Planetary ring
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 spectacular planetary rings known are those around Saturn, but the other three gas giants of the solar system (Jupiter, Uranus and Neptune) possess ring systems of their own.
Recent reports[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.
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Overview
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.[4]
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.[5] 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' Epsilon ring also has two shepherd satellites, Cordelia and Ophelia, acting as inner and outer shepherds respectively.[6] Both moons are well within Uranus' synchronous orbit radius, and their orbits are therefore slowly decaying due to tidal deceleration.[7]
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.[8] It is thought[9] 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.[10]
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.[11][12]
After the impact of Theia and before the coalescence of the Moon, it is generally assumed that the Earth had a ring system.
History of Earth rings
In an article published in Nature in 1980, NASA astronomer Dr. John A. O'Keefe, credited with the discovery of Earth's "pear shape" using U.S. Vanguard satellite data collected in the late 1950s, theorized the "O'Keefe Earth ring." He hypothesized it to be a Saturn-like ring of tektites that could explain the Eocene extinctions of 35 million years ago.
That ring was thought to have cut out as much as a third of all sunshine reaching the Earth's surface. It is also thought that it was in existence for between one and several million years. Colder winters, and the extinction of hundreds of species including horses in Europe and plankton in the Caribbean, marked the end of the geologic Eocene period in Earth's history, occurring approximately 35 million years ago.
In Science Frontiers (Issue #76, Jul-Aug 1991), "In the past, the Earth had a ring system just like Saturn, Uranus and Neptune," according to a Danish astronomer. He has gone so far as to say that our planet boasted rings on 16 separate occasions in the past 2,800 years.
As recently as September 2002, Sandia National Laboratories published an article by two University of New Mexico researchers who again reinforced the notion of past "Earth rings."
In that study, the authors supposed the existence of a thin system of rings created by large meteoritic impacts, which caused material to get ejected back out into space and then gather into a debris ring formation along the Earth's equatorial plane. They again suggested that such a ring system, perhaps of similar opacity to Saturn's B-ring, may have caused past climatic changes on the Earth by blotting out sunlight and casting a cooling shadow over the equator for hundreds of thousands of years.
Since then, there have been many proponents in the global space community who advocated that an artificial ring system could help us to cool off a runaway greenhouse effect resulting from current global warming. The article "Earth Rings for Planetary Environment Control," authored by four researchers from the World Space Congress and published in the Smithsonian/NASA ADS system, is most notable in that respect. It argues in favor of many advantages of having a ring around the Earth, such as controlling climate temperature, reducing the intensity of the Van Allen radiation belts, making dangerous near-Earth flying asteroids more useful, providing night-time illumination without power, and creating an artificial ionosphere for radio communications.
Visual comparison
See also
- Rings of Jupiter
- Rings of Saturn
- Rings of Uranus
- Rings of Neptune
- Rings of Rhea
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. 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.
- ↑ "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 402052332, 9781402052330.
- ↑ Esposito, L. W. (2002). "Planetary rings". Reports On Progress In Physics 65: 1741–1783. 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: 69–77. doi:10.1006/icar.2001.6597. http://adsabs.harvard.edu/abs/2001Icar..151...69K.
- ↑ 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: 1102–1104. doi:10.1126/science.282.5391.1102. PMID 9804544. http://ads.ari.uni-heidelberg.de/abs/1998Sci...282.1102S.
- ↑ Steffl, Andrew J.; S. Alan Stern. First Constraints on Rings in the Pluto System. astro-ph/0608036. http://arxiv.org/abs/astro-ph/0608036.
- ↑ Holsapple, K. A. (December 2001). "Equilibrium Configurations of Solid Cohesionless Bodies". Icarus 154 (2): 432–448. doi:10.1006/icar.2001.6683. http://adsabs.harvard.edu/abs/2001Icar..154..432H. Retrieved 2007-12-13.
- ↑ Gürtler, J. & Dorschner, J: "Das Sonnensystem", Barth (1993), ISBN 3-335-00281-4
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