Rings of Uranus

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Scheme of the Uranus ring moon system. The solid lines indicate rings; dashed lines represent the orbits of the moons.
Uranus with its rings (Hubble Space Telescope, 1998)

The planet Uranus is surrounded by a system of planetary rings , which in its variation and complexity does not come close to the much larger orbits of the rings of Saturn , but can still be classified in front of the simpler structures of the Jupiter and Neptune rings . The first rings of Uranus were discovered on March 10, 1977 by James L. Elliot , Edward W. Dunham and Douglas J. Mink . Although the astronomer Wilhelm Herschel had already reported on the observation of rings 200 years earlier , today's astronomers doubt that, given their dark and pale appearance, it was possible with the means of the time to actually perceive the ring system. Two more rings were discovered in 1986 in images taken of the planet by the Voyager 2 spacecraft , and an additional pair of rings were found in photos taken by the Hubble Space Telescope between 2003 and 2005 .

Since then, 13 independent rings of the Uranus ring system are known. In order of their distance from the planet, they are called 1986U2R / ζ , 6, 5, 4, α , β , η , γ , δ , λ , ε , ν and μ . Their radii are 38,000 km for the 1986U2R / ζ ring and 98,000 km for the μ ring. Additional dull bands of dust and incomplete arches could be observed between the main rings. The rings are extremely dark, so the spherical albedo of the ring particles does not exceed 2 percent. They are likely made up of frozen water that has combined with some dark, radiation-absorbing organic components.

Most of the Uranus rings are opaque and only a few kilometers wide. The ring system consists of small objects, the majority of which have a diameter between 0.2 and 20 m. Some of the rings are optically very small: the extended and matt rings 1986U2R / ζ, μ and ν consist of thin dust particles, while the narrow and also matt λ ring is composed of larger objects. The relative absence of dust within the ring system can be explained by the air resistance that the extensive exosphere of Uranus brings with it through its corona .

It is believed that the rings of Uranus are not older than 600 million years and are therefore relatively young. The ring system probably consists of the remains of a large number of moons, which originally orbited the planet before they collided with each other a long time ago. After collisions, the moons broke up into countless parts, which then survived as the narrow and optically dense rings visible today and now surround the planet in strictly defined orbits.

The process of how the narrow rings are held in their shape is still not fully understood. Initially, it was believed that each narrow ring is associated with a pair of nearby so-called shepherd moons that support its shape. However, when Voyager 2 flew by in 1986, only one pair of shepherds ( Cordelia and Ophelia ) were able to find an influence on the brightest ring (ε).

discovery

The first mention of a ring system surrounding Uranus comes from the 18th century and can be found in the notes of Friedrich Wilhelm Herschel , in which he wrote down the findings from his observations of the planet. These contained the following passage:

"February 22, 1789: A ring was suspected." (Translated: "February 22, 1789: Suspected a ring.") 

Herschel drew a narrow diagram of the ring and noted that it "tends a little towards red". The Keck telescope in Hawaii was able to confirm this, at least with regard to the ν ring. Herschel's notes were published in the Royal Society Journal in 1797 . Over the years, serious doubts arose as to whether Herschel could have seen anything like it, while hundreds of other astronomers had not seen anything like it. Nonetheless, there are legitimate objections to the fact that Herschel was indeed able to give a precise description of the dimensions of the ν-ring in relation to Uranus, its changes in the movement of Uranus around the sun and its color appearance. In the following two centuries, between 1797 and 1977, the rings of Uranus were rarely, if at all, mentioned in scientific papers.

