Rings of Jupiter

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A schema of Jupiter's ring system showing the four main components. For simplicity, Metis and Adrastea are depicted as sharing their orbit. (In reality, Metis is very slightly closer to Jupiter.) PIA01627 Ringe.jpg
A schema of Jupiter's ring system showing the four main components. For simplicity, Metis and Adrastea are depicted as sharing their orbit. (In reality, Metis is very slightly closer to Jupiter.)

The rings of Jupiter are a system of faint planetary rings. The Jovian rings were the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. The main ring was discovered in 1979 by the Voyager 1 space probe [1] and the system was more thoroughly investigated in the 1990s by the Galileo orbiter. [2] The main ring has also been observed by the Hubble Space Telescope and from Earth for several years. [3] Ground-based observation of the rings requires the largest available telescopes. [4]

Contents

The Jovian ring system is faint and consists mainly of dust. [1] [5] 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 and Thebe. [6]

The main and halo rings consist of dust ejected from the moons Metis, Adrastea and perhaps smaller, unobserved 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 in color. [3] The size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius about 15 μm in all rings except the halo. [8] The halo ring is probably dominated by submicrometre dust. The total mass of the ring system (including unresolved parent bodies) is poorly constrained, but is probably in the range of 1011 to 1016 kg. [9] The age of the ring system is also not known, but it is possible that it has existed since the formation of Jupiter. [9]

A ring or ring arc appears to exist close to the moon Himalia's orbit. One explanation is that a small moon recently crashed into Himalia and the force of the impact ejected the material that forms the ring.

Discovery and structure

Jupiter's ring system was the third to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed on 4 March 1979 by the Voyager 1 space probe. [1] [10] It is composed of 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 after the moons of whose material they are composed: Amalthea and Thebe. [6] The principal attributes of the known Jovian Rings are listed in the table. [2] [5] [6] [8]

In 2022, dynamical simulations suggested that the relative meagreness of Jupiter's ring system, compared to that of the smaller Saturn, is due to destabilising resonances created by the Galilean satellites. [11]

NameRadius (km)Width (km)Thickness (km) Optical depth [lower-alpha 1] (in τ)Dust fractionMass, kgNotes
Halo ring92,000–122,50030,50012,500~1 × 10−6100% —
Main ring122,500–129,0006,50030–3005.9 × 10−6~25%107– 109 (dust)
1011– 1016 (large particles)		Bounded by Adrastea
Amalthea gossamer ring129,000–182,00053,0002,000~1 × 10−7100%107– 109Connected with Amalthea
Thebe gossamer ring129,000–226,00097,0008,400~3 × 10−8100%107– 109Connected with Thebe. There is an extension beyond the orbit of Thebe.

Main ring

Appearance and structure

Mosaic of Jovian ring images with a scheme showing ring and satellite locations Jovian Ring System PIA01623.jpg
Mosaic of Jovian ring images with a scheme showing ring and satellite locations
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. Jovian main ring New Horizons 050107 10.jpg
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.
Metis orbiting at the edge of Jupiter's main ring, as imaged by the New Horizons spacecraft in 2007 PIA09921 Shepherd Moons.gif
Metis orbiting at the edge of Jupiter's main ring, as imaged by the New Horizons spacecraft in 2007

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 129,000 km (1.806 RJ;RJ = equatorial radius of Jupiter or 71,398 km) and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea. [2] [5] Its inner edge is not marked by any satellite and is located at about 122,500 km (1.72 RJ). [2]

Thus the width of the main ring is around 6,500 km. The appearance of the main ring depends on the viewing geometry. [9] In forward-scattered light [lower-alpha 2] the brightness of the main ring begins to decrease steeply at 128,600 km (just inward of the Adrastean orbit) and reaches the background level at 129,300 km—just outward of the Adrastean orbit. [2] Therefore, Adrastea at 129,000 km clearly shepherds the ring. [2] [5] 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 Metidian orbit 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. [2] [5] In forward-scattered light all Jovian rings are especially bright.

