View source for Rings of Jupiter

[[Image:PIA01627 Ringe.jpg|thumb|400px|right|A schema of Jupiter's ring system showing the four main components]]
The planet [[Jupiter]] has a system of [[planetary ring|ring]]s, known as the '''rings of Jupiter''' or the '''Jovian ring system'''. It was the third ring system to be discovered in the [[Solar System]], after those of [[Rings of Saturn|Saturn]] and [[Rings of Uranus|Uranus]]. It was first observed in 1979 by the ''[[Voyager 1]]'' [[space probe]]<ref name=Smith1979/> and thoroughly investigated in the 1990s by the ''[[Galileo (spacecraft)|Galileo]]'' orbiter.<ref name=Ockert-Bell1999/> It has also been observed by the [[Hubble Space Telescope]] and from [[Earth]] for the past 25 years.<ref name=Meier1999/> Ground-based observations of the rings require the largest available [[telescopes]].<ref name=dePater1999/>

The Jovian ring system is faint and consists mainly of [[dust]].<ref name=Smith1979/><ref name=Burns1987/> It has four main components: a thick inner [[torus]] of particles known as the "halo ring"; a relatively bright, exceptionally thin "main ring"; and two wide, thick and faint outer "gossamer rings", named for the moons of whose material they are composed: [[Amalthea (moon)|Amalthea]] and [[Thebe (moon)|Thebe]].<ref name=Esposito2002/> 

The main and halo rings consist of dust ejected from the [[moons]] [[Metis (moon)|Metis]], [[Adrastea (moon)|Adrastea]] and other unobserved parent bodies as the result of high-velocity impacts.<ref name=Ockert-Bell1999/> High-resolution images obtained in February and March 2007 by the ''[[New Horizons]]'' spacecraft revealed a rich fine structure in the main ring.<ref name=Morring2007/> 

In visible and near-[[infrared]] light, the rings have a reddish color, except the halo ring, which is neutral or blue in color.<ref name=Meier1999/> The size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius about 15 [[micrometre|μm]] in all rings except the halo.<ref name=Throop2004/> The halo ring is probably dominated by submicrometre dust. The total mass of the ring system (including unresolved parent bodies) is poorly known, but is probably in the range of 10<sup>11</sup>&nbsp;to 10<sup>16</sup>&nbsp;kg.<ref name=Burns2004/> The age of the ring system is not known, but it may have existed since the formation of Jupiter.<ref name=Burns2004/>

==Discovery and structure==
The rings of Jupiter was the third ring system to be discovered in the [[Solar System]], after those of [[Rings of Saturn|Saturn]] and [[Rings of Uranus|Uranus]]. It was first observed in 1979 by the ''[[Voyager 1]]'' [[space probe]].<ref name=Smith1979>{{cite journal |last=Smith |first=B. A. |coauthors=Soderblom, L. A.; Johnson, T. V.; ''et al.'' |title=The Jupiter System through the Eyes of Voyager 1 |journal=Science |year=1979 |volume=204 |pages=951&ndash;957, 960–972 |url=http://adsabs.harvard.edu/abs/1979Sci...204..951S |doi=10.1126/science.204.4396.951 |pmid=17800430}}</ref> It comprises four main components: a thick inner [[torus]] of particles known as the "halo ring"; a relatively bright, exceptionally thin "main ring"; and two wide, thick and faint outer "gossamer rings", named for the moons of whose material they are composed: [[Amalthea (moon)|Amalthea]] and [[Thebe (moon)|Thebe]].<ref name=Esposito2002>{{cite journal |last=Esposito |first=L. W. |authorlink=Larry W. Esposito |title=Planetary rings |journal=Reports On Progress In Physics |year=2002 |volume=65 |pages=1741&ndash;1783 |url=http://www.iop.org/EJ/abstract/0034-4885/65/12/201 |doi=10.1088/0034-4885/65/12/201}}</ref> The principal attributes of the known Jovian Rings are listed in the table.<ref name=Ockert-Bell1999/><ref name=Burns1987>{{cite journal |last=Showalter |first=M. A. |coauthors=Burns, J. A.; Cuzzi, J. N.; [[James B. Pollack|Pollack, J. B.]] |title=Jupiter's Ring System: New Results on Structure and Particle Properties |journal=Icarus |year=1987 |volume=69 |issue=3 |pages=458&ndash;498 |doi=10.1016/0019-1035(87)90018-2 |url=http://adsabs.harvard.edu/abs/1987Icar...69..458S}}</ref><ref name=Esposito2002/><ref name=Throop2004/>
{| class="wikitable" 
|-
!                    Name
! abbr="Radius"    | Radius (km)
! abbr="Width"     | Width (km)
! abbr="Thickness" | Thickness (km)
!                    [[Optical depth]]{{Ref_label|C|c|none}}
!                    Dust fraction
!                    Notes
|-
! Halo ring
| 92&nbsp;000&ndash;122&nbsp;500 || 30&nbsp;500||12&nbsp;500||~1{{Esp|&minus;6}}|| 100% ||
|-
! Main ring
| 122&nbsp;500&ndash;129&nbsp;000 || 6&nbsp;500||30&ndash;300||5.9{{Esp|&minus;6}}|| ~25%||Bounded by [[Adrastea (moon)|Adrastea]]
|-
! Amalthea gossamer ring 
| 129&nbsp;000&ndash;182&nbsp;000 || 53&nbsp;000||2&nbsp;000||~1{{Esp|&minus;7}}||100%||Connected with [[Amalthea (moon)|Amalthea]]
|-
! Thebe gossamer ring
| 129&nbsp;000&ndash;226&nbsp;000 || 97&nbsp;000||8&nbsp;400||~3{{Esp|&minus;8}}||100%||Connected with [[Thebe (moon)|Thebe]]. There is an extension beyond the orbit of Thebe.
|}