Animation of an occultation of the star SAO 158687 by Uranus
Click on the picture to start
The Kuiper Airborne Observatory in flight

The undisputed discovery of the Uranus rings can finally be attributed to the astronomers James L. Elliot, Edward W. Dunham and Douglas J. Mink on March 10, 1977, who succeeded in sighting the rings with the help of the Kuiper Airborne Observatory . However, this event only came about through a chance observation . They originally planned to study the atmosphere of Uranus by observing the occultation ( occultation ) of the star SAO 158687 by the planet. When they analyzed their observations, they discovered that the star had been shown to have briefly disappeared five times before and five times after it passed through the planet. From this they deduced that a system of narrow rings must exist around the planet. The five occultation events they observed were identified in their papers with the Greek letters α, β, γ, δ and ε. This designation has been retained to this day as a designation for the rings. Later they found four more rings; one between rings β and γ and three within the α ring. They called the first η-ring, the latter received, according to the numbering of the occultation events, the designation ring 4, 5 and 6. After the rings of Saturn it was the second ring system that had been discovered within our solar system.

When the Voyager 2 space probe crossed the Uranus system in 1986, the first image documents were created showing the rings from above. Two more matte rings were discovered, bringing the total number of rings to eleven. Between 2003 and 2005, the Hubble space telescope was able to detect another, previously invisible pair of rings, which now leads to the number of rings known today. With the discovery of these outer rings, the previously known radius of the ring system also doubled. Hubble's images continued to reveal two small satellites, one of which, the moon Mab , shares its orbit with the newly discovered, outermost ring.

Basic properties

The inner Uranus rings: The light outer ring is the epsilon ring, next to it 8 other rings are visible. (Voyager 2, 1986, distance 2.52 million km)

As already mentioned, the ring system of Uranus according to the current state of knowledge consists of 13 clearly delimitable rings. They are classified according to their distance from the planet as 1986U2R / ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ They can be divided into three groups:

  • the nine narrow main rings (6, 5, 4, α, β, η, γ, δ, ε), 
  • the two dust rings (1986U2R / ζ, λ) 
  • as well as the two outer rings (μ, ν). 

The rings of Uranus consist mainly of macroscopic particles with some dust added. Dust could be detected in the 1986U2R / ζ-, η-, δ-, λ-, ν- and the μ-ring. In addition to these well-known rings, numerous optically thin dust bands and other matt rings can exist between them. Such matte rings and dust bands can, however, only exist temporarily or be composed of a number of separate arcs, which can sometimes be made out during occultation observations. For example, some of them were visible in 2007 during a special astronomical event when the ring surfaces crossed several times when viewed from the earth. Also in the photos of Voyager 2 , which were taken with a geometric forward scatter, a series of dust bands could be made out between the rings. All of the rings of Uranus continued to show some variations in brightness when viewed at an azimuthal angle.

The rings are each made of extremely dark substances. The geometric albedo of the ring particles never exceeds a value of 5 to 6 percent, while the spherical albedo is even lower, at around 2 percent. With a phase angle between the sun-object and observation position-object lines of almost zero, there is a clear increase in the albedo of the ring particles, the value of which increases significantly here. This means that, conversely, their albedo is far lower if they are already observed slightly outside the opposition area. The rings appear slightly red in the ultraviolet and visible parts of the spectrum and gray in the near infrared . They have no recognizable specific spectral characteristics . The chemical composition of the ring particles is still unknown today. However, it is certain that they cannot be made of pure ice like the rings of Saturn , since they are too dark for this and even appear darker than the inner moons of Uranus . This suggests that they may be a mixture of ice and dark ingredients. Although the nature of these constituents is unclear, they could be organic compounds that are significantly darkened by charged particles emitted by the magnetosphere of Uranus. It can be assumed that the ring particles consist of heavily worked chunks, which initially show similarities to the nature of the inner moons.

Overall, the ring system of Uranus is neither comparable with the dull, dusty Jupiter rings nor with the broad and complex ring structure of Saturn, in which some ring bands consist of very light material and chunks of ice. Nevertheless, there is certainly a similarity to some parts of the last-mentioned ring system. For example, the ε-ring and the F-ring of Saturn are both narrow, relatively dark and each protected by a pair of moons. The newly discovered outer rings of Uranus in turn have properties that match the outer G and E rings of Saturn. In the broad rings of Saturn there are narrow ringlets as in the narrow rings of Uranus. In addition, bands of dust could be observed between the main rings, as they also occur with the rings of Jupiter. In contrast to this is the ring system of Neptune , which is similar to that of Uranus, but is less complex, definitely darker and more dusty. In addition, the Neptune rings are positioned much further from their planet.

