Rings of Rhea

Short description: Possible rings around Saturn's moon Rhea

An artist's impression of Rhea's rings. The density of the particles has been greatly exaggerated.[1]

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.[2]

In November 2005 the Cassini orbiter found that Saturn's magnetosphere is depleted of energetic electrons near Rhea.[3] According to the discovery team, the pattern of depletion is best explained by assuming the electrons are absorbed by solid material in the form of an equatorial disk of particles perhaps several decimeters to approximately a meter in diameter and that contains several denser rings or arcs. Subsequent targeted optical searches of the putative ring plane from several angles by Cassini's narrow-angle camera failed to find any evidence of the expected ring material, and in August 2010 it was announced that Rhea was unlikely to have rings,[4] and that the reason for the depletion pattern, which is unique to Rhea, is unknown.[5][6] However, an equatorial chain of bluish marks on the Rhean surface suggests past impacts of deorbiting ring material and leaves the question unresolved.[7]

## Contents

* 1 Detection
* 2 Interpretation
* 3 Physics
* 4 See also
* 5 References
* 6 External links

## Detection

Comparison of MIMI readings at Rhea and Tethys, marking possible rings. Magnetic interference is more turbulent at Rhea than at Tethys, so its shadow is not as clear-cut.

Voyager 1 observed a broad depletion of energetic electrons trapped in Saturn's magnetic field downstream from Rhea in 1980. These measurements, which were never explained, were made at a greater distance than the Cassini data.

On November 26, 2005, Cassini made the one targeted Rhea flyby of its primary mission. It passed within 500 km of Rhea's surface, downstream of Saturn's magnetic field, and observed the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was an abrupt cutoff of energetic electrons as Cassini crossed into the moons' plasma shadows (the regions where the moons themselves blocked the magnetospheric plasma from reaching Cassini).[2][8] However, in the case of Rhea, the electron plasma started to drop off slightly at eight times that distance, and decreased gradually until the expected sharp drop off as Cassini entered Rhea's plasma shadow. The extended distance corresponds to Rhea's Hill sphere, the distance of 7.7 times Rhea's radius inside of which orbits are dominated by Rhea's rather than Saturn's gravity. When Cassini emerged from Rhea's plasma shadow, the reverse pattern occurred: A sharp surge in energetic electrons, then a gradual increase out to Rhea's Hill-sphere radius.

These readings are similar to those of Enceladus, where water venting from its south pole absorbs the electron plasma. However, in the case of Rhea, the absorption pattern is symmetrical. In addition, the Magnetospheric Imaging Instrument (MIMI) observed that this gentle gradient was punctuated by three sharp drops in plasma flow on each side of the moon, a pattern that was also nearly symmetrical.[2][8]

In August 2007, Cassini passed through Rhea's plasma shadow again, but further downstream. Its readings were similar to those of Voyager 1\. Two years later, in October 2009, it was announced that a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around Rhea's circumference, within 2 degrees of the equator, may represent further evidence for a ring. The spots presumably represent the impact points of deorbiting ring material.[9]

There are no images or direct observations of the material thought to be absorbing the plasma, but the likely candidates would be difficult to detect directly. Further observations during Cassini's targeted flyby on March 2, 2010[8] found no evidence of orbiting ring material.[4]

## Interpretation

A 100 s exposure of a back-lit Rhea failed to find any evidence of rings, whether they are too tenuous or do not scatter enough light to be detected. This viewing geometry is attuned to detecting dust-sized particles, so a ring made of larger debris is possible. The sun-lit crescent is on the lower side; the gibbous illumination on the left is planetshine.

Possible Rhean rings Ring | Orbital radius (km)

|

Disk | < 5900
1 | ≈ 1615
2 | ≈ 1800
3 | ≈ 2020

Cassini's flyby trajectory makes interpretation of the magnetic readings difficult.

The obvious candidates for magnetospheric plasma-absorbing matter are neutral gas and dust, but the quantities required to explain the observed depletion are far greater than Cassini's measurements allow. Therefore the discoverers, led by Geraint Jones of the Cassini MIMI team, argue that the depletions must be caused by solid particles orbiting Rhea:

> "An analysis of the electron data indicates that this obstacle is most likely in the form of a low optical depth disk of material near Rhea’s equatorial plane and that the disk contains solid bodies up to ~1 m in size."[2]

The simplest explanation for the symmetrical punctuations in plasma flow are "extended arcs or rings of material" orbiting Rhea in its equatorial plane. These symmetric dips bear some similarity to the method by which the rings of Uranus were discovered in 1977.[10] The slight deviations from absolute symmetry may be due to "a modest tilt to the local magnetic field" or "common plasma flow deviations" rather than to asymmetry of the rings themselves, which may be circular.

Not all scientists are convinced that the observed signatures are caused by a ring system. No rings have been seen in images, which puts a very low limit on dust-sized particles. Furthermore, a ring of boulders would be expected to generate dust that would likely have been seen in the images.[11]

## Physics

The fresh ice (dark) can be seen spanning the equator in this image comparing infrared and green wavelengths.