==Main ring==
===Appearance and structure===
[[Image:Jovian Ring System PIA01623.jpg|thumb|300px|Mosaic of Jovian ring images with a scheme showing ring and satellite locations]]

The narrow and relatively thin main ring is the brightest part of [[Jupiter]]'s ring system. Its outer edge is located at a radius of about 129&nbsp;000&nbsp;km (1.806 ''R''<sub>J</sub>; ''R''<sub>J</sub> = equatorial radius of Jupiter or 71&nbsp;398&nbsp;km) and coincides with the orbit of Jupiter's smallest inner satellite, [[Adrastea (moon)|Adrastea]].<ref name=Ockert-Bell1999/><ref name=Burns1987/> Its inner edge is not marked by any satellite and is located at about 122&nbsp;500&nbsp;km (1.72 ''R''<sub>J</sub>).<ref name=Ockert-Bell1999>{{cite journal |last=Ockert-Bell |first=M. E. |coauthors=Burns, J. A.; Daubar, I. J.; ''et al.'' |title=The Structure of Jupiter’s Ring System as Revealed by the Galileo Imaging Experiment |journal=Icarus |year=1999 |volume=138 |pages=188&ndash;213 |doi=10.1006/icar.1998.6072 |url= http://adsabs.harvard.edu/abs/1999Icar..138..188O}}</ref> 

Thus the width of the main ring is around 6&nbsp;500&nbsp;km. The appearance of the main ring depends on the viewing geometry.<ref name=Burns2004/> In forward-scattered light{{Ref_label|A|a|none}} the brightness of the main ring begins to decrease steeply at 128&nbsp;600&nbsp;km (just inward of Adrastea's orbit) and reaches the background level at 129&nbsp;300&nbsp;km&mdash;just outward of Adrastean orbit.<ref name=Ockert-Bell1999/> Therefore [[Adrastea (moon)|Adrastea]] at 129&nbsp;000&nbsp;km clearly shepherds the ring.<ref name=Ockert-Bell1999/><ref name=Burns1987/> The brightness continues to increase in the direction of [[Jupiter]] and has a maximum near the ring’s center at 126&nbsp;000&nbsp;km, although there is a pronounced gap (notch) near the orbit of [[Metis (moon)|Metis]] at 128&nbsp;000&nbsp;km.<ref name=Ockert-Bell1999/> The inner boundary of the main ring, in contrast, appears to fade off slowly from 124&nbsp;000 to 120&nbsp;000&nbsp;km, merging into the halo ring.<ref name=Ockert-Bell1999/><ref name=Burns1987/> In forward-scattered light all Jovian rings are especially bright.
[[Image:Jovian main ring New Horizons 050107 10.jpg|thumb|300px|right|The upper image shows the main ring in back-scattered light as seen by the ''New Horizons'' spacecraft. The fine structure of its outer part is visible. The lower image shows the main ring in forward-scattered light demonstrating its lack of any structure except the Metis notch.]]

