Rings of Jupiter - AbsoluteAstronomy.com

The planet Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

has a system of ring

Planetary ring

A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

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

Solar System

The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, after those of Saturn

Rings of Saturn

The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

and Uranus

Rings of Uranus

The planet Uranus has a system of rings 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 Douglas J. Mink...

. It was first observed in 1979 by the Voyager 1

Voyager 1

The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

space probe

Space probe

A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

and thoroughly investigated in the 1990s by the Galileo orbiter. It has also been observed by the Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

and from Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

for the past 23 years. Ground-based observations of the rings require the largest available telescopes.

The Jovian ring system is faint and consists mainly of dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

. It has four main components: a thick inner torus

Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

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

Amalthea (moon)

Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '....

and Thebe

Thebe (moon)

Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter...

.

The main and halo rings consist of dust ejected from the moons Metis

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

, Adrastea

Adrastea (moon)

Adrastea , also known as ', is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than...

and other unobserved parent bodies as the result of high-velocity impacts. High-resolution images obtained in February and March 2007 by the New Horizons

New Horizons

New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...

spacecraft revealed a rich fine structure in the main ring.

In visible and near-infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

light, the rings have a reddish color, except the halo ring, which is neutral or blue in color. The size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius about 15 μm

Micrometre

A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

in all rings except the halo. 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 10¹¹ to 10¹⁶ kg. The age of the ring system is not known, but it may have existed since the formation of Jupiter.

A ring could possibly exist around the moon Himalia

Himalia (moon)

Himalia is the largest irregular satellite of Jupiter, the sixth largest overall in size, and the fifth largest in mass. It was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904 and is named after the nymph Himalia, who bore three sons of Zeus .- Discovery...

's orbit, which would have been created if S/2000 J 11

S/2000 J 11

S/2000 J 11 was an object believed to be the second-outermost prograde irregular satellite of Jupiter. It was discovered by a team of astronomers from the University of Hawaii led by Scott S. Sheppard in 2000....

had indeed crashed into Himalia.

Discovery and structure

The rings of Jupiter was the third ring system to be discovered in the Solar System

Solar System

, after those of Saturn

Rings of Saturn

and Uranus

Rings of Uranus

. It was first observed in 1979 by the Voyager 1

Voyager 1

space probe

Space probe

. It comprises four main components: a thick inner torus

Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

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

Amalthea (moon)

and Thebe

Thebe (moon)

. The principal attributes of the known Jovian Rings are listed in the table.

Name	Radius (km)	Width (km)	Thickness (km)	Optical depth Optical depth Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...	Dust fraction (in τ)	Mass, kg	Notes
Halo ring	92 000–122 500	30 500	12 500	~1	100%	—
Main ring	122 500–129 000	6 500	30–300	5.9	~25%	10⁷– 10⁹ (dust) 10¹¹– 10¹⁶ (large particles)	Bounded by Adrastea Adrastea (moon) Adrastea , also known as ', is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than...
Amalthea gossamer ring	129 000–182 000	53 000	2 000	~1	100%	10⁷– 10⁹	Connected with Amalthea Amalthea (moon) Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '....
Thebe gossamer ring	129 000–226 000	97 000	8 400	~3	100%	10⁷– 10⁹	Connected with Thebe Thebe (moon) Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter... . There is an extension beyond the orbit of Thebe.

Appearance and structure

The narrow and relatively thin main ring is the brightest part of Jupiter

Jupiter

's ring system. Its outer edge is located at a radius of about 129 000 km (1.806 R_J; R_J = equatorial radius of Jupiter or 71 398 km) and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea

Adrastea (moon)

. Its inner edge is not marked by any satellite and is located at about 122 500 km (1.72 R_J).

Thus the width of the main ring is around 6 500 km. The appearance of the main ring depends on the viewing geometry. In forward-scattered 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. Therefore Adrastea

Adrastea (moon)

at 129 000 km clearly shepherds the ring. The brightness continues to increase in the direction of Jupiter

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

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

at 128 000 km. 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. In forward-scattered light all Jovian rings are especially bright.

In back-scattered light the situation is different. The outer boundary of the main ring, located at 129 100 km, or slightly beyond the orbit of Adrastea

Adrastea (moon)

, is very steep. 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. 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

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

thus forming the Metis notch. Inward of Metis's orbit the brightness of the ring rises much less than in forward-scattered light. 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. 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

New Horizons

in February–March 2007. The early observations by Hubble Space Telescope

Hubble Space Telescope

(HST), Keck and the Cassini spacecraft failed to detect it, probably due to insufficient spatial resolution. However the fine structure was observed by the Keck telescope using adaptive optics

Adaptive optics

Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

in 2002–2003.

Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30 km. In the side scatter geometry the ring thickness is 80–160 km, increasing somewhat in the direction of Jupiter

Jupiter

. The ring appears to be much thicker in the forward-scattered light—about 300 km. 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. The bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo.

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.