In back-scattered light [lower-alpha 3] the situation is different. The outer boundary of the main ring, located at 129,100 km, or slightly beyond the orbit of Adrastea, is 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 about 128,500 km. [9] 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 Metidian orbit, forming the Metis notch. [9] Inward of the orbit of Metis, 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] [12] 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. [7] [13] The early observations by Hubble Space Telescope (HST), [3] Keck [4] and the Cassini spacecraft failed to detect it, probably due to insufficient spatial resolution. [8] However the fine structure was observed by the Keck telescope using adaptive optics in 2002–2003. [14]

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—about 300 km. [2] One of the discoveries of the Galileo orbiter was the bloom of the main ring—a faint, relatively thick (about 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]

In February–March 2007 New Horizons spacecraft conducted a deep search for new small moons inside the main ring. [15] While no satellites larger than 0.5 km were 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. [15] The conclusion, that they are clumps and not small moons, is based on their azimuthally extended appearance. They subtend 0.1–0.3° along the ring, which correspond to 1,000–3,000 km. [15] 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 resonances with Metis. [15] They may be wavelike structures excited by this interaction.

Spectra and particle size distribution

Image of the main ring obtained by Galileo in forward-scattered light. The Metis notch is clearly visible. Main Ring Galeleo forward PIA00538.jpg
Image of the main ring obtained by Galileo in forward-scattered light. The Metis notch is clearly visible.

Spectra of the main ring obtained by the HST, [3] Keck, [16] Galileo [17] 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, although the Cassini observations yielded evidence for absorption bands near 0.8 μm and 2.2 μm. [8] The spectra of the main ring are very similar to Adrastea [3] and Amalthea. [16]

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] [12] However, larger bodies are required to explain the strong back-scattering and fine structure in the bright outer part of the main ring. [9] [12]

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] [18] [19]

where n(r) dr is a number of particles with radii between r and r + dr and 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 around 15 μm. [8] [17]

The power law mentioned above allows estimation of the optical depth [lower-alpha 1] 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 about 5000 km². [lower-alpha 4] [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, excluding Metis and Adrastea, is 1011−1016 kg. It depends on their maximum size— the upper value corresponds to about 1 km maximum diameter. [9] These masses can be compared with masses of Adrastea, which is about 2 × 1015 kg, [9] Amalthea, about 2 × 1018 kg, [20] 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. [19] The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring bounded by the orbit of Adrastea. [9] In contrast, large particles, which scatter in the back direction, are confined in a number of ringlets between the Metidian and Adrastean orbits. [9] [12]

Origin and age

Schematic illustrating the formation of Jupiter's rings R08 satorb full.jpg
Schematic illustrating the 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 Jovian magnetosphere. [19] [21] Volatile materials such as ices, for example, evaporate quickly. The lifetime of dust particles in the ring is from 100 to 1,000 years, [9] [21] so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 0.5 km [15] and between the same large bodies and high velocity particles coming from outside the Jovian system. [9] [21] This parent body population is confined to the narrow—about 1,000 km—and bright outer part of the main ring, and includes Metis and Adrastea. [9] [12] The largest parent bodies must be less than 0.5 km in size. The upper limit on their size was obtained by New Horizons spacecraft. [15] The previous upper limit, obtained from HST [3] [12] and Cassini [8] observations, was near 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] [21] 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]

Vertical corrugations

Images from the Galileo and New Horizons space probes show the presence of two sets of spiraling vertical corrugations in the main ring. These waves became more tightly wound over time at the rate expected for differential nodal regression in Jupiter's gravity field. Extrapolating backwards, the more prominent of the two sets of waves appears to have been excited in 1995, around the time of the impact of Comet Shoemaker-Levy 9 with Jupiter, while the smaller set appears to date to the first half of 1990. [22] [23] [24] Galileo's November 1996 observations are consistent with wavelengths of 1920 ± 150 and 630 ± 20 km, and vertical amplitudes of 2.4 ± 0.7 and 0.6 ± 0.2 km, for the larger and smaller sets of waves, respectively. [24] The formation of the larger set of waves can be explained if the ring was impacted by a cloud of particles released by the comet with a total mass on the order of 2–5 × 1012 kg, which would have tilted the ring out of the equatorial plane by 2 km. [24] A similar spiraling wave pattern that tightens over time [25] has been observed by Cassini in Saturns's C and D rings. [26]