In back-scattered light{{Ref_label|B|b|none}} the situation is different. The outer boundary of the main ring, located at 129&nbsp;100&nbsp;km, or slightly beyond the orbit of [[Adrastea (moon)|Adrastea]], is actually very steep.<ref name=Burns2004/> The orbit of the moon is marked by a gap in the ring so there is a thin ringlet just outside its orbit. There is another ringlet just inside Adrastean orbit followed by a gap of unknown origin located at about 128&nbsp;500&nbsp;km.<ref name=Burns2004/> The third ringlet is found inward of the central gap outside the orbit of Metis. The ring’s brightness drops sharply just outward of the orbit of [[Metis (moon)|Metis]] thus forming the Metis notch.<ref name=Burns2004>{{cite encyclopedia|last=Burns|first=J.A.|coauthors=Simonelli, D. P.;Showalter, M.R. et al.|title=Jupiter’s Ring-Moon System |encyclopedia=Jupiter: The Planet, Satellites and Magnetosphere|year=2004|publisher=Cambridge University Press|editor= Bagenal, F.; Dowling, T.E.; McKinnon, W.B.|url=http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf| format=pdf}}</ref> Inward of Metis's orbit the brightness of the ring rises much less than in forward-scattered light.<ref name=dePater1999/> So in the back-scattered geometry the main ring appears to consist of two different parts: a narrow outer part extending from 128&nbsp;000 to 129&nbsp;000&nbsp;km, which itself includes three narrow ringlets separated by notches, and a fainter inner part from 122&nbsp;500 to 128&nbsp;000&nbsp;km, which lacks any visible structure like in the forward-scattering geometry.<ref name=Burns2004/><ref name=Showalter2005>{{cite conference |last=Showalter |first=M. R. |coauthors=Burns, J. A.; de Pater, I.; ''et al.'' |title=Updates On The Dusty Rings Of Jupiter, Uranus And Neptune |Date=26&ndash;28 September 2005 |booktitle=Proceedings of the Conference held September 26&ndash;28, 2005 in Kaua'i, Hawaii. LPI Contribution No. 1280 |pages=130 |url=http://adsabs.harvard.edu/abs/2005LPICo1280..130S}}</ref> The Metis notch serves as their boundary. The fine structure of the main ring was discovered in data from the ''[[Galileo (spacecraft)|Galileo]]'' orbiter and is clearly visible in back-scattered images obtained from ''[[New Horizons]]'' in February&ndash;March 2007.<ref name=Morring2007>{{cite journal |last=Morring |first=F. |title=Ring Leader |journal=Aviation Week&Space Technology |date=May 7, 2007 |pages=80&ndash;83}}</ref><ref name="New Horizons">{{cite web|title=Jupiter's Rings: Sharpest View |date=May 1, 2007 |publisher=NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute |url=http://pluto.jhuapl.edu/gallery/missionPhotos/pages/050107/050107_10.html |accessdate=2007-05-31}}</ref> However observations by [[Hubble Space Telescope]] (HST),<ref name=Meier1999>{{cite journal |last=Meier |first=R. |coauthors=Smith, B. A.; Owen, T. C.; ''et al.'' |title=Near Infrared Photometry of the Jovian Ring and Adrastea |journal=Icarus |year=1999 |volume=141 |pages=253&ndash;262 |doi=10.1006/icar.1999.6172 |url=http://adsabs.harvard.edu/abs/1999Icar..141..253M}}</ref> [[W. M. Keck Observatory|Keck]]<ref name=dePater1999>{{cite journal |last=de Pater |first=I. |coauthors=Showalter, M. R.; Burns, J. A.; ''et al.'' |title=Keck Infrared Observations of Jupiter’s Ring System near Earth’s 1997 Ring Plane Crossing |journal=Icarus |year=1999 |volume=138 |pages=214&ndash;223|doi=10.1006/icar.1998.6068|url=http://www.astro.umd.edu/~hamilton/research/reprints/DePater99.pdf| format=pdf}}</ref> and the ''[[Cassini Huygens|Cassini]]'' spacecraft failed to detect it, probably due to insufficient spatial resolution.<ref name=Throop2004>{{cite journal |last=Throop |first=H. B. |coauthors=[[Carolyn C. Porco|Porco, C. C.]]; West, R. A.; ''et al.'' |title=The Jovian Rings: New Results Derived from Cassini, Galileo, Voyager, and Earth-based Observations |journal=Icarus |year=2004 |volume=172 |pages=59&ndash;77 |doi=10.1016/j.icarus.2003.12.020 |url=http://ciclops.org/media/sp/2007/2687_7449_0.pdf|format=pdf}}</ref>

Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30&nbsp;km.<ref name=Burns1987/> In the side scatter geometry the ring thickness is 80&ndash;160&nbsp;km, increasing somewhat in the direction of [[Jupiter]].<ref name=Ockert-Bell1999/><ref name=Throop2004/> The ring appears to be much thicker in the forward-scattered light&mdash;about 300&nbsp;km.<ref name=Ockert-Bell1999/> One of the discoveries of the ''[[Galileo (spacecraft)|Galileo]]'' orbiter was the bloom of the main ring&mdash;a faint, relatively thick (about 600&nbsp;km) cloud of material which surrounds its inner part.<ref name=Ockert-Bell1999/> The bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo.<ref name=Ockert-Bell1999/>

Detailed analysis of the ''[[Galileo (spacecraft)|Galileo]]'' images revealed longitudinal variations of the main ring’s brightness unconnected with the viewing geometry. The Galileo images also showed some patchiness in the ring on the scales 500&ndash;1000&nbsp;km.<ref name=Ockert-Bell1999/><ref name=Burns2004/>

In February&ndash;March 2007 ''[[New Horizons]]'' spacecraft conducted a deep search for new small moons inside the main ring.<ref name=Showalter2007/> While no satellites larger than 0.5&nbsp;km was found, the cameras of the spacecraft detected seven small clumps of ring particles. They orbit just inside the orbit of Adrastea inside a dense ringlet.<ref name=Showalter2007/> The conclusion, that they are clumps and not small moons, is based on their [[azimuthal]]ly extended appearance. They subtend 0.1&ndash;0.3° along the ring, which correspond to 1000&ndash;3000&nbsp;km.<ref name=Showalter2007/> The clumps are divided into two groups of five and two members, respectively. The nature of the clumps is not clear, but their orbits are close to 115:116 and 114:115 [[orbital resonance|resonance]]s with [[Metis (moon)|Metis]].<ref name=Showalter2007/> So they can be wave like structures excited by this interaction.