In February–March 2007 New Horizons

New Horizons

spacecraft conducted a deep search for new small moons inside the main ring. While no satellites larger than 0.5 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. 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 1000–3000 km. 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 resonance

Orbital resonance

In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...

s with Metis

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

. They may be wavelike structures excited by this interaction.

Spectra and particle size distribution

Spectra

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

of the main ring obtained by the HST

Hubble Space Telescope

, Keck, Galileo and Cassini have shown that particles forming it are red, i.e. their albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

is higher at longer wavelengths. The existing spectra span the range 0.5–2.5 μm. 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. The spectra of the main ring are very similar to Adrastea

Adrastea (moon)

and Amalthea

Amalthea (moon)

.

The properties of the main ring can be explained by the hypothesis that it contains significant amounts of dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

with 0.1–10 μm particle sizes. This explains the stronger forward-scattering of light as compared to back-scattering. However, larger bodies are required to explain the strong back-scattering and fine structure in the bright outer part of the main ring.

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

Power law

A power law is a special kind of mathematical relationship between two quantities. When the frequency of an event varies as a power of some attribute of that event , the frequency is said to follow a power law. For instance, the number of cities having a certain population size is found to vary...

where n(r) dr is a number of particles with radii

Radius

In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

between r and r + dr and

is a normalizing parameter chosen to match the known total light flux

Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

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. The distribution of large bodies in the mm–km size range is undetermined presently. The light scattering in this model is dominated by particles with r around 15 μm.

The power law mentioned above allows estimation of the optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

of the main ring:

for the large bodies and

for the dust. This optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

means that the total cross section of all particles inside the ring is about 5000 km². The particles in the main ring are expected to have aspherical shapes. The total mass of the dust is estimated to be 10⁷−10⁹ kg. The mass of large bodies, excluding Metis

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

and Adrastea

Adrastea (moon)

, is 10¹¹−10¹⁶ kg. It depends on their maximum size— the upper value corresponds to about 1 km maximum diameter. These masses can be compared with masses of Adrastea

Adrastea (moon)

, which is about 2 kg, Amalthea

Amalthea (moon)

— about 2 kg and Earth's Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

—7.4 kg.

The presence of two populations of particles in the main ring explains why its appearance depends on the viewing geometry. The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring bounded by the orbit of Adrastea. 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.

Origin and age

The dust is constantly being removed from the main ring by a combination of Poynting–Robertson drag and electromagnetic forces from the Jovian magnetosphere. Volatile materials, for example ices, evaporate quickly. The lifetime of dust particles in the ring is from 100 to 1000 years, so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 0.5 km and between the same large bodies and high velocity particles coming from outside the Jovian system. This parent body population is confined to the narrow—about 1000 km—and bright outer part of the main ring, and includes Metis

Metis (moon)

Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

and Adrastea

Adrastea (moon)

. The largest parent bodies must be less than 0.5 km in size. The upper limit on their size was obtained by New Horizons

New Horizons

spacecraft. The previous upper limit, obtained from HST

Hubble Space Telescope

and Cassini observations, was near 4 km. The dust produced in collisions retains approximately the same orbital elements as the parent bodies and slowly spirals in the direction of Jupiter

Jupiter

forming the faint (in back-scattered light) innermost part of the main ring and halo ring. 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

Jupiter

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 is appears to have been excited in 1995, around the time of the impact of Comet Shoemaker-Levy 9

Comet Shoemaker-Levy 9

Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by...

with Jupiter, while the smaller set appears to date to the first half of 1990. Galileos November 1996 observations are consistent with wavelengths of and , and vertical amplitudes of and , for the larger and smaller sets of waves, respectively. 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 x 10¹² kg, which would have tilted the ring out of the equatorial plane by 2 km. A similar spiraling wave pattern that tightens over time has been observed by Cassini in Saturns's C and D rings.

Appearance and structure

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 122 500 km (1.72 R_J). 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. The inner boundary of the halo is relatively sharp and located at the radius 100 000 km (1.4 R_J), but some material is present further inward to approximately 92 000 km. Thus the width of the halo ring is about 30 000 km. Its shape resembles a thick torus without clear internal structure. 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. While its surface brightness is much less than that of the main ring, its vertically (perpendicular to the ring plane) integrated photon flux

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 of the form z^−0.6 to z^−1.5, where z is altitude over the ring plane. The halo’s appearance in the back-scattered light, as observed by Keck and HST

Hubble Space Telescope

, 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.

The spectral properties

Spectrum

of the halo ring are different from the main ring. The flux

Flux

distribution in the range 0.5–2.5 μm is flatter than in the main ring; the halo is not red and may even be blue.

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. Parts of the halo located far from the ring plane may consist of submicrometre dust. 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

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

is comparable with that of the dust in the main ring. The large thickness of the halo can be attributed to the excitation of orbital inclinations and eccentricities

Orbital eccentricity

The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

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. As Poynting–Robertson drag 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. The halo ring’s inner boundary is not far from the strongest 2:1 Lorentz resonance. In this resonance the excitation is probably very significant, forcing particles to plunge into the Jovian atmosphere thus defining a sharp inner boundary. Being derived from the main ring, the halo has the same age.