Halo ring

Appearance and structure

False color image of the halo ring obtained by Galileo in forward-scattered light Jovian Halo Ring PIA00658.jpg
False color image of the halo ring obtained by Galileo in forward-scattered light

The halo ring is the innermost and the vertically thickest Jovian ring. Its outer edge coincides with the inner boundary of the main ring approximately at the radius 122500 km (1.72 RJ). [2] [5] 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 10000 km over the ring plane. [2] [4] The inner boundary of the halo is relatively sharp and located at the radius 100000 km (1.4 RJ), [4] but some material is present further inward to approximately 92000 km. [2] Thus the width of the halo ring is about 30000 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 20000 km, the halo's brightness is strongly concentrated towards the ring plane and follows a power law of the form 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 observed by Keck [4] and HST, [3] is 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. [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. [16]

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 μm. [3] [9] [18] Parts of the halo located far from the ring plane may consist of submicrometre dust. [3] [4] [9] 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. [lower-alpha 5] [19] [27] [28] As Poynting–Robertson drag [19] [21] 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. [19] [27] [28] 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]

Gossamer rings

Amalthea gossamer ring

Image of the gossamer rings obtained by Galileo in forward-scattered light Jovian Gosamer Rings PIA00659.jpg
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 at 182000 km (2.54 RJ) to about 129000 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. [lower-alpha 6] [4] The Amalthea gossamer ring is actually the brightest near its top and bottom edges and becomes gradually brighter towards Jupiter; one of the edges is often brighter than another. [29] The outer boundary of the ring is relatively steep; [2] the ring's brightness drops abruptly just inward of the orbit of Amalthea, [2] although it may have a small extension beyond the orbit of the satellite ending near 4:3 resonance with Thebe. [14] In forward-scattered light the ring appears to be about 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. [12] Back-scattering images show additional structure in the ring: a peak in the brightness just inside the Amalthean orbit and confined to the top or bottom edge of the ring. [4] [14]

In 2002–2003 Galileo spacecraft had two passes through the gossamer rings. During them its dust counter detected dust particles in the size range 0.2–5 μm. [30] [31] In addition, the Galileo spacecraft's star scanner detected small, discrete bodies (< 1 km) near Amalthea. [32] These may represent collisional debris generated from impacts with this satellite.

The detection of the Amalthea gossamer ring from the ground, in 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 ± 0.5. [12] [31] The optical depth of this ring is about 10−7, which is an order of magnitude lower than that of the main ring, but the total mass of the dust (107–109 kg) is comparable. [6] [21] [31]

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 Thebean orbit at 226000 km (3.11 RJ) to about 129000 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. [lower-alpha 6] [4] The Thebe gossamer ring is brightest near its top and bottom edges and gradually becomes brighter towards Jupiter—much like the Amalthea ring. [29] The outer boundary of the ring is not especially steep, stretching over 15000 km. [2] There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to 280000 km (3.75 RJ) and called the Thebe Extension. [2] [31] In forward-scattered light the ring appears to be about 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 a peak of brightness just inside the orbit of Thebe. [4] In 2002–2003 the dust counter of the Galileo spacecraft detected dust particles in the size range 0.2–5 μm—similar to those in the Amalthea ring—and confirmed the results obtained from imaging. [30] [31]

The optical depth of the Thebe gossamer ring is about 3 × 10−8, which is three times lower than the Amalthea gossamer ring, but the total mass of the dust is the same—about 107–109 kg. [6] [21] [31] However the particle size distribution of the dust is somewhat shallower than in the Amalthea ring. It follows a power law with q < 2. In the Thebe extension the parameter q may be even smaller. [31]

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. [21] 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. [21] These particles initially retain the same orbits as their moons but then gradually spiral inward by Poynting–Robertson drag. [21] 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. [29]

