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Rings of Jupiter
From Wikipedia, the free encyclopedia
The planet Jupiter has a system of rings, 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 Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe[1] and thoroughly investigated in the 1990s by the Galileo orbiter.[2] It has also been observed by the Hubble Space Telescope and from Earth for the past 25 years.[3] Ground-based observations of the rings require the largest available telescopes.[4]
The Jovian ring system is faint and consists mainly of dust.[1][5] 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 and Thebe.[6]
The main and halo rings consist of dust ejected from the moons Metis, Adrastea and other unobserved parent bodies as the result of high-velocity impacts.[2] High-resolution images obtained in February and March 2007 by the New Horizons spacecraft revealed a rich fine structure in the main ring.[7]
In visible and near-infrared light, the rings have a reddish color, except the halo ring, which is neutral or blue.[3] The size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius ~15 μm in all rings except the halo.[8] The halo ring is probably dominated by submicron dust. The total mass of the ring system (including unobserved parent bodies) is about 1016 kg, which is comparable with the mass of Adrastea.[9] The age of the ring system is not known, but it may have existed since the formation of Jupiter.[9]
Contents |
The principal attributes of the four known Jovian Rings are listed in the table.[5][2][6][8]
| Name | Radius (km) | Width (km) | Thickness (km) | Optical depth | Dust fraction | Notes |
|---|---|---|---|---|---|---|
| Halo ring | 92,000–122,500 | 30,500 | 12500 | ~1×10−6 | 100% | |
| Main ring | 122,500–129,000 | 6,500 | 30–300 | 5.9×10−6 | ~25% | Bounded by Adrastea |
| Amalthea gossamer ring | 129,000–182,000 | 53,000 | 2000 | ~1×10−7 | 100% | Connected with Amalthea |
| Thebe gossamer ring | 129,000–226,000 | 97,000 | 8400 | ~3×10−8 | 100% | Connected with Thebe. There is an extension beyond the orbit of Thebe. |
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 1.806 RJ (~129,000 km; RJ = equatorial radius of Jupiter or 71,398 km) and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea.[5][2] Its inner edge is not marked by any satellite and is located at ~122,500 km (1.72 RJ).[2]
Thus the width of the main ring is ~6,500 km. The appearance of the main ring depends on the viewing geometry.[9] In forward-scattered light (light scattered at a small angle relative to solar light) the brightness of the main ring begins to decrease steeply at 128,600 km (just inward of Adrastea's orbit) and reaches the background level at 129,300 km (just outward of Adrastean orbit).[2] Therefore Adrastea at 129,000 km clearly shepherds the ring.[5][2] The brightness continues to increase in the direction of Jupiter and has a maximum near the ring’s center at 126,000 km, although there is a pronounced gap (notch) near the orbit of Metis at 128,000 km.[2] The inner boundary of the main ring, in contrast, appears to fade off slowly from 124,000 to 120,000 km, merging into the halo ring.[5][2] In forward-scattered light all Jovian rings are especially bright.
In back-scattered light (the light scattered at an angle close to 180° relative to solar light) the situation is different. The outer boundary of the main ring, located at 129,100 km, or slightly beyond the orbit of Adrastea, is actually very steep.[9] 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 ~128,500 km.[9] The third ringlet is found inward of the central gap outside the Metisian orbit. The ring’s brightness drops sharply just outward of the orbit of Metis thus forming the Metis notch.[9] Inward of the Metisian orbit the brightness of the ring rises much less than in forward-scattered light.[4] So in the back-scattered geometry the main ring appears to consist of two different parts: a narrow outer part extending from 128,000 to 129,000 km, which itself includes three narrow ringlets separated by notches, and a fainter inner part from 122,500 to 128,000 km, which lacks any visible structure like in the forward-scattering geometry.[9][10] The Metis notch serves as their boundary. The fine structure of the main ring was discovered in data from the Galileo orbiter and is clearly visible in back-scattered images obtained from New Horizons in February-March 2007.[11][7] However observations by Hubble Space Telescope (HST),[3] Keck[4] and the Cassini spacecraft[8] failed to detect it, probably due to insufficient spatial resolution.
Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30 km.[5] In the side scatter geometry the ring thickness is 80–160 km, increasing somewhat in the direction of Jupiter.[2][8] The ring appears to be much thicker in the forward-scattered light (~300 km).[2] One of the discoveries of the Galileo orbiter was the bloom of the main ring – a faint, relatively thick (~600 km) cloud of material which surrounds its inner part.[2] The bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo.[2]
Detailed analysis of the 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 500–1000 km.[2][9]
Spectra of the main ring obtained by the HST,[3] Keck,[12] Galileo[13] and Cassini[8] have shown that particles forming it are red, i.e. their albedo is higher at longer wavelengths. The existing spectra span the range 0.5–2.5 μm.[8] No spectral features have been found so far which can be attributed to particular chemical compounds. The spectra of the main ring are actually very similar to Adrastea[3] and Amalthea.[12]
The properties of the main ring can be explained by the hypothesis that it contains significant amounts of dust with 0.1–10 μm particle sizes. This explains the stronger forward-scattering of light as compared to back-scattering.[9][10] However, larger bodies are required to explain the strong back-scattering and fine structure in the bright outer part of the main ring.[9][10]
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[8][14][15]
where n(r) dr is a number of particles with radii between r and r + dr and A is a normalizing parameter chosen to match the known total light flux from the ring. The parameter q is 2.0 ± 0.2 for particles with r <15 ± 0.3 μm and q = 5±1 for those with r > 15 ± 0.3 μm.[8] The distribution of large bodies in the mm–km size range is undetermined presently.[9] The light scattering in this model is dominated by particles with r ~15 μm.[8][13]
The power law mentioned above allows estimation of the optical depth 
of the main ring: 
for the large bodies and 
for the dust.[8] This optical depth means that the total cross section of all particles inside the ring is ~5000 km² (compare it with ~1500 km² total cross section of Metis and Adrastea).[9] The particles in the main ring are expected to have aspherical shapes.[8] The total mass of the dust is estimated to be 107−109 kg.[9] The mass of large bodies is 1011−1016 kg (excluding Metis and Adrastea) depending on their maximum size (~1 km for the upper value).[9] These masses can be compared with masses of Adrastea ~2×1015 kg,[9] Amalthea ~2×1018 kg[16] and Earth's Moon 7.4×1022 kg.
The presence of two populations of particles in the main ring explains why its appearance depends on the viewing geometry.[15] The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring (excluding the Metis notch) bounded by the orbit of Adrastea.[9] 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.[9][10]
The dust is constantly being removed from the main ring by a combination of Poynting-Robertson drag and electromagnetic forces from the Jovian magnetosphere.[17][15] Volatile compounds (i.e. ices) also evaporate quickly. The lifetime of dust particles in the ring is ~100 years,[9][17] so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 10 km and between the same large bodies and high velocity particles coming from outside the Jovian system.[9][17] This parent body population is confined to the narrow (~1000 km) bright outer part of the main ring, which includes Metis and Adrastea and may contain additional unknown kilometer-sized objects.[9][10] The upper limit on their size, obtained from HST[10][3] and Cassini[8] observations, is ~4 km.[9] 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.[9][17]
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.[6]
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 122,500 km (1.72 RJ).[5][2] 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 10,000 km over the ring plane.[4][2] The inner boundary of the halo is relatively sharp and located at the radius 100,000 km (1.4 RJ),[4] but some material is present further inward to approximately 92,000 km.[2] Thus the width of the halo ring is about 30,000 km. Its shape resembles a thick torus without clear internal structure.[9] 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.[2] 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 20,000 km, the halo’s brightness is strongly concentrated towards the ring plane and follows a power law (proportional to z−0.6 to z−1.5),[9] where z is altitude over the ring plane. The halo’s appearance in the back-scattered light (as seen in Keck[4] and HST[3] observations) is basically the same, but its total photon flux is several times lower than that of the main ring and is more strongly concentrated near the ring plane.[9]
The spectral properties of the halo ring are different from the main ring. The flux distribution in the range 0.5–2.5 μm is flatter than in the main ring;[3] the halo is not red and may even be blue.[12]
The optical properties of the halo ring can be explained by the hypothesis that it comprises only dust with particle sizes less than 15 μm.[9][3][14] Parts of the halo located far from the ring plane may consist of submicron dust.[4][9][3] 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 