===Spectra and particle size distribution===
[[Image:Main Ring Galeleo forward PIA00538.jpg|thumb|300px|right|Image of the main ring obtained by Galileo in forward-scattered light. The Metis notch is clearly visible.]]
[[spectrum|Spectra]] of the main ring obtained by the [[Hubble Space Telescope|HST]],<ref name=Meier1999/> [[W. M. Keck Observatory|Keck]],<ref name=Wong2006>{{cite journal |last=Wong |first=M. H. |coauthors=de Pater, I.; Showalter, M. R.; ''et al.'' |title=Ground-based Near Infrared Spectroscopy of Jupiter’s Ring and Moons |journal=Icarus |year=2006 |volume=185 |pages=403&ndash;415 |doi=10.1016/j.icarus.2006.07.007 |url=http://adsabs.harvard.edu/abs/2006Icar..185..403W}}</ref> [[Galileo (spacecraft)|Galileo]]<ref name=McMuldroch2000>{{cite journal |last=McMuldroch |first=S. |coauthors=Pilortz, S. H.; Danielson, J. E.; ''et al.'' |title=Galileo NIMS Near-Infrared Observations of Jupiter’s Ring System |journal=Icarus |year=2000 |volume=146 |pages=1&ndash;11 |doi=10.1006/icar.2000.6343 |url=http://adsabs.harvard.edu/abs/2000Icar..146....1M}}</ref> and ''[[Cassini Huygens|Cassini]]''<ref name=Throop2004/> have shown that particles forming it are red, i.e. their [[albedo]] is higher at longer wavelengths. The existing spectra span the range 0.5&ndash;2.5 μm.<ref name=Throop2004/> No spectral features have been found so far which can be attributed to particular chemical compounds, although the Cassini observations yielded evidence for absorption bands near 0.8&nbsp;μm and 2.2&nbsp;μm.<ref name=Throop2004/> The spectra of the main ring are actually very similar to [[Adrastea (moon)|Adrastea]]<ref name=Meier1999/> and [[Amalthea (moon)|Amalthea]].<ref name=Wong2006/>

The properties of the main ring can be explained by the hypothesis that it contains significant amounts of [[dust]] with 0.1&ndash;10&nbsp;μm particle sizes. This explains the stronger forward-scattering of light as compared to back-scattering.<ref name=Burns2004/><ref name=Showalter2005/> However, larger bodies are required to explain the strong back-scattering and fine structure in the bright outer part of the main ring.<ref name=Burns2004/><ref name=Showalter2005/> 

Analysis of available phase and spectral data leads to a conclusion that the size distribution of small particles in the main ring obeys a [[power law]]<ref name=Throop2004/><ref name=Brooks2004>{{cite journal |last=Brooks |first=S. M. |coauthors=Esposito, L. W.; Showalter, M. R.; ''et al.'' |title=The Size Distribution of Jupiter’s Main Ring from Galileo Imaging and Spectroscopy |journal=Icarus |year=2004 |volume=170 |pages=35&ndash;57 |doi=10.1016/j.icarus.2004.03.003 |url=http://adsabs.harvard.edu/abs/2004Icar..170...35B}}</ref><ref name=Burns2001>{{cite encyclopedia|last=Burns|first=J.A.|coauthors=Hamilton, D.P.; Showalter, M.R.|title=Dusty Rings and Circumplanetary Dust: Observations and Simple Physics |encyclopedia=Interplanetary Dust|year=2001 |publisher=Springer |place=Berlin |editor=Grun, E.; Gustafson, B. A. S.; Dermott, S. T.; Fechtig H. |pages=641&ndash;725|url=http://www.astro.umd.edu/~hamilton/research/preprints/BurHamSho01.pdf|format=pdf}}</ref>

:<math>n(r)=A\times r^{-q}</math>

where ''n''(''r'')&nbsp;''dr'' is a number of particles with [[radius|radii]] between ''r'' and ''r''&nbsp;+&nbsp;''dr'' and <math>A</math> is a normalizing parameter chosen to match the known total light [[flux]] from the ring. The parameter ''q'' is 2.0&nbsp;±&nbsp;0.2 for particles with ''r''&nbsp;<&nbsp;15&nbsp;±&nbsp;0.3&nbsp;μm and ''q'' = 5&nbsp;±&nbsp;1 for those with ''r''&nbsp;>&nbsp;15&nbsp;±&nbsp;0.3&nbsp;μm.<ref name=Throop2004/> The distribution of large bodies in the mm&ndash;km size range is undetermined presently.<ref name=Burns2004/> The light scattering in this model is dominated by particles with ''r'' around 15&nbsp;μm.<ref name=Throop2004/><ref name=McMuldroch2000/> 

The power law mentioned above allows estimation of the [[optical depth]]{{Ref_label|C|c|none}} <math>\scriptstyle\tau</math> of the main ring: <math>\scriptstyle\tau_l\,=\,4.7\times 10^{-6}</math> for the large bodies and <math>\scriptstyle \tau_s = 1.3\times 10^{-6}</math> for the dust.<ref name=Throop2004/> This [[optical depth]] means that the total cross section of all particles inside the ring is about 5000&nbsp;km².{{Ref_label|D|d|none}}<ref name=Burns2004/> The particles in the main ring are expected to have aspherical shapes.<ref name=Throop2004/> The total mass of the dust is estimated to be 10<sup>7</sup>&minus;10<sup>9</sup>&nbsp;kg.<ref name=Burns2004/> The mass of large bodies, excluding [[Metis (moon)|Metis]] and [[Adrastea (moon)|Adrastea]], is 10<sup>11</sup>&minus;10<sup>16</sup>&nbsp;kg. It depends on their maximum size&mdash; the upper value corresponds to about 1&nbsp;km maximum diameter.<ref name=Burns2004/> These masses can be compared with masses of [[Adrastea (moon)|Adrastea]], which is about 2{{Esp|15}}&nbsp;kg,<ref name=Burns2004/> [[Amalthea (moon)|Amalthea]]&mdash; about 2{{Esp|18}}&nbsp;kg<ref name=Anderson2005>{{cite journal |last=Anderson |authorlink=John D. Anderson |first=J. D. |coauthors=Johnson, T. V.; Shubert, G.; ''et al.'' |title=Amalthea’s Density Is Less Than That of Water |journal=Science |year=2005 |volume=308 |pages=1291&ndash;1293 |doi=10.1126/science.1110422 |url=http://adsabs.harvard.edu/abs/2005Sci...308.1291A |pmid=15919987}}</ref> and Earth's [[Moon]]&mdash;7.4{{Esp|22}}&nbsp;kg.