Narrow main rings

ε ring

A close-up view of the ε-ring (Voyager 2, 1986, distance 1.12 million km)

The ε-ring is the brightest and densest section of the Uranus ring system. He alone is responsible for two thirds of the total light that is reflected by the rings. While its orbit has the greatest eccentricity of all Uranus rings and thus corresponds least to a circle-like orbit, it has a negligible inclination . Due to its eccentricity, the brightness with which it is perceived varies in the course of its orbit. The radiation intensity of the ring is highest near the apse and lowest near the periapsis. The brightness ratio between maximum and minimum is between 2.5 m and 3.0 m . These fluctuations are related to the change in ring size, which ranges between 19.7 km at the periapsis and 96.4 km at the apoapsis. As a result, the shading between the particles is reduced at the points where the ring widens, which is why more of them become visible, which then leads to an increase in brightness in these sections. The deviations in the ring widths were measured on the basis of Voyager 2 images, on which the ε-ring ring was resolved by the cameras of the probe with only one other. The course observed in this way indicates that the ring is not optically thin. Indeed, occultation observations made from both the Earth and the spacecraft show that the optical depth varies between 0.5 and 2.5, with its greatest value near the periapsis. The equivalent depth of the ε-ring is about 47 km and is almost constant along its orbit.

A close up view of the (top to bottom) δ, γ, η, β and α rings of Uranus. In the η-ring, the optically thin but wide component is evident.

The exact value of the geometric thickness of the ε-ring is not known, although the ring can certainly be regarded as very thin. Some estimates suggest that its thickness is less than 150 m. Despite such an extremely small vertical diameter, it still consists of several different particle layers. The ε-ring is actually an overcrowded site of objects whose fill factor near the apoapsis is estimated by various sources to be between 0.008 and 0.06, which means that 0.8 to 6 percent of the ring area is filled with solids . The mean size of the ring particles is around 0.2 to 20 m, the mean distance between them being 4.5 times their radii. The ring is nearly free of interstellar dust , presumably due to the aerodynamic drag exerted by the outermost atmospheric corona of Uranus. Due to the razor-thin nature of the ε-ring, it appears almost invisible when you look at its "edge", which was the case in 2007 when a crossing of the ring planes was observed.

During a radio occultation experiment, the Voyager 2 spacecraft received a strange signal that came from the ε-ring. The signal looked like a large increase in forward scatter, which occurred at a wavelength of 3.6 cm near the apoapsis of the ring. Such strong scattering angles indicate the existence of a larger coherent structure. The fact that the ε-ring has such a fine structure could subsequently also be confirmed in various subsequent occultation observations. The ring appears to consist of a number of matt and optically dense ringlets, some of which are probably composed of incomplete arches.

The ε-ring is also known to be associated with both an inner and an outer shepherd's moon , Cordelia and Ophelia . The inner edge of the ring is in a 24:25 resonance with Cordelia, the outer edge in turn has an orbital resonance of 14:13 with Ophelia. The masses of the moons must be at least three times the mass of the ring so that it can be effectively kept within its confines. The mass of the ε-ring is estimated at around 10 16  kg.

δ ring

Comparison of the Uranus rings in the forward and backscattered light ( Voyager 2 , 1986)

The δ-ring has a circular shape and is slightly inclined. The sharp outer edge of the δ ring has a 23:22 resonance to Cordelia. Its optical depth and breadth show significant, inexplicable azimuthal deviations, i.e. inconsistent values, if one observes it along the horizontal plane. One possible explanation for this is that the ring has a wave-like structure in azimuth, which is caused by a small moon inside. In addition, the δ-ring consists of two components, a narrow, optically thin component and a wide inner edge strip that has only a small optical depth. The width of the narrow area is 4.1 ... 6.1 km and the equivalent depth is about 2.2 km, which is comparable to an optical depth of 0.3 ... 0.6. The broad ring component, on the other hand, has a width of 10… 12 km and its equivalent depth is almost 0.3 km, which indicates an equally small optical depth of 3 · 10 −2 . However, this information is based only on data from occultation observations, since the images from Voyager 2 were unable to resolve the δ-ring in sufficient detail. When observing the space probe in forward-scattered light, the δ-ring appears relatively bright, which indicates the presence of dust in its broad area. This wide area is also geometrically thicker than the narrow one. This fact is supported by observations of the ring plane intersection from 2007, when the δ-ring increased in brightness, which corresponds to the behavior of a simultaneously geometrically thick but optically thin ring.