is comparable with that of the dust in the main ring.[5][9] The large thickness of the halo can be attributed to the excitation of orbital inclinations and 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 (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).[18][19][15] As Poynting-Robertson drag[17][15] causes particles to slowly drift towards Jupiter, their orbital inclinations are excited while passing through it. The bloom of the main ring may be a beginning of the halo.[9] The halo ring’s inner boundary is not far from the strongest 2:1 Lorentz resonance.[18][19][15] In this resonance the excitation is probably very significant, forcing particles to plunge into the Jovian atmosphere thus defining a sharp inner boundary.[9]
Being derived from the main ring, the halo has the same age.[9]
The Amalthea gossamer ring is a very faint structure with a rectangular cross section, stretching from the orbit of Amalthea at 182,000 km (2.54 RJ) to about 129,000 km (1.80 RJ).[2][9] Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo.[2] The thickness of the ring is approximately 2300 km near the orbit of Amalthea and slightly decreases in the direction of Jupiter.[4] The Amalthea gossamer ring is actually brightest near its top and bottom edges and becomes gradually brighter towards Jupiter.[2] The outer boundary of the ring is relatively steep, especially at the top edge.[2] There is a pronounced shelf in its radial profile just inward of the orbit of Amalthea.[2] In forward-scattered light the ring appears to be ~30 times fainter than the main ring.[2] In back-scattered light it has been detected only by the Keck telescope[4] and the ACS (Advanced Camera for Surveys) on HST.[10] Back-scattering images show additional structure in the ring: an increase of the brightness just inside the orbit of Amalthea.[4]
The detection of the Amalthea gossamer ring from the ground in addition to Galileo images has 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±0.5.[10] The optical depth of this ring is ~10-7 (an order of magnitude lower than the main ring) but the total mass of the dust (107–109 kg) is comparable.[6][17]
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 at 226,000 km (3.11 RJ) to about 129,000 km (1.80 RJ).[2][9] Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo.[2] The thickness of the ring is approximately 8400 km near the orbit of Thebe and slightly decreases in the direction of the planet.[4] The Thebe gossamer ring is actually brightest near its top and bottom edges and gradually becomes brighter towards Jupiter.[2] The outer boundary of the ring is not especially steep, stretching over 15,000 km.[2] There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to 260,000 km (3.50 RJ) and called Thebe Extension.[2] In forward-scattered light the ring appears to be ~3 times fainter than the Amalthea gossamer ring.[2] In back-scattered light it has been detected only by the Keck telescope.[4] Back-scattering images show an increase in brightness just inside the orbit of Thebe.[4]
The optical depth of the Thebe gossamer ring is ~3×10-8 (three times lower than the Amalthea gossamer ring), but the total mass of the dust (~107–9 kg) is the same.[6][17] In 2002–2003 the Galileo orbiter passed through the Thebe gossamer ring, enabling the first direct detection of its dust.[20] These measurements revealed the particles with sizes 0.2–3 μm thus confirming the dust composition of the gossamer rings.
The dust in the gossamer rings originates in essentially the same way as that in the main ring and halo.[17] Its sources are the inner Jovian moons Amalthea and Thebe respectively. High velocity impacts by projectiles coming from outside the Jovian system eject dust particles from their surfaces.[17] These particles initially retain the same orbits as their moons but then gradually spiral inward by Poynting-Robertson drag.[17] The thickness of the gossamer rings is determined by vertical excursions of the moons due to their nonzero orbital inclinations.[9] 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.[9]
The existence of the Jovian rings was inferred from observations of the planetary radiation belts by Pioneer 11 spacecraft in 1975.[21] In 1979 the Voyager 1 spacecraft obtained a single overexposed image of the ring system.[1] More extensive imaging was conducted by Voyager 2 in the same year, which allowed rough determination of the ring’s structure.[5] The superior quality of the images obtained by the Galileo orbiter between 1994 and 2003 greatly extended the existing knowledge about the Jovian rings.[2] Ground-based observation of the rings by the Keck[4] telescope in 1997 and 2002 and the HST in 1999[3] revealed the rich structure visible in back-scattered light. Images transmitted by the New Horizons spacecraft in February-March 2007[11] allowed observation of the fine structure in the main ring for the first time. In 2000, the Cassini spacecraft en route to Saturn conducted extensive observations of the Jovian ring system.[22] Future missions to the Jovian system will provide additional information about the rings.[23]
- ^ a b c Smith, B. A.; Soderblom, L. A.; Johnson, T. V.; et al. (1979). "The Jupiter System through the Eyes of Voyager 1". Science 204: 951–957, 960–972.
