ring
(riŋ)
(computer science) A cyclic arrangement of data elements, usually including a specified entry pointer.
(design engineering) A tie member or chain link; tension or compression applied through the center of the ring produces bending moment, shear, and normal force on radial sections.
(mathematics) An algebraic system with two operations called multiplication and addition; the system is a commutative group relative to addition, and multiplication is associative, and is distributive with respect to addition. A ring of sets is a collection of sets where the union and difference of any two members is also a member.
(organic chemistry) A closed loop of bonded atoms in a chemical structure, for example, benzene or cyclohexane.
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planetary ring
An annulus of material, in the form of countless numbers of small objects, in orbit around a planet. Ring matter—whether it is dust particles or huge boulders—will orbit a planet without coalescing into a single body if it remains within a certain distance, known as the Roche limit. Any bodies that orbit outside this limit won't be adversely affected by the gravity of the parent planet and may therefore accrete into a larger body. All four giant planets in the solar system have ring systems, though only Saturn's is bright enough to be seen at visible wavelengths from Earth. The various rings have different proportions of ice and rock, and different distribution of particle sizes.
A tie member or chain link. Tension or compression applied through the center of a ring produces bending moment, shear, and normal force on radial sections. Because shear stress is zero at the boundaries of the section where bending stress is maximum, it is usually neglected.
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(1) The ringer line in an early telephone cable. See tip and ring.
(2) A privilege level in the computer. When software is assigned to a ring, it may be limited to executing certain instructions in the computer. Ring 0 has the highest privilege and can access all instructions. The operating system or the virtual memory monitor (VMM) resides in ring 0.
Applications typically reside in ring 3, which has a lower priority, and are prohibited from executing instructions that address the hardware. If an application attempts to execute a prohibited instruction, an error indication (fault) is generated. Rings 1 and 2 are available in some computers, but may or may not be used. See virtual machine monitor.
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ring, in astronomy, relatively thin band of rocks and dust and ice particles that orbit around a planet in the planet's equatorial plane. All four of the giant planets in the solar system-Jupiter, Saturn, Uranus, and Neptune- have rings, although only those of Saturn are easily visible. The origin of the rings is unknown. One theory is that they may have been formed from moons that were shattered by the impact of comets and meteoroids. Another holds that they might be the remnants of moons or comets that came within the planet's Roche limit and were broken up by gravitational forces. In the case of the E ring, it is now known that geyserlike eruptions on Enceladus are a source of the material in the ring.
Saturn has seven rings designated alphabetically as A through G in the order of their discovery. Two additional rings, designated as R/2004 S1 and R/2004 S2 were discovered in images returned to earth from the Cassini space probe in 2004. In 2009 the Spitzer Space Telescope discovered an enormous but faint dust ring that originates in material removed from the moon Phoebe by impacts. From the planet outward, the rings are D, C, B, A, R/2004 S1, R/2004 S2, F, G, E, and the Phoebe ring. With named gaps occupying the space between several of the rings, Saturn's rings are a highly complex structure stretching almost 167,770 mi (270,000 km) from the planet's center to the farthest edge of ring E; the Phoebe ring extends from 3.7 to 7.4 million mi (6 to 12 million km). The rings are not perfectly circular, and the gaps are not completely empty. The Columbo and Maxwell Gaps separate the C and B rings, the Cassini Division and Huygens Gap separate the B and A rings, and the Encke Division and Keeler Gap separate the A and R/2004 S1 rings. Except for the A and B rings, which are separated primarily by the 2,920-mi-wide (4,700-km) Cassini Division, and the Phoebe ring, the rings are relatively close to one another. Most of the rings appear to be composed of small pieces of water ice mixed with a small amount of rocky material in a wide range of particle sizes, from 1 in. (2.5 cm) to 33 ft (10 m)-although there may be an occasional object as large as a mile (1.6 km) in diameter. The Phoebe ring is composed of dust particles about 10 microns in size. Data returned by Cassini indicates that the rings are not uniform; for example, the B ring is very different from the A and C rings (which are similar to one another) found on either side of it. The Phoebe ring is tilted at a 27° angle from the plane of the other rings and, unlike the other rings, orbits Saturn with a retrograde motion. Several of Saturn's small moons appear to be shepherd satellites, maintaining the shape of the rings through gravitational interactions, and there are also ring arcs associated with several moons.