γ ring

The γ-ring can be described as narrow, optically deep and as slightly eccentric . Its orbital tilt is close to zero. The width of the ring varies between 3.6 and 4.7 km, although its equivalent depth is uniformly 3.3 km. The optical depth of the γ-ring is between 0.7 and 0.9. During the ring plane crossing in 2007, the γ-ring disappeared, which leads to the realization that it must be just as thin as the ε-ring and also seems to be dust-free. The significant azimuthally deviating values, which are shown in the width and the optical depth of the γ-ring, also resemble the properties of the ε-ring. The mechanism that keeps such a narrow ring within its limits cannot yet be explained. Independently of this, it was found that the sharp inner edge of the γ-ring is in a 6: 5 orbit resonance with Ophelia.

η ring

The η-ring has an orbital eccentricity and inclination that is practically zero. Like the δ-ring, it can be divided into two areas, a narrow, optically dense component and a wide outer band of shallow optical depth. The width of the narrow component is 1.9 ... 2.7 km and the equivalent depth is about 0.42 km, which is consistent with an optical depth of about 0.16 ... 0.25. The wide area has an extension of about 40 km and its equivalent depth is close to 0.85 km, which indicates a shallow optical depth of 2 · 10 −2 . This could also be resolved in this way on images from Voyager 2 . In forward-scattered light the η-ring appears bright, from which the presence of a not inconsiderable amount of dust within this ring can be deduced, which is probably mainly to be found in the broad component. This part of the ring is geometrically much thicker than the narrow component. This conclusion is supported by the observations made during the intersection of the ring planes in 2007, when the brightness of the η ring increased and it briefly became the second brightest part of the ring system. This is in agreement with the behavior of a geometrically thick but at the same time optically thin ring. Like the majority of the other rings, the η-ring also shows azimuthal deviations when observing the optical depth and width. In some places it even happens that the narrow component disappears completely.

α and β rings

After the ε ring, the α and β rings are the next brightest of all Uranus rings. Like the ε-ring, they show a uniform change in their brightness and width. Its brightest and widest section is at a distance of about 30 ° from the apoapsis, while the darkest and narrowest part is located 30 ° from the periapsis. The α and β rings have a considerable eccentricity of their orbits and a not insignificant inclination. Their widths are between 4.8 and 10 km and 6.1 and 11.4 km, respectively. The equivalent depths are 3.29 km and 2.14 km, from which an optical depth of 0.3 to 0.7 and 0.2 to 0.35 can be derived. During the ring plane crossing in 2007, the rings disappeared, which suggests that, like the ε-ring, they are extremely thin and dust-free. In the same event, a dense and optically thin stripe was discovered just outside the β-ring, which had previously been seen on the images from Voyager 2. The masses of the α and β rings are each estimated to be about 5 · 10 15  kg, which corresponds to about half the mass of the ε ring.

Rings 6, 5 and 4

Rings 6, 5 and 4 are the innermost and darkest of the narrow Uranus rings. They are also the rings with the greatest inclination. The extent of its orbital eccentricity is only exceeded by that of the ε-ring. Their inclinations (0.06 °, 0.05 ° and 0.03 °) were large enough for Voyager 2 to be able to resolve their individual layers, which spread over 15 ... 46 km, over the equatorial plane of Uranus. Rings 6, 5 and 4 are also the narrowest rings on the planet and have thicknesses of 1.6 ... 2.2 km, 1.9 ... 4.9 km and 2.4 ... 4.4 km. Their equivalent depths are 0.41 km, 0.91 km and 0.71 km, which are values ​​for the optical depth of 0.18 ... 0.25, 0.18 ... 0.48 and 0.16 ... 0.3 corresponds. Since they are very narrow and free of dust, they were not visible at all when the ring plane crossed in 2007.