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae Ockert-Bell, M. E.; Burns, J. A.; Daubar, I. J.; et al. (1999). "The Structure of Jupiter’s Ring System as Revealed by the Galileo Imaging Experiment". Icarus 138: 188–213. doi:10.1006/icar.1998.6072.
- ^ a b c d e f g h i j k Meier, R.; Smith, B. A.; Owen, T. C.; et al. (1999). "Near Infrared Photometry of the Jovian Ring and Adrastea". Icarus 141: 253–262. doi:10.1006/icar.1999.6172.
- ^ a b c d e f g h i j k l m n de Pater, I.; Showalter, M. R.; Burns, J. A.; et al. (1999). "Keck Infrared Observations of Jupiter’s Ring System near Earth’s 1997 Ring Plane Crossing". Icarus 138: 214–223. doi:10.1006/icar.1998.6068.
- ^ a b c d e f g h i Showalter, M. A.; Burns, J. A.; Cuzzi, J. N.; Pollack, J. B. (1987). "Jupiter's Ring System: New Results on Structure and Particle Properties". Icarus 69 (3): 458–498. doi:10.1016/0019-1035(87)90018-2.
- ^ a b c d e Esposito, L. W. (2002). "Planetary rings". Reports On Progress In Physics 65: 1741–1783.
- ^ a b Morring, F. (May 7, 2007). "Ring Leader". Aviation Week&Space Technology: 80–83.
- ^ a b c d e f g h i j k l Throop, H. B.; Porco, C. C.; West, R. A.; et al. (2004). "The Jovian Rings: New Results Derived from Cassini, Galileo, Voyager, and Earth-based Observations". Icarus 172: 59–77. doi:10.1016/j.icarus.2003.12.020.
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai Burns, J. A.; D. P. Simonelli & M. R. Showalter et al. (2004), "Jupiter’s Ring-Moon System", in Bagenal, F.; Dowling, T. E.; McKinnon, W. B., Jupiter: The Planet, Satellites and Magnetosphere, Cambridge University Press, <http://www.astro.umd.edu/~hamilton/research/preprints/BurSimSho03.pdf>
- ^ a b c d e f g h Showalter, M. R.; Burns, J. A.; de Pater, I.; et al.. "Updates On The Dusty Rings Of Jupiter, Uranus And Neptune". Proceedings of the Conference held September 26–28, 2005 in Kaua'i, Hawaii. LPI Contribution No. 1280: 130.
- ^ a b Jupiter's Rings: Sharpest View. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (May 1, 2007). Retrieved on 2007-05-31.
- ^ a b c Wong, M. H.; de Pater, I.; Showalter, M. R.; et al. (2006). "Ground-based Near Infrared Spectroscopy of Jupiter’s Ring and Moons". Icarus 185: 403–415. doi:10.1016/j.icarus.2006.07.007.
- ^ a b McMuldroch, S.; Pilortz, S. H.; Danielson, J. E.; et al. (2000). "Galileo NIMS Near-Infrared Observations of Jupiter’s Ring System". Icarus 146: 1–11. doi:10.1006/icar.2000.6343.
- ^ a b Brooks, S. M.; Esposito, L. W.; Showalter, M. R.; et al. (2004). "The Size Distribution of Jupiter’s Main Ring from Galileo Imaging and Spectroscopy". Icarus 170: 35–57. doi:10.1016/j.icarus.2004.03.003.