Fresh blue ice at Rhea's equator suggest impacts from deorbiting ring material.

Simulations suggest that solid bodies can stably orbit Rhea near its equatorial plane over astronomical timescales. They may not be stable around Dione and Tethys because those moons are so much closer to Saturn, and therefore have much smaller Hill spheres, or around Titan because of drag from its dense atmosphere.[2]

Several suggestions have been made for the possible origin of rings. An impact could have ejected material into orbit; this could have happened as recently as 70 million years ago. A small body could have been disrupted when caught in orbit about Rhea. In either case, the debris would eventually have settled into circular equatorial orbits. Given the possibility of long-term orbital stability, however, it is possible that they survive from the formation of Rhea itself.[2]

For discrete rings to persist, something must confine them. Suggestions include moonlets or clumps of material within the disk, similar to those observed within Saturn's A ring.[2]

## See also

* Subsatellite

## References

1. ↑ "Rhea's Rings". NASA. 2008-04-15. http://www.nasa.gov/multimedia/imagegallery/image_feature_1062.html.
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Jones, Geraint H. (March 2008). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science (AAAS) 319 (5868): 1380–1384. doi:10.1126/science.1151524. PMID 18323452. Bibcode: 2008Sci...319.1380J. https://pdfs.semanticscholar.org/0d74/d7801493544d2f60b90f17c64d89fcf8594e.pdf.
3. ↑ Hecht, Jeff (2008-03-06). "Saturn satellite reveals first moon rings". New Scientist. https://www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html.
4. ↑ 4.0 4.1 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. doi:10.1029/2010GL043663. Bibcode: 2010GeoRL..3714205T.
5. ↑ Gold, Lauren (2010-08-02). "No Rings Around Saturn's Rhea". Space Daily. http://www.spacedaily.com/reports/No_Rings_Around_Saturn_Rhea_999.html.
6. ↑ Kerr, Richard A. (2010-06-25). "The Moon Rings That Never Were". ScienceNow. http://news.sciencemag.org/sciencenow/2010/06/the-moon-rings-that-never-were.html.
7. ↑ "Cassini Catches Saturn Moons in Paintball Fight". NASA/JPL. http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20101007.
8. ↑ 8.0 8.1 8.2 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.
9. ↑ Lakdawalla, E. (5 October 2009). "Another possible piece of evidence for a Rhea ring". The Planetary Society Blog. Planetary Society. http://planetary.org/blog/article/00002137/.
10. ↑ "Saturn's Moon Rhea Also May Have Rings". NASA. 2008-03-06. http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html.
11. ↑ Kerr, Richard A. (March 2008). "News of the Week: Electron Shadow Hints at Invisible Rings Around a Moon". Science (AAAS) 319 (5868): 1325. doi:10.1126/science.319.5868.1325. PMID 18323426. http://www.sciencemag.org/cgi/content/summary/319/5868/1325.

## External links

* NASA podcast

* v
* t
* e

Solar System

* Sun
* Mercury
* Venus
* Earth
* Mars
* Ceres
* Jupiter
* Saturn
* Uranus
* Neptune
* Pluto
* Haumea
* Makemake
* Eris

| Planets|

* Terrestrial planets
* Mercury
* Venus
* Earth
* Mars

* Giant planets
* Jupiter
* Saturn
* Uranus
* Neptune

* Dwarf planets
* Ceres
* Pluto
* Haumea
* Makemake
* Eris

|

Rings|

* Jovian
* Saturnian (Rhean)
* Charikloan
* Chironean
* Uranian
* Neptunian
* Haumean

Moons|

* Terrestrial
* Moon
* other near-Earth objects
* Martian
* Phobos
* Deimos
* Jovian
* Ganymede
* Callisto
* Io
* Europa
* all 79
* Saturnian
* Titan
* Rhea
* Iapetus
* Dione
* Tethys
* Enceladus
* Mimas
* Hyperion
* Phoebe
* all 82
* Uranian
* Titania
* Oberon
* Umbriel
* Ariel
* Miranda
* all 27
* Neptunian
* Triton
* Proteus
* Nereid
* all 14
* Plutonian
* Charon
* Nix
* Hydra
* Kerberos
* Styx
* Eridian
* Dysnomia
* Haumean
* Hiʻiaka
* Namaka
* Makemakean
* S/2015 (136472) 1

Exploration
(outline)|

* Colonization
* Discovery
* astronomy
* historical models
* timeline
* Human spaceflight
* space stations
* list
* Space probes
* timeline
* list

* Mercury
* Venus
* Moon
* mining
* Mars
* Ceres
* Asteroids
* mining
* Comets
* Jupiter
* Saturn
* Uranus
* Neptune
* Pluto
* Deep space

Hypothetical
objects|

* Fifth giant
* Nemesis
* Phaeton
* Planet Nine
* Planet V
* Planet X
* Theia
* Tyche
* Vulcan
* Vulcanoids
* Subsatellites