Dust rings

λ ring

High phase angle (172.5 °) image of the inner rings of Saturn. In the forward- scattered light, the dust bands can be made visible, which cannot be seen on other images. (Voyager 2, 1986, exposure time 96 s)

The λ ring was one of two rings discovered by the Voyager 2 probe in 1986. It is a narrow, matt ring that is positioned within the ε-ring between its inner edge and the shepherd's moon Cordelia . The moon ensures that a dust-free strip is created within the λ-ring. When viewed in backscattered light , the λ-ring appears extremely narrow, between 1 and 2 km and its equivalent depth is between 0.1 and 0.2 km at a wavelength of 2.2 μm, while the optical depth has a value of 0, 1 to 0.2 reached. It shows a strong dependence on the wavelength, which is atypical for the ring system of the planet. The equivalent depth in the ultraviolet part of the spectrum is greater than 0.36 km, which explains why the λ-ring was originally only detected by Voyager 2 during a stellar occultation in the UV region. Using a stellar occultation, however, it could not be detected until 1996, also at a wavelength of 2.2 μm.

However, the appearance of the λ ring changed drastically when it was observed in forward-scattered light in 1986. In this light the ring became the brightest part of the Uranus ring system and even outshone the ε ring. These observations in combination with the wavelength depend on the optical depth and indicate that the λ-ring contains a significant amount of small dust particles of a few micrometers in size. The optical depth of the dust is between 10 −4 and 10 −3 . Further observations by the Keck telescope confirmed this conclusion during a plane crossing of the rings in 2007, since the λ-ring again became one of the brightest parts of the ring system.

Detailed analyzes of the images from Voyager 2 also revealed azimuthal deviations in the brightness of the λ-ring. These variations appear to be periodic, similar to a standing wave . The origin of such a fine structure within the λ-ring is still not understood.

1986U2R / ζ ring

Image that led to the discovery of the 1986U2R / ζ ring

In 1986, Voyager 2 revealed a broad and dull layer of bodies inside Ring 6. This ring, identified as independent, was temporarily given the designation 1986U2R. It had an optical depth of 10 −3 or less and appeared extremely dull. In fact, he was only visible in one of the photos taken by Voyager 2 . The distance of the ring from the center of Uranus was between 37,000 km and 39,500 km on this image, or in other words, it was only 12,000 km above its clouds. No further observations were made until 2003/2004, when the Keck telescope rediscovered a broad and matt layer of bodies just inside ring 6. This ring, identified as independent, was given the name ζ-ring. The position of the ζ ring differs significantly from what the scientists observed in 1986. In contrast to the 1986U2R ring discovered at the time, it is now at a distance from the center of Uranus between 37,850 km and 41,350 km. In addition, an internal, gradually fading expansion can be seen, which reaches up to a distance line of 32,600 km.

The ζ ring was observed again during the ring plane crossing in 2007 when it became the brightest part of the ring system and shone brighter than all the other rings combined. The equivalent depth of this ring is given as just under 1 km (0.6 km for the inside extension), while the optical depth should again be less than 10 −3 . Some aspects that differ between the 1986U2R and the ζ-ring can possibly be explained by the different observation angles with which one examined their geometry. During the observations between 2003 and 2007, the rings were observed in rear scattering geometry, whereas in 1986 they were observed in lateral scattering geometry. In addition, it cannot be ruled out that changes have occurred over the past 20 years that were reflected in the expansion of the dust that dominates the ring.