The presence of two populations of particles in the main ring explains why its appearance depends on the viewing geometry.<ref name=Burns2001/> The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring bounded by the orbit of Adrastea.<ref name=Burns2004/> In contrast, large particles, which scatter in the back direction, are confined inside the region between the orbits of Metis and Adrastea in a number of ringlets.<ref name=Burns2004/><ref name=Showalter2005/>

===Origin and age ===
[[Image:R08 satorb full.jpg|thumb|right|300px|Formation of Jupiter's rings]]
The dust is constantly being removed from the main ring by a combination of [[Poynting-Robertson drag]] and electromagnetic forces from the [[Jupiter#Magnetosphere|Jovian magnetosphere]].<ref name=Burns2001/><ref name=Burns1999>{{cite journal |last=Burns |first=J. A.| coauthors=Showalter, M. R.; Hamilton, D. P.; ''et al.'' |title=The Formation of Jupiter's Faint Rings |journal=Science |year=1999 |volume=284 |pages=1146&ndash;1150 |doi=10.1126/science.284.5417.1146 |url=http://www.astro.umd.edu/~hamilton/research/reprints/BurnsShowHam99.pdf|format=pdf |pmid=10325220}}</ref> Volatile materials, for example ices, evaporate quickly. The lifetime of dust particles in the ring is from 100 to 1000&nbsp;years,<ref name=Burns2004/><ref name=Burns1999/> so the dust must be continuously replenished in the collisions between large bodies with sizes from 1&nbsp;cm to 0.5&nbsp;km<ref name=Showalter2007>{{cite journal|last=Showalter|first=Mark R.|coauthors=Cheng, Andrew F.; Weaver, Harold A.; et al.|title= Clump Detections and Limits on Moons in Jupiter’s Ring System|journal=Science|year=2007|volume=318|pages=232&ndash;234|doi=10.1126/science.1147647| url=http://adsabs.harvard.edu/abs/2007Sci...318..232S|pmid= 17932287 }}</ref> and between the same large bodies and high velocity particles coming from outside the Jovian system.<ref name=Burns2004/><ref name=Burns1999/> This parent body population is confined to the narrow&mdash;about 1000&nbsp;km&mdash;and bright outer part of the main ring, and includes [[Metis (moon)|Metis]] and [[Adrastea (moon)|Adrastea]].<ref name=Burns2004/><ref name=Showalter2005/> The largest parent bodies must be less than 0.5&nbsp;km in size. The upper limit on their size was obtained by ''[[New Horizons]]'' spacecraft.<ref name=Showalter2007/> The previous upper limit, obtained from [[Hubble Space Telescope|HST]]<ref name=Meier1999/><ref name=Showalter2005/> and ''[[Cassini Huygens|Cassini]]''<ref name=Throop2004/> observations, was near 4&nbsp;km.<ref name=Burns2004/> The dust produced in collisions retains approximately the same orbital elements as the parent bodies and slowly spirals in the direction of [[Jupiter]] forming the faint (in back-scattered light) innermost part of the main ring and halo ring.<ref name=Burns2004/><ref name=Burns1999/> The age of the main ring is currently unknown, but it may be the last remnant of a past population of small bodies near [[Jupiter]].<ref name=Esposito2002/>

==Halo ring==
===Appearance and structure===
[[Image:Jovian Halo Ring PIA00658.jpg|thumb|300px|right|False color image of the halo ring obtained by Galileo in forward-scattered light]]
The halo ring is the innermost and thickest Jovian ring. Its outer edge coincides with the inner boundary of the main ring approximately at the radius 122&nbsp;500&nbsp;km (1.72 ''R''<sub>J</sub>).<ref name=Ockert-Bell1999/><ref name=Burns1987/> From this radius the ring becomes rapidly thicker towards Jupiter. The true vertical extent of the halo is not known but the presence of its material was detected as high as 10&nbsp;000&nbsp;km over the ring plane.<ref name=Ockert-Bell1999/><ref name=dePater1999/> The inner boundary of the halo is relatively sharp and located at the radius 100&nbsp;000&nbsp;km (1.4 ''R''<sub>J</sub>),<ref name=dePater1999/> but some material is present further inward to approximately 92&nbsp;000&nbsp;km.<ref name=Ockert-Bell1999/> Thus the width of the halo ring is about 30&nbsp;000&nbsp;km. Its shape resembles a thick torus without clear internal structure.<ref name=Burns2004/> In contrast to the main ring, the halo's appearance depends only slightly on the viewing geometry.

The halo ring appears brightest in forward-scattered light, in which it was extensively imaged by ''[[Galileo (spacecraft)|Galileo]]''.<ref name=Ockert-Bell1999/> While its surface brightness is much less than that of the main ring, its vertically (perpendicular to the ring plane) integrated photon [[flux]] is comparable due to its much larger thickness. Despite a claimed vertical extent of more than 20&nbsp;000&nbsp;km, the halo’s brightness is strongly concentrated towards the ring plane and follows a power law of the form ''z''<sup>&minus;0.6</sup> to ''z''<sup>&minus;1.5</sup>,<ref name=Burns2004/> where ''z'' is altitude over the ring plane. The halo’s appearance in the back-scattered light, as observed by [[W. M. Keck Observatory|Keck]]<ref name=dePater1999/> and [[Hubble Space Telescope|HST]],<ref name=Meier1999/> is basically the same. However its total photon flux is several times lower than that of the main ring and is more strongly concentrated near the ring plane than in the forward-scattered light.<ref name=Burns2004/>