Lists|

* Comets
* Dwarf planets (possible)
* Gravitationally rounded objects
* Minor planets
* names
* Natural satellites
* Solar System models
* Solar System objects
* by size
* by discovery date

| Small Solar
System bodies|

* Comets
* Damocloids
* Meteoroids
* Minor planets
* moons
* Planetesimal

* Mercury-crossers
* Venus-crossers
* Venus trojans
* Near-Earth objects
* Earth-crossers
* Earth trojans
* Mars-crossers
* Mars trojans

* Asteroid belt
* Asteroids
* Ceres
* Pallas
* Juno
* Vesta
* first 1000
* families
* exceptional
* Kirkwood gap
* Main-belt comets

* Jupiter-crossers
* Jupiter trojans
* Centaurs
* Saturn-crossers
* Uranus-crossers
* Uranus trojans
* Neptune-crossers
* Neptune trojans
* Cis-Neptunian objects
* Trans-Neptunian objects

* Plutoids
* Kuiper belt
* Cubewanos
* Plutinos
* Detached objects
* Hills cloud
* Oort cloud
* Scattered disc
* Sednoids

|

Formation
and evolution|

* Accretion
* Accretion disk
* Asteroid belt
* Circumplanetary disk
* Circumstellar disc
* Circumstellar envelope
* Cosmic dust
* Debris disk
* Detached object
* Disrupted planet
* Excretion disk
* Exoplanetary Circumstellar Environments and Disk Explorer
* Exozodiacal dust
* Extraterrestrial materials
* Extraterrestrial sample curation
* Giant-impact hypothesis
* Gravitational collapse
* Hills cloud
* Interplanetary dust cloud
* Interplanetary medium
* Interplanetary space
* Interstellar cloud
* Interstellar dust
* Interstellar medium
* Interstellar space
* Kuiper belt
* List of interstellar and circumstellar molecules
* Merging stars
* Molecular cloud
* Nebular hypothesis
* Oort cloud
* Outer space
* Planetary migration
* Planetary system
* Planetesimal
* Planet formation
* Protoplanetary disk
* Ring system
* Rubble pile
* Sample-return mission
* Scattered disc
* Star formation

* Outline of the Solar System
*   *   *

Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe
Each arrow (→) may be read as "within" or "part of".

* v
* t
* e

Planetary rings

Planets|

* Rings of Jupiter
* Rings of Saturn
* Rings of Uranus
* Rings of Neptune
* Rings of exoplanets

Other bodies|

* Rings of Chariklo
* Rings of Chiron
* Rings of Rhea
* Rings of Haumea

Solar|

* Asteroid belt
* Kuiper belt

Related topics|

* Gas torus

* v
* t
* e

Moons of Saturn

Listed in approximately increasing distance from Saturn.

Ring shepherds|

* S/2009 S 1
* Ring moonlets
* Pan
* Daphnis
* Atlas
* Prometheus
* Pandora

Co-orbitals|

* Epimetheus
* Janus

G Ring|

* Aegaeon

Alkyonides|

* Methone
* Anthe
* Pallene

Inner large
(with trojans)|

* Mimas
* Enceladus
* Tethys
* Telesto
* Calypso
* Dione
* Helene
* Polydeuces

Outer large|

* Rhea
* Titan
* Hyperion
* Iapetus

Inuit group|

* Kiviuq
* Ijiraq
* Paaliaq
* S/2004 S 29
* S/2004 S 31
* Siarnaq
* Tarqeq

Norse group|

* Phoebe
* Skathi
* S/2004 S 37
* S/2007 S 2
* Skoll
* S/2004 S 13(?)
* Greip
* Hyrrokkin
* Jarnsaxa
* Mundilfari
* S/2006 S 1
* S/2004 S 17(?)
* Bergelmir
* Narvi
* S/2004 S 20
* Suttungr
* Hati
* S/2004 S 12(?)
* S/2004 S 27
* Farbauti
* Thrymr
* S/2004 S 30
* Aegir
* S/2007 S 3(?)
* Bestla
* S/2004 S 7(?)
* S/2004 S 22(?)
* S/2004 S 23(?)
* S/2004 S 25(?)
* S/2004 S 32(?)
* S/2006 S 3
* S/2004 S 38(?)
* Fenrir
* S/2004 S 28(?)
* Surtur
* Kari
* S/2004 S 35(?)
* Ymir
* S/2004 S 21(?)
* Loge
* S/2004 S 36(?)
* S/2004 S 39(?)
* S/2004 S 33(?)
* S/2004 S 34(?)
* Fornjot
* S/2004 S 26(?)

Gallic group|

* Albiorix
* Bebhionn
* Erriapus
* Tarvos

Prograde outer satellites|

* S/2004 S 24

* Rings of Saturn
* Cassini–Huygens
* Themis
* Chiron
* S/2004 S 6
* S/2004 S 4
* S/2004 S 3
* In fiction

0.00

(0 votes)

Original source: https://en.wikipedia.org/wiki/Rings of Rhea. Read more

|

Retrieved from "https://handwiki.org/wiki/index.php?title=Astronomy:Rings_of_Rhea&oldid=3351210"
*[v]: View this template
*[t]: Discuss this template
*[e]: Edit this template