More dust belt

In addition to the rings 1986U2R / ζ and λ, there are other extremely matt dust bands within the Uranus ring system. These are invisible during occultation because they have negligible optical depth, although they appear bright in forward-scattered light. The photographs of Voyager 2 , taken in forward-scattered light, thus revealed the presence of bright dust bands that exist between the λ and δ rings, between the η and β rings, and between the α ring and ring 4 . Many of these bands were rediscovered in backscattered light during observations with the Keck telescope in 2003 and 2004 and during the ring plane crossing in 2007, with their exact positions and relative brightnesses differing from previous observations by Voyager . The optical depth of the dust bands is 10 −5 or less. The particle size distribution of the dust particles is based on the power law with p  = 2.5 ± 0.5.

The outer ring system

μ and ν rings of Uranus (R / 2003 U1 and U2) ( Hubble Space Telescope , 2005)

Between 2003 and 2005, the Hubble Space Telescope discovered a pair of previously unknown rings, now known as the outer ring system, increasing the number of known rings on the planet to 13. These rings were subsequently given the names μ- and ν-rings. The μ-ring is the outer of the two. It is twice as far from the planet as the bright η ring, for example. The outer rings differ from the inner narrow rings in many ways. They are 17,000 km and 3,800 km wide and very dull. The highest values ​​of the optical depths are 8.5 · 10 −6 and 5.4 · 10 −6 . The resulting equivalent depths are set at 140 m and 12 m. The rings are also characterized by a triangular radiating brightness profile.

The greatest brightness of the μ-ring is almost exactly on the orbit of the small Uranus moon Mab , which is presumably the source of the ring particles. The ν-ring is positioned between the moons Portia and Rosalind , but has no own moons within its orbital area. A follow-up examination of the photos taken by Voyager 2 in the forward-scattered light clearly revealed the μ and ν rings here too. In this view, the rings appear much wider, which suggests that they are made up of many microscopic dust particles. The outer rings of Uranus are very similar to the G and E rings of Saturn. The G-Ring also lacks any observable original bodies, while the E-Ring is extremely wide and receives dust from its Beiond Enceladus .

The μ-ring probably consists almost entirely of dust without containing large particles. This hypothesis is supported by observations through the Keck telescope, through which the μ-ring in the near-infrared range at 2.2 μm, in contrast to the ν-ring, could not be made out. This absence means that the μ-ring appears blue in its color, from which it can be concluded that it is mainly dominated by very small (a few micrometers) dust particles. The dust itself probably consists of ice. In contrast, the ν ring appears in red.

Movement and origin

A color-highlighted scheme of the inner rings based on images from Voyager 2

The mechanism that acts on the narrow Uranus rings and fixes them within their limits is not understood. Without one that holds the ring particles together, the rings would have to spread out very quickly in all directions and spread out in space. The lifetime of the Uranus rings without such a mechanism would have to be less than 1 million years. The most frequently cited model for such a limitation, proposed by Goldreich and Tremaine , assumes that a pair of nearby moons, outer or inner shepherd moons, interact with their gravity to form a ring and represent a reinforcing or provides a weakening angular momentum in the ring particles. The shepherd moons hold the particles in place while they slowly but steadily remove themselves from the rings. In order to be effective, the mass of the adjacent probe must exceed the mass of the rings by at least a factor of 2 or better 3. This mechanism can be observed in the case of the ε-ring, on which Cordelia and Ophelia act as inner and outer shepherd moons, respectively. In addition, Cordelia is an outer shepherd's moon to the δ-ring, while Ophelia as an outer adjacent moon influences the γ-ring. Nevertheless, no moon larger than 10 km could be made out in the vicinity of other rings. The current distance between Cordelia and Ophelia and the ε-ring can be used as a guide to estimate the age of the rings. The calculations show that the ε-ring cannot be older than 600 million years.