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] 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. [33] The same forces can explain a dip in the particle distribution and ring's brightness, which occurs between the orbits of Amalthea and Thebe. [31] [33]

The peak in the brightness just inside of the Amalthea's orbit and, therefore, the vertical asymmetry the Amalthea gossamer ring may be due to the dust particles trapped at the leading (L4) and trailing (L5) Lagrange points of this moon. [29] The particles may also follow horseshoe orbits between the Lagrangian points. [14] The dust 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. [29]

Himalia ring

Composite of six New Horizons images of the possible Himalia ring. The double exposure of Himalia is circled. The arrow points to Jupiter. Himalia ring.jpg
Composite of six New Horizons images of the possible Himalia ring. The double exposure of Himalia is circled. The arrow points to Jupiter.

In September 2006, as NASA's New Horizons mission to Pluto approached Jupiter for a gravity assist, it photographed what appeared to be a faint, previously unknown planetary ring or ring arc, parallel with and slightly inside the orbit of the irregular satellite Himalia. The amount of material in the part of the ring or arc imaged by New Horizons was at least 0.04 km3, assuming it had the same albedo as Himalia. If the ring (arc) is debris from Himalia, it must have formed quite recently, given the century-scale precession of the Himalian orbit. It is possible that the ring could be debris from the impact of a very small undiscovered moon into Himalia, suggesting that Jupiter might continue to gain and lose small moons through collisions. [34]

Exploration

The existence of the Jovian rings was inferred from observations of the planetary radiation belts by Pioneer 11 spacecraft in 1975. [35] 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 1995 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 [13] 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. [36] Future missions to the Jovian system will provide additional information about the rings. [37]

See also

Notes

  1. 1 2 The normal optical depth is the ratio between the total cross section of the ring's particles to the square area of the ring. [8]
  2. ↑ The forward-scattered light is the light scattered at a small angle relative to solar light.
  3. ↑ The back-scattered light is the light scattered at an angle close to 180° relative to solar light.
  4. ↑ ^ It should be compared with approximately 1700 km² total cross section of Metis and Adrastea. [9]
  5. ↑ 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. [27]
  6. 1 2 The thickness of the gossamer rings is defined here as the distance between peaks of brightness at their top and bottom edges. [29]

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There are 95 moons of Jupiter with confirmed orbits as of 5 February 2024. This number does not include a number of meter-sized moonlets thought to be shed from the inner moons, nor hundreds of possible kilometer-sized outer irregular moons that were only briefly captured by telescopes. All together, Jupiter's moons form a satellite system called the Jovian system. The most massive of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 91 known moons and the rings together comprising just 0.003% of the total orbiting mass.

<span class="mw-page-title-main">Rings of Saturn</span>

The rings of Saturn are the most extensive and complex ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirely of water ice, with a trace component of rocky material. There is still no consensus as to their mechanism of formation. Although theoretical models indicated that the rings were likely to have formed early in the Solar System's history, newer data from Cassini suggested they formed relatively late.

<span class="mw-page-title-main">Rings of Uranus</span>

The rings of Uranus are intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Jessica Mink. William Herschel had also reported observing rings in 1789; modern astronomers are divided on whether he could have seen them, as they are very dark and faint.

<span class="mw-page-title-main">Rings of Neptune</span>

The rings of Neptune consist primarily of five principal rings. They were first discovered by simultaneous observations of a stellar occultation on 22 July 1984 by André Brahic's and William B. Hubbard's teams at La Silla Observatory (ESO) and at Cerro Tololo Interamerican Observatory in Chile. They were eventually imaged in 1989 by the Voyager 2 spacecraft. At their densest, they are comparable to the less dense portions of Saturn's main rings such as the C ring and the Cassini Division, but much of Neptune's ring system is quite faint and dusty, in some aspects more closely resembling the rings of Jupiter. Neptune's rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams. Neptune also has a faint unnamed ring coincident with the orbit of the moon Galatea. Three other moons orbit between the rings: Naiad, Thalassa and Despina.