Amalthea gossamer ring

The Amalthea gossamer ring is a very faint structure with a rectangular cross section, stretching from the orbit of Amalthea

Amalthea (moon)

at 182 000 km (2.54 RJ) to about 129 000 km (1.80 R_J). Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo. The thickness of the ring is approximately 2300 km near the orbit of Amalthea and slightly decreases in the direction of Jupiter

Jupiter

. 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. The outer boundary of the ring is relatively steep; the ring's brightness drops abruptly just inward of the orbit of Amalthea

Amalthea (moon)

, although it may have a small extension beyond the orbit of the satellite ending near 4:3 resonance with Thebe. In forward-scattered light the ring appears to be about 30 times fainter than the main ring. In back-scattered light it has been detected only by the Keck telescope and the ACS (Advanced Camera for Surveys

Advanced Camera for Surveys

The Advanced Camera for Surveys is a third generation axial instrument aboard the Hubble Space Telescope . The initial design and scientific capabilities of ACS were defined by a team based at Johns Hopkins University. ACS was assembled and tested extensively at Ball Aerospace & Technologies Corp...

) on HST

Hubble Space Telescope

. Back-scattering images show additional structure in the ring: a peak in the brightness just inside the orbit of Amalthea

Amalthea (moon)

confined to the top or bottom edge of the ring.

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. In addition, the Galileo spacecraft's star scanner detected small, discrete bodies (< 1 km) near Amalthea. 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. The optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

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.

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

Thebe (moon)

at 226 000 km (3.11 R_J) to about 129 000 km (1.80 R_J;). Its inner boundary is not clearly defined because of the presence of the much brighter main ring and halo. The thickness of the ring is approximately 8400 km near the orbit of Thebe and slightly decreases in the direction of the planet. The Thebe gossamer ring is brightest near its top and bottom edges and gradually becomes brighter towards Jupiter

Jupiter

—much like the Amalthea ring. The outer boundary of the ring is not especially steep, stretching over 15 000 km. There is a barely visible continuation of the ring beyond the orbit of Thebe, extending up to 280 000 km (3.75 R_J) and called Thebe Extension. In forward-scattered light the ring appears to be about 3 times fainter than the Amalthea gossamer ring. In back-scattered light it has been detected only by the Keck telescope. Back-scattering images show a peak of brightness just inside the orbit of Thebe. 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.

The optical depth

Optical depth

Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

of the Thebe gossamer ring is about 3, which is three times lower than the Amalthea gossamer ring, but the total mass of the dust is the same—about 10⁷–10⁹ kg. 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.

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. Its sources are the inner Jovian moons Amalthea

Amalthea (moon)

and Thebe

Thebe (moon)

respectively. High velocity impacts by projectiles coming from outside the Jovian system eject dust particles from their surfaces. These particles initially retain the same orbits as their moons but then gradually spiral inward by Poynting–Robertson drag. The thickness of the gossamer rings is determined by vertical excursions of the moons due to their nonzero orbital inclinations. This hypothesis naturally explains almost all observable properties of the rings: rectangular cross-section, decrease of thickness in the direction of Jupiter

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

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. The particles may also follow horseshoe orbits between the Lagrangian points. 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.

Himalia ring

The small moon S/2000 J 11

S/2000 J 11

, 4 kilometres in diameter, has gone missing since its discovery in 2000. One theory is that it has crashed into the much larger moon Himalia

Himalia (moon)

, 170 kilometres in diameter, creating a faint ring. This possible ring appears as a faint streak near Himalia in images from NASA's New Horizons

New Horizons

mission to Pluto

Pluto

Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

. This suggests that Jupiter sometimes gains and loses small moons through collisions.

Exploration

The existence of the Jovian rings was inferred from observations of the planetary radiation belts by Pioneer 11

Pioneer 11

Pioneer 11 is a 259-kilogram robotic space probe launched by NASA on April 6, 1973 to study the asteroid belt, the environment around Jupiter and Saturn, solar wind, cosmic rays, and eventually the far reaches of the solar system and heliosphere...

spacecraft in 1975. In 1979 the Voyager 1

Voyager 1

spacecraft obtained a single overexposed image of the ring system. More extensive imaging was conducted by Voyager 2

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

in the same year, which allowed rough determination of the ring’s structure. The superior quality of the images obtained by the Galileo orbiter between 1995 and 2003 greatly extended the existing knowledge about the Jovian rings. Ground-based observation of the rings by the Keck telescope in 1997 and 2002 and the HST

Hubble Space Telescope

in 1999 revealed the rich structure visible in back-scattered light. Images transmitted by the New Horizons spacecraft in February–March 2007 allowed observation of the fine structure in the main ring for the first time. In 2000, the Cassini spacecraft en route to Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

conducted extensive observations of the Jovian ring system. Future missions to the Jovian system will provide additional information about the rings.

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