Jupiter's rings are similar to those of Saturn but much smaller and fainter. The main ring is about 4,300 mi (7,000 km) wide and has an abrupt outer boundary 80,000 mi (128,940 km) from the center of the planet. The inner main ring is formed from dust and ice particles kicked up when meteoroids collide with the small Jovian satellites Metus and Adrastea. The particles then spiral slowly in toward Jupiter. At its inner edge the main ring merges into the halo. A broad, faint band of dust and particles, the halo is about 6,200 mi (10,000 km) thick and stretches halfway from the main ring down to the top of Jupiter's atmosphere. A pair of broad, faint gossamer rings are located just outside the main ring, one bounded by the orbit of the Jovian shepherd satellite Amalthea and the other by the orbit Thebe.
Uranus has a thin elliptical band of eleven faint, narrow rings composed of ice, rock, and dust. Stretching outward from the planet, the rings are named 1986 U2R, Six, Five, Four, Alpha, Beta, Eta, Gamma, Delta, 1986 U1R, and Epsilon; the distance from the planetary center to the Epsilon ring is 31,750 mi (51,140 km). The rings are distinctly different from those of Jupiter and Saturn. A tenuous distribution of fine dust is scattered throughout the ring system, and the rings all are the same flat, dark color (perhaps from methane or black-carbon ice coating the rock), unlike Saturn's bright rings. The nine main rings consist of a single layer of particles, the monolayer, which had not previously been seen in planetary rings; the particles are kept from drifting away by several shepherd satellites. Because there are ringlets and incomplete rings and a varying opacity in several rings, it is believed that the Uranian ring system may be the remnants of a small moon.
Neptune has four almost circular faint rings composed of small rocks and dust. The rings are not uniform in density and thickness; the thicker parts of the rings are called ring arcs. Stretching outward from the planet, the rings are named Galle, Leverrier (whose outer extension is called Lassel), Arago, and Adams (which includes the ring arcs Liberty, Equality, and Fraternity); the distance from the planetary center to the Adams ring is 39,000 mi (62,930 km). The forces responsible for the development of ring arcs and ring extensions are not well understood, but shepherd satellites and gravitational forces attributable to Neptune's moons are thought to play a significant role. Earth-based observations indicate that the rings are less stable than was originally believed.
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Planetary ring
"Shepherd moon" redirects here. For the Enya album, see Shepherd Moons.
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region. The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system (Jupiter, Uranus and Neptune) possess ring systems of their own.
Reports in March 2008[1][2][3] have suggested that the Saturnian moon Rhea may have its own tenuous ring system, which would make it the only moon known to possess a ring system. A later study published in 2010 revealed that imaging of Rhea from the Cassini mission was inconsistent with the predicted properties of the rings, suggesting that some other mechanism is responsible for the magnetic effects that had led to the ring hypothesis.[4]
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Overview
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There are three ways that planetary rings (the rings around planets) have been proposed to have formed: from material of the protoplanetary disk that was within the Roche limit of the planet and thus could not coalesce to form moons; from the debris of a moon that was disrupted by a large impact; or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet's Roche limit. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it now appears that Saturn's rings might be quite old, dating to the early days of the Solar system.[5]
The composition of ring particles varies; they may be silicate or icy dust. Larger rocks and boulders may also be present, and in 2007 tidal effects from eight 'moonlets' only a few hundred meters across were detected within Saturn's rings.
Sometimes rings will have "shepherd" moons, small moons that orbit near the outer edges of rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.