- ^ a b c d e f Burns, J. A.; D. P. Hamilton & M. R. Showalter (2001), [http://www.astro.umd.edu/~hamilton/research/preprints/BurHamSho01.pdf "Dusty Rings and Circumplanetary Dust: Observations and Simple Physics"], written at Berlin, in Grun, E.; Gustafson, B. A. S.; Dermott, S. T.; Fechtig H., Interplanetary Dust, Springer, 641–725, <http://www.astro.umd.edu/~hamilton/research/preprints/BurHamSho01.pdf>
- ^ Anderson, J. D.; Johnson, T. V.; Shubert, G.; et al. (2005). "Amalthea’s Density Is Less Than That of Water". Science 308: 1291–1293. doi:10.1126/science.1110422.
- ^ a b c d e f g h i j Burns, J. A.; Showalter, M. R.; Hamilton, D. P.; et al. (1999). "The Formation of Jupiter's Faint Rings". Science 284: 1146–1150. doi:10.1126/science.284.5417.1146.
- ^ a b Hamilton, D. P. (1994). "A Comparison of Lorentz, Planetary Gravitational, and Satellite Gravitational Resonances". Icarus 109: 221–240.
- ^ a b Burns, J. A.; Schaffer, L. E.; Greenberg, R. J.; et al. (1985). "Lorentz Resonances and the Structure of the Jovian Ring". Nature 316: 115–119.
- ^ Krüger, H.; Grün, E.; Hamilton, D. P.. "Galileo In-Situ Dust Measurements in Jupiter's Gossamer Rings". 35th COSPAR Scientific Assembly: 1582.
- ^ Fillius, R. W.; McIlwain, C. E.; Mogro-Campero, A. (1975). "Radiation Belts of Jupiter - A Second Look". Science 188: 465–467.
- ^ Brown, R. H.; Baines, K. H.; Bellucci, G.; et al. (2003). "Observations with the Visual and Infrared Mapping Spectrometer (VIMS) during Cassini’s Flyby of Jupiter". Icarus 164: 461–470. doi:10.1016/S0019-1035(03)00134-9.
- ^ Juno - NASA New Frontiers Mission to Jupiter. Retrieved on 2007-06-06.
- Jupiter Rings Fact Sheet
- Jupiter's Rings by NASA's Solar System Exploration
- NASA Pioneer project page
- NASA Voyager project page
- NASA Galileo project page
- NASA Cassini project space
- New Horizont project page
- Planetary Ring Node: Jupiter's Ring System
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| Rings of Jupiter • Rings of Saturn • Rings of Uranus • Rings of Neptune |
| See also: gas torus |
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|---|---|---|
| Major Moons | Io · Europa · Ganymede · Callisto |  |
| Characteristics | Cloud pattern on Jupiter · Great Red Spot · Jupiter's magnetosphere · Oval BA · Rings of Jupiter · Moons | |
| Exploration | Pioneer program · Voyager program · Galileo (spacecraft) · Juno (spacecraft) · Europa Orbiter | |
| Other | Jupiter-crosser asteroid · Earthly Branches · Colonization · Jupiter mass | |
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|---|---|
| Listed in increasing distance from Jupiter. Temporary names in italics. | |
| Amalthea group | Metis · Adrastea · Amalthea · Thebe |
| Galilean moons | Io · Europa · Ganymede · Callisto |
| Themisto | |
| Himalia group | Leda · Himalia · Lysithea · Elara · S/2000 J 11 |
| Carpo · S/2003 J 12 | |
| Ananke group | Ananke · Praxidike · Harpalyke · Iocaste · Euanthe · Thyone (core) Euporie · S/2003 J 3 · S/2003 J 18 · Thelxinoe · Helike · Orthosie · S/2003 J 16 · Hermippe · Mneme · S/2003 J 15 (peripheral) |
| Carme group | S/2003 J 17 · S/2003 J 10 · Pasithee · Chaldene · Arche · Isonoe · Erinome · Kale · Aitne · Taygete · S/2003 J 9 · Carme · S/2003 J 5 · S/2003 J 19 · Kalyke · Eukelade · Kallichore |
| Pasiphaë group | Eurydome · S/2003 J 23 · Hegemone · Pasiphaë · Sponde · Cyllene · Megaclite · S/2003 J 4 · Callirrhoe · Sinope · Autonoe · Aoede · Kore |
| S/2003 J 2 | |
| Rings of Jupiter | |
