Planetary ring

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"Shepherd moon" redirects here. For the Enya album, see Shepherd Moons.
File:PIA07712 - F ring animation.gif
The moons Prometheus and Pandora shepherd the F ring of Saturn.

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.

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]

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.[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' Epsilon 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]

After the impact of Theia and before the coalescence of the Moon, it is generally assumed[citation needed] that the Earth had a ring system.

Visual comparison

File:Main Ring Galeleo forward PIA00538.jpg
A Galileo image of Jupiter's main ring.

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A Cassini mosaic of Saturn's rings.
Enlarge
A Cassini mosaic of Saturn's rings.
File:FDS 26852.19 Rings of Uranus.png
A Voyager 2 image of Uranus' rings.
File:PIA02202 Neptune's full rings.jpg
A pair of Voyager 2 images of Neptune's rings.

See also

Notes

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External links

Planetary rings
Planets
Moons
Related topics
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ar:قمر راعي

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zh:行星环
  1. ↑ http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA - Saturn's Moon Rhea Also May Have Rings
  2. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  3. ↑ Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". The Planetary Society web site. Planetary Society. Retrieved 2008-03-09.  External link in |work= (help)
  4. ↑ Matthew S. Tiscareno, Joseph A. Burns, Jeffrey N. Cuzzi, Matthew M. Hedman (2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters. 37 (14): L14205. Bibcode:2010GeoRL..3714205T. doi:10.1029/2010GL043663. 
  5. ↑ "Saturn's Rings May Be Old Timers". NASA (News Release 2007-149). December 12, 2007. Retrieved 2008-04-11. 
  6. ↑ Gunter Faure, Teresa M. Mensing (2007). Introduction to Planetary Science: The Geological Perspective. Springer. ISBN 402052332, 9781402052330 Check |isbn= value: invalid character (help). 
  7. ↑ Esposito, L. W. (2002). "Planetary rings". Reports on Progress in Physics. 65: 1741–1783. doi:10.1088/0034-4885/65/12/201. 
  8. ↑ 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. 
  9. ↑ 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. 
  10. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  11. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  12. ↑ Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.
  13. ↑ Gürtler, J. & Dorschner, J: "Das Sonnensystem", Barth (1993), ISBN 3-335-00281-4
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