The [[spectrum|spectral properties]] of the halo ring are different from the main ring. The [[flux]] distribution in the range 0.5&ndash;2.5&nbsp;μm is flatter than in the main ring;<ref name=Meier1999/> the halo is not red and may even be blue.<ref name=Wong2006/>

===Origin of the halo ring===
The optical properties of the halo ring can be explained by the hypothesis that it comprises only dust with particle sizes less than 15&nbsp;μm.<ref name=Meier1999/><ref name=Burns2004/><ref name=Brooks2004/> Parts of the halo located far from the ring plane may consist of submicrometre dust.<ref name=Meier1999/><ref name=dePater1999/><ref name=Burns2004/> This dusty composition explains the much stronger forward-scattering, bluer colors and lack of visible structure in the halo. The dust probably originates in the main ring, a claim supported by the fact that the halo’s [[optical depth]] <math>\scriptstyle\tau_s\,\sim\,10^{-6}</math> is comparable with that of the dust in the main ring.<ref name=Burns1987/><ref name=Burns2004/> The large thickness of the halo can be attributed to the excitation of [[orbital inclination]]s and [[orbital eccentricity|eccentricities]] of dust particles by the electromagnetic forces in the Jovian magnetosphere. The outer boundary of the halo ring coincides with location of a strong 3:2 Lorentz resonance.{{Ref_label|E|e|none}}<ref name=Burns2001/><ref name=Hamilton1994>{{cite journal|last=Hamilton|first=D. P.|title=A Comparison of Lorentz, Planetary Gravitational, and Satellite Gravitational Resonances| journal=Icarus|year=1994|volume=109|pages=221&ndash;240 |url=http://www.astro.umd.edu/~hamilton/research/reprints/Ham94.pdf| format=pdf|doi=10.1006/icar.1994.1089}}</ref><ref name=Burns1985>{{cite journal|last=Burns|first=J.A.|coauthors=Schaffer, L. E.; Greenberg, R. J. ''et al.'' |title=Lorentz Resonances and the Structure of the Jovian Ring|year=1985|journal=Nature |volume=316|pages=115&ndash;119|url=http://adsabs.harvard.edu/abs/1985Natur.316..115B |doi=10.1038/316115a0}}</ref> As [[Poynting-Robertson drag]]<ref name=Burns2001/><ref name=Burns1999/> causes particles to slowly drift towards Jupiter, their [[orbital inclination]]s are excited while passing through it. The bloom of the main ring may be a beginning of the halo.<ref name=Burns2004/> The halo ring’s inner boundary is not far from the strongest 2:1 Lorentz resonance.<ref name=Burns2001/><ref name=Hamilton1994/><ref name=Burns1985/> In this resonance the excitation is probably very significant, forcing particles to plunge into the Jovian atmosphere thus defining a sharp inner boundary.<ref name=Burns2004/> Being derived from the main ring, the halo has the same age.<ref name=Burns2004/>

==Gossamer rings==
===Amalthea gossamer ring===
[[Image:Jovian Gosamer Rings PIA00659.jpg|thumb|300px|left|Image of the gossamer rings obtained by Galileo in forward-scattered light]]
The Amalthea gossamer ring is a very faint structure with a rectangular cross section, stretching from the orbit of [[Amalthea (moon)|Amalthea]] at 182&nbsp;000&nbsp;km (2.54 ''R''<small>J</small>) to about 129&nbsp;000&nbsp;km (1.80 ''R''<sub>J</sub>).<ref name=Ockert-Bell1999/><ref name=Burns2004/> Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo.<ref name=Ockert-Bell1999/> The thickness of the ring is approximately 2300&nbsp;km near the orbit of Amalthea and slightly decreases in the direction of [[Jupiter]].{{Ref_label|F|f|none}}<ref name=dePater1999/> The Amalthea gossamer ring is actually the brightest near its top and bottom edges and becomes gradually brighter towards Jupiter; its top edge is brighter than the bottom edge.<ref name=Showalter2008>{{cite journal|last=Showalter|first=Mark R.|coauthors=de Pater, Imke; Verbanac, Guili et al.|title= Properties and dynamics of Jupiter’s gossamer rings from Galileo, Voyager, Hubble and Keck images|journal=Icarus|year=2008|volume=195|pages=361&ndash;377|doi=10.1016/j.icarus.2007.12.012| url=http://www.astro.umd.edu/~hamilton/research/reprints/ShoPatVer08.pdf|format=pdf}}</ref> The outer boundary of the ring is relatively steep, especially at the top edge.<ref name=Ockert-Bell1999/> There is a sharp drop in the brightness just inward of the orbit of [[Amalthea (moon)|Amalthea]] with an additional shelf-like structure.<ref name=Ockert-Bell1999/> In forward-scattered light the ring appears to be about 30 times fainter than the main ring.<ref name=Ockert-Bell1999/> In back-scattered light it has been detected only by the [[W. M. Keck Observatory|Keck]] telescope<ref name=dePater1999/> and the ACS (Advanced Camera for Surveys) on [[Hubble Space Telescope|HST]].<ref name=Showalter2005/> Back-scattering images show additional structure in the ring: a peak in the brightness just inside the orbit of [[Amalthea (moon)|Amalthea]].<ref name=dePater1999/><ref name=Showalter2008/> In 2002&ndash;2003 Galileo spacecraft had two passes through the gossamer rings. Its dust counter detected dust particles in the size range 0.2&ndash;5&nbsp;μm and confirmed the results obtained form imaging.<ref name=Kruger2003/><ref name=Kruger2008>{{cite journal|last=Kruger|first=Harald|coauthors=Hamilton, Duglas P.Moissl, Richard; and Grun, Eberhard|title=Galileo In-Situ Dust Measurements inJupiter’s Gossamer Rings|journal=Icarus, submitted|year=2008| url=http://adsabs.harvard.edu/abs/2008arXiv0803.2849K}}</ref>

