Rings of 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 and the system was more thoroughly investigated in the 1990s by the Galileo orbiter. The main ring has also been observed by the Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes.
The Jovian ring system is faint and consists mainly of dust. 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.
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. High-resolution images obtained in February and March 2007 by the New Horizons spacecraft revealed a rich fine structure in the main ring.
In visible and near-infrared 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 in all rings except the halo. The halo ring is probably dominated by submicrometre dust. The total mass of the ring system is poorly constrained, but is probably in the range of 1011 to 1016 kg. The age of the ring system is also not known, but it is possible that it has existed since the formation of Jupiter.
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. 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. The principal attributes of the known Jovian Rings are listed in the table.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.
| Name | Radius | Width | Thickness | Optical depth | Dust fraction | Mass, kg | Notes |
| Halo ring | – | ~1 | 100% | — | |||
| Main ring | – | 30–300 | 5.9 | ~25% | 107– 109 1011– 1016 | Bounded by Adrastea | |
| Amalthea gossamer ring | – | ~1 | 100% | 107– 109 | Connected with Amalthea | ||
| Thebe gossamer ring | – | ~3 | 100% | 107– 109 | Connected with Thebe. There is an extension beyond the orbit of Thebe. |
Main ring
Appearance and structure
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 and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea. Its inner edge is not marked by any satellite and is located at about .Thus the width of the main ring is around. 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 and reaches the background level at —just outward of the Adrastean orbit. Therefore, Adrastea at clearly shepherds the ring. The brightness continues to increase in the direction of Jupiter and has a maximum near the ring's center at, although there is a pronounced gap near the Metidian orbit at. The inner boundary of the main ring, in contrast, appears to fade off slowly from to, 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, or slightly beyond the orbit of Adrastea, 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. 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. Inward of the orbit of Metis, 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 to, which itself includes three narrow ringlets separated by notches, and a fainter inner part from to, 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 in February–March 2007. The early observations by Hubble Space Telescope, 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 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. 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 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 spacecraft conducted a deep search for new small moons inside the main ring. 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. 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 –. 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. They may be wavelike structures excited by this interaction.
Spectra and particle size distribution
of the main ring obtained by the HST, Keck, Galileo and Cassini 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. 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 and Amalthea.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. 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
where n ''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. 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 of the main ring: for the large bodies and for the dust. This optical depth 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 and Adrastea, 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, which is about 2 kg, Amalthea, about 2 kg, and Earth's Moon, 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 in a number of ringlets between the Metidian and Adrastean orbits.