<span class="mw-page-title-main">Io (moon)</span> Innermost of the four Galilean moons of Jupiter

Io, or Jupiter I, is the innermost and second-smallest of the four Galilean moons of the planet Jupiter. Slightly larger than Earth's moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, the strongest surface gravity of any moon, and the lowest amount of water by atomic ratio of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers.

In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton. Inner satellites are distinguished from other regular satellites by their proximity to the parent planet, their short orbital periods, their low mass, small size, and irregular shapes.

<span class="mw-page-title-main">Exploration of Jupiter</span> Overview of the exploration of Jupiter the planet and its moons

The exploration of Jupiter has been conducted via close observations by automated spacecraft. It began with the arrival of Pioneer 10 into the Jovian system in 1973, and, as of 2023, has continued with eight further spacecraft missions in the vicinity of Jupiter. All of these missions were undertaken by the National Aeronautics and Space Administration (NASA), and all but two were flybys taking detailed observations without landing or entering orbit. These probes make Jupiter the most visited of the Solar System's outer planets as all missions to the outer Solar System have used Jupiter flybys. On 5 July 2016, spacecraft Juno arrived and entered the planet's orbit—the second craft ever to do so. Sending a craft to Jupiter is difficult, mostly due to large fuel requirements and the effects of the planet's harsh radiation environment.

<span class="mw-page-title-main">Exploration of Io</span> Overview of the exploration of Io, Jupiters innermost Galilean and third-largest moon

The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light. Based on ephemerides produced by astronomer Giovanni Cassini and others, Pierre-Simon Laplace created a mathematical theory to explain the resonant orbits of three of Jupiter's moons, Io, Europa, and Ganymede. This resonance was later found to have a profound effect on the geologies of these moons. Improved telescope technology in the late 19th and 20th centuries allowed astronomers to resolve large-scale surface features on Io as well as to estimate its diameter and mass.

<span class="mw-page-title-main">Thor (volcano)</span> Active volcano on Jupiters moon Io

Thor is an active volcano on Jupiter's moon Io. It is located on Io's anti-Jupiter hemisphere at 39.15°N 133.14°W. A major eruption with high thermal emission and a large, volcanic plume was observed during a Galileo flyby on August 6, 2001, when the spacecraft flew through the outer portions of the plume allowing for direct sampling. The eruption continued into Galileo's next flyby in October 2001. As seen during high-resolution images taken during the eruption, Thor consists of a series of dark lava flows emanating from a set of nearby volcanic depressions. Before the eruption, the area consisted of red-brown plains, composed of irradiated sulfur, typical of Io's mid- to high-northern latitudes and a set of yellow flows, possibly consisting of sulfur or silicate flows covered by diffuse sulfur deposits. During the New Horizons encounter in February 2007, Thor was still active, with the spacecraft observing thermal emission in the near-infrared and a volcanic plume at the volcano.

<span class="mw-page-title-main">Mark R. Showalter</span> American astronomer

Mark Robert Showalter is a senior research scientist at the SETI Institute. He is the discoverer of six moons and three planetary rings. He is the Principal Investigator of NASA's Planetary Data System Rings Node, a co-investigator on the Cassini–Huygens mission to Saturn, and works closely with the New Horizons mission to Pluto.

<span class="mw-page-title-main">Zethus (crater)</span> Large crater on Thebe

Zethus is a large crater on Jupiter's moon Thebe. The crater is named after Zethus, the husband of the nymph Thebe in Greek mythology, and is the only named surface feature on Thebe. It measures 40 km (25 mi) across in diameter, making quite large when compared to Thebe itself. Zethus, like many other craters on Thebe and Amalthea has bright spots located near its rim. Zethus never faces Jupiter due to being tidally locked to Jupiter in a way that Thebe's long axis points towards Jupiter.

<span class="mw-page-title-main">Dust astronomy</span> Branch of astronomy

Dust astronomy is a subfield of astronomy that uses the information contained in individual cosmic dust particles ranging from their dynamical state to its isotopic, elemental, molecular, and mineralogical composition in order to obtain information on the astronomical objects occurring in outer space. Dust astronomy overlaps with the fields of Planetary science, Cosmochemistry, and Astrobiology.

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