Since the Uranus rings seem to be very young, it is necessary that they have to be constantly renewed by fragments that arise from collisions of larger chunks. The estimation of the lifetime shows that they can hardly be the collision remnants of a single moon the size of puck , whose lifetime is a few billion years. In contrast, the lifetime of a smaller satellite is much shorter. For that to happen, all of the present inner moons and rings would have to be the end product of the destruction of various satellites roughly the size of Puck and broken apart over the past four and a half billion years. Any such breakup would have triggered a collision cascade, whereby almost any larger body would quickly crumble into much smaller particles, including dust. Under certain circumstances, they lost most of their mass and only those particles remained in position that could be stabilized by mutual resonances and shepherd moons. The end product of this decay would eventually explain the formation of a narrow ring system as presented to us by Uranus. A few tiny moons still have to be embedded within the rings today. The maximum size of these tiny moons is probably no more than 10 km.

The origin of the dust bands, however, is less difficult to explain. The dust has a very short lifetime in the range of 100 to 1000 years, but is constantly renewed by collisions between larger ring particles, tiny moons and meteoroids from outside the uranium system. The belts of the original tiny moons and particles themselves are invisible because they have only a shallow optical depth, while the dust only reveals itself in forward-scattered light. In the case of the narrow main rings and the belts of tiny moons that create the dust bands, it is assumed that they are distributed in different particle sizes. The main rings consist mostly of centimeter-sized and to a lesser extent meter-sized bodies. Such propagation expands the area interspersed with material and surrounding the rings and leads to a high optical density which can then be observed in back-scattered light. In contrast, the dust bands consist of relatively few larger particles, which in turn results in their low optical depth.

exploration

When the Voyager 2 spacecraft passed Uranus in January 1986, the most thorough investigation of the ring system to date began. In the process, two new rings were discovered, λ and 1986U2R, which increased the total number of the previously known Uranus rings to eleven. The rings were studied using the analysis data from radiometric , ultraviolet and optical occultation. Voyager 2 photographed the rings in different positions relative to the sun, created images in backscattered, forwardscattered and sideways scattered light. The analysis of these images made it possible to derive the entire phase function, as well as the geometric and bond albedo of the ring particles. Two more rings, ε and η, could also be made out in the pictures, which expresses the complex and fine structure of the ring system even more clearly. Further analysis of the Voyager photos led to the discovery of 10 inner moons of Uranus , including the two shepherd moons of the ε ring, Cordelia and Ophelia.

List of properties

The table below lists the properties of the rings of Uranus :

Surname Radius
(km) 		Width
km 		Equivalent
depth (km) 		Optical
depth 		Eccentricity
(10 −3 ) 		Slope
(") 		Remarks
ζ c 32,000 ... 37,850 03500 00.6 ≈ 10 −4 ? 00? inside expansion of the ζ-ring
1986U2R 37,000 ... 39,500 02500 0? <10 −3 ? 00? matt dust ring
ζ 37,850 ... 41,350 03500 01 <10 −3 ? 00?
6th 41,837 00001.6 ... 02.2 00.41 0.18 ... 0.25 1.0 223
5 42,234 00001.9 ... 04.9 00.91 0.18 ... 0.48 1.9 194
4th 42,570 00002.4 ... 04.4 00.71 0.16 ... 0.30 1.1 115
α 44,718 00004.8 ... 10.0 03.39 00.3 ... 0.70 0.8 054
β 45,661 00006.1 ... 11.4 02.14 0.20 ... 0.35 0.4 018th
η 47,175 00001.9 ... 02.7 00.42 0.16 ... 0.25 0 004th
η c 47.176 00040 00.85 02 · 10 −2 0 004th outer broad component of the η ring
γ 47,627 00003.6 ... 04.7 03.3 00.7 ... 0.90 0.1 007th
δ c 48,300 00010 ... 12 00.3 03 · 10 −2 0 004th inner broad component of the δ-ring
δ 48,300 00004.1 ... 06.1 02.2 00.3 ... 0.60 0 004th
λ 50.023 00001 ... 02 00.2 00.1 ... 0.20 0? 000? matt dust ring
ε 51,149 00019.7 ... 96.4 47 00.5 ... 2.50 7.9 000 kept within limits by Cordelia and Ophelia
ν 66.100 ... 69.900 03800 00.012 05.4 · 10 −6 ? 00? between Portia and Rosalind , greatest brightness at 