The detection of the Amalthea gossamer ring from the ground, in ''[[Galileo (spacecraft)|Galileo]]'' images and the direct dust measurements have allowed the determination of the particle size distribution, which appears to follow the same power law as the dust in the main ring with ''q''=2&nbsp;±&nbsp;0.5.<ref name=Showalter2005/><ref name=Kruger2008/> The [[optical depth]] of this ring is about 10<sup>&minus;7</sup>, which is an order of magnitude lower than that of the main ring, but the total mass of the dust&mdash;10<sup>7</sup>&ndash;10<sup>9</sup>&nbsp;kg&mdash;is comparable.<ref name=Esposito2002/><ref name=Burns1999/><ref name=Kruger2008/>

===Thebe gossamer ring===
The Thebe gossamer ring is the faintest Jovian ring. It appears as a very faint structure with a rectangular cross section, stretching from the orbit of [[Thebe (moon)|Thebe]] at 226&nbsp;000&nbsp;km (3.11 ''R''<sub>J</sub>) to about 129&nbsp;000&nbsp;km (1.80 ''R''<sub>J</sub>;).<ref name=Ockert-Bell1999/><ref name=Burns2004/> Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo.<ref name=Ockert-Bell1999/> The thickness of the ring is approximately 8400&nbsp;km near the orbit of Thebe and slightly decreases in the direction of the planet.{{Ref_label|F|f|none}}<ref name=dePater1999/> The Thebe gossamer ring is brightest near its top and bottom edges and gradually becomes brighter towards [[Jupiter]]&mdash;much like the Amalthea ring.<ref name=Showalter2008/> The outer boundary of the ring is not especially steep, stretching over 15&nbsp;000&nbsp;km.<ref name=Ockert-Bell1999/> There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to 280&nbsp;000&nbsp;km (3.75 ''R''<sub>J</sub>) and called Thebe Extension.<ref name=Ockert-Bell1999/><ref name=Kruger2008/> In forward-scattered light the ring appears to be about 3 times fainter than the Amalthea gossamer ring.<ref name=Ockert-Bell1999/> In back-scattered light it has been detected only by the [[W. M. Keck Observatory|Keck]] telescope.<ref name=dePater1999/> Back-scattering images show a peak of brightness just inside the orbit of Thebe.<ref name=dePater1999/> In 2002&ndash;2003 the dust counter of the Galileo spacecraft detected dust particles in the size range 0.2&ndash;5&nbsp;μm&mdash;similar to those in the Amalthea ring&mdash;and confirmed the results obtained form imaging.<ref name=Kruger2003>{{cite conference |last=Krüger |first=H. |coauthors=Grün, E.; Hamilton, D. P. |title=Galileo In-Situ Dust Measurements in Jupiter's Gossamer Rings |Date=18&ndash;25 July 2004 |booktitle=35th COSPAR Scientific Assembly |pages=1582 |url=http://adsabs.harvard.edu/abs/2004cosp...35.1582K}}</ref><ref name=Kruger2008/>

The [[optical depth]] of the Thebe gossamer ring is about 3{{Esp|&minus;8}}, which is three times lower than the Amalthea gossamer ring, but the total mass of the dust is the same&mdash;about 10<sup>7&ndash;9</sup> kg.<ref name=Esposito2002/><ref name=Burns1999/><ref name=Kruger2008/> However the particle size distribution of the dust is somewhat shallower than in the Amalthea ring. It follows a power law with q&nbsp;<&nbsp;2. In the Thebe extension the parameter q may be even smaller.<ref name=Kruger2008/>

===Origin of the gossamer rings===
The dust in the gossamer rings originates in essentially the same way as that in the main ring and halo.<ref name=Burns1999/> Its sources are the inner Jovian moons [[Amalthea (moon)|Amalthea]] and [[Thebe (moon)|Thebe]] respectively. High velocity impacts by projectiles coming from outside the Jovian system eject dust particles from their surfaces.<ref name=Burns1999/> These particles initially retain the same orbits as their moons but then gradually spiral inward by [[Poynting-Robertson drag]].<ref name=Burns1999/> The thickness of the gossamer rings is determined by vertical excursions of the moons due to their nonzero [[orbital inclination]]s.<ref name=Burns2004/> This hypothesis naturally explains almost all observable properties of the rings: rectangular cross-section, decrease of thickness in the direction of [[Jupiter]] and brightening of the top and bottom edges of the rings. 