Several of Jupiter's small innermost moons, namely Metis and Adrastea, are within Jupiter's ring system and are also within Jupiter's Roche limit.[6] It is possible that these rings are composed of material that is being pulled off of these two bodies by Jupiter's tidal forces, possibly facilitated by impacts of ring material on their surfaces.
Uranus' ε ring also has two shepherd satellites, Cordelia and Ophelia, acting as inner and outer shepherds respectively.[7] Both moons are well within Uranus' synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration.[8]
Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2's images showed them to be complete rings with bright clumps.[9] It is thought[10] that the gravitational influence of the shepherd moon Galatea and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.
Pluto is not known to have any ring systems. However, some astronomers think that the New Horizons probe might find a ring system when it visits in 2015.[11]
It is also predicted that Phobos, a moon of Mars, will break up and form into a planetary ring in about 50 million years due to its low orbit.[12][13]
Visual comparison
See also
Notes
- ^ http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA – Saturn's Moon Rhea Also May Have Rings
- ^ Jones, G. H.; et al. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science (AAAS) 319 (5868): 1380–1384. Bibcode 2008Sci...319.1380J. doi:10.1126/science.1151524. PMID 18323452. http://www.sciencemag.org/cgi/content/short/319/5868/1380.
- ^ Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". The Planetary Society web site. Planetary Society. http://planetary.org/news/2008/0306_A_Ringed_Moon_of_Saturn_Cassini.html. Retrieved 2008-03-09.
- ^ Tiscareno, Matthew S.; Burns, Joseph A.; Cuzzi, Jeffrey N.; Hedman, Matthew M. (2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters 37 (14): L14205. Bibcode 2010GeoRL..3714205T. doi:10.1029/2010GL043663.
- ^ "Saturn's Rings May Be Old Timers". NASA (News Release 2007-149). December 12, 2007. http://www.nasa.gov/mission_pages/cassini/media/cassini20071212.html. Retrieved 2008-04-11.
- ^ Gunter Faure, Teresa M. Mensing (2007). Introduction to Planetary Science: The Geological Perspective. Springer. ISBN 978-1-4020-5233-0.
- ^ Esposito, L. W. (2002). "Planetary rings". Reports on Progress in Physics 65 (12): 1741–1783. Bibcode 2002RPPh...65.1741E. doi:10.1088/0034-4885/65/12/201. http://www.iop.org/EJ/abstract/0034-4885/65/12/201.
- ^ Karkoschka, Erich (2001). "Voyager's Eleventh Discovery of a Satellite of Uranus and Photometry and the First Size Measurements of Nine Satellites". Icarus 151 (1): 69–77. Bibcode 2001Icar..151...69K. doi:10.1006/icar.2001.6597.
- ^ Miner, Ellis D., Wessen, Randii R., Cuzzi, Jeffrey N. (2007). "Present knowledge of the Neptune ring system". Planetary Ring System. Springer Praxis Books. ISBN 978-0-387-34177-4.
- ^ Salo, Heikki; Hanninen, Jyrki (1998). "Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles". Science 282 (5391): 1102–1104. Bibcode 1998Sci...282.1102S. doi:10.1126/science.282.5391.1102. PMID 9804544.
- ^ Steffl, Andrew J.; S. Alan Stern (2007). "First Constraints on Rings in the Pluto System". The Astronomical Journal 133 (4): 1485–1489. arXiv:astro-ph/0608036. Bibcode 2007AJ....133.1485S. doi:10.1086/511770.
- ^ Holsapple, K. A. (December 2001). "Equilibrium Configurations of Solid Cohesionless Bodies". Icarus 154 (2): 432–448. Bibcode 2001Icar..154..432H. doi:10.1006/icar.2001.6683.
- ^ Gürtler, J. & Dorschner, J: "Das Sonnensystem", Barth (1993), ISBN 3-335-00281-4
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