. He had been making observations of the planet Saturn from around 1652 with the aim of explaining its appearance. His hypothesis was written up in "De corpore saturni," in which he came close to suggesting the planet had a ring. However Wren was unsure whether the ring was independent of the planet, or physically attached to it. Before Wren's theory was published Christiaan Huygens presented his theory of the rings of Saturn. Immediately Wren recognised this as a better hypothesis than his own and "De corpore saturni" was never published.
   BULLET::::- Rings of Saturn nomenclature from the USGS planetary nomenclature page
  
  BULLET::::- Biggest Ring Around Saturn Just Got Supersized (retrieved 2017-12-20 from Space.com)
  
  BULLET::::- Everything a Curious Mind Should Know About Planetary Ring Systems with Dr Mark Showalter (Waseem Akhtar podcast with Mark Showalter)
  
  BULLET::::- High-resolution animation by Seán Doran of the backlit rings
  
  BULLET::::- High-resolution animation by Kevin M. Gill of a flyover of the outer B Ring at equinox (it starts getting less uniform after the first minute); see Rings album for more
  