However some properties have so far gone unexplained, like the Thebe Extension, which may be due to unseen bodies outside Thebe's orbit, and structures visible in the back-scattered light.<ref name=Burns2004/> One possible explanation of the Thebe extension is influence of the electromagnetic forces from the Jovian magnetosphere. When the dust enters the shadow behind Jupiter, it loses its electrical charge fairly quickly. Since the small dust particles partially corotate with the planet, they will move outward during the shadow pass creating an outward extension of the Thebe gossamer ring.<ref name=Hamilton2008>{{cite journal|last=Hamilton|first=Douglas P.|coauthors=Kruger, Harold|title=The sculpting of Jupiter’s gossamer rings by its shadow|journal=Nature|year=2008|volume=453|pages=72&ndash;75|doi=10.1038/nature06886| url=http://www.astro.umd.edu/~hamilton/research/reprints/HamKru08.pdf|format=pdf}}</ref> The same forces can explain a dip in the particle distribution and ring's brightness, which occurs between the orbits of Amalthea and Thebe.<ref name=Kruger2008/><ref name=Hamilton2008/>

The analysis of gossamer ring's images revealed that a peak in the brightness just inside the Amalthea's orbit may be due to the dust particles trapped at the leading (L<sub>4</sub>) and trailing (L<sub>5</sub>) [[Lagrange point]]s of Amalthea. The higher brightness of the observed top edge of the Amalthea gossamer ring may also be caused by the this trapped dust. The particles may be present at the leading and trailing Lagrange points of Thebe as well. This discovery implies that there are two particle populations in the gossamer rings: one slowly drifts in the direction of Jupiter as described above, while another remains near a source moon trapped in 1:1 resonance with it.<ref name=Showalter2008/>

==Exploration==
The existence of the Jovian rings was inferred from observations of the planetary [[radiation belt]]s by [[Pioneer 11]] spacecraft in 1975.<ref name=Fillius1976>{{cite journal |last=Fillius |first=R. W. |coauthors=McIlwain, C. E.; Mogro-Campero, A. |title=Radiation Belts of Jupiter - A Second Look |journal=Science |year=1975 |volume=188 |pages=465&ndash;467 |url=http://adsabs.harvard.edu/abs/1975Sci...188..465F |doi=10.1126/science.188.4187.465 |pmid=17734363 }}</ref> In 1979 the ''[[Voyager 1]]'' spacecraft obtained a single overexposed image of the ring system.<ref name=Smith1979/> More extensive imaging was conducted by ''[[Voyager 2]]'' in the same year, which allowed rough determination of the ring’s structure.<ref name=Burns1987/> The superior quality of the images obtained by the ''[[Galileo (spacecraft)|Galileo]]'' orbiter between 1995 and 2003 greatly extended the existing knowledge about the Jovian rings.<ref name=Ockert-Bell1999/> Ground-based observation of the rings by the [[W. M. Keck Observatory|Keck]]<ref name=dePater1999/> telescope in 1997 and 2002 and the [[Hubble Space Telescope|HST]] in 1999<ref name=Meier1999/> revealed the rich structure visible in back-scattered light. Images transmitted by the ''New Horizons'' spacecraft in February&ndash;March 2007<ref name="New Horizons"/> allowed observation of the fine structure in the main ring for the first time. In 2000, the ''[[Cassini Huygens|Cassini]]'' spacecraft en route to [[Saturn]] conducted extensive observations of the Jovian ring system.<ref name=Brown2003>{{cite journal |last=Brown |first=R. H. |coauthors=Baines, K. H.; Bellucci, G.; ''et al.'' |title=Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini’s Flyby of Jupiter |year=2003 |journal=Icarus |volume=164 |pages=461&ndash;470 |doi=10.1016/S0019-1035(03)00134-9 |url=http://adsabs.harvard.edu/abs/2003Icar..164..461B}}</ref> Future missions to the Jovian system will provide additional information about the rings.<ref name=Juno>{{cite web|title=Juno - NASA New Frontiers Mission to Jupiter |url=http://juno.wisc.edu/ |accessdate=2007-06-06}}</ref>

==Notes==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}} The forward-scattered light is the light scattered at a small angle relative to solar light.
<li>{{Note_label|B|b|none}} The back-scattered light is the light scattered at an angle close to 180° relative to solar light.
<li>{{Note_label|C|c|none}} The normal optical depth is the ratio between the total [[cross-section]] of the ring's particles to the square area of the ring.<ref name=Throop2004/>
<li>{{Note_label|D|d|none}} It should be compared with approximately 1700&nbsp;km² total cross section of [[Metis (moon)|Metis]] and [[Adrastea (moon)|Adrastea]].<ref name=Burns2004/>
<li>{{Note_label|E|e|none}} Lorentz resonance is a resonance between particle's orbital motion and rotation of planetary magnetosphere, when the ratio of their periods is a [[rational number]].<ref name=Hamilton1994/>
<li>{{Note_label|F|f|none}} The thickness of the gossamer rings is defined here as the distance between peaks of brightness at their top and bottom edges.<ref name=Showalter2008/>
</li>
</ol>
</div>

== See also ==
*[[Moons of Jupiter]]

==References==
{{reflist|2}}

==External links==
{{commonscat|Jupiter (rings)}}
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupringfact.html Jupiter Rings Fact Sheet]
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Jupiter&Display=Rings Jupiter's Rings] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://web.archive.org/web/20060206090452/spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhome.html NASA Pioneer project page]
* [http://voyager.jpl.nasa.gov NASA Voyager project page]
* [http://www2.jpl.nasa.gov/galileo NASA Galileo project page]
* [http://saturn.jpl.nasa.gov/home/index.cfm NASA Cassini project space]
* [http://pluto.jhuapl.edu New Horizont project page]
* [http://pds-rings.seti.org/jupiter/ Planetary Ring Node: Jupiter's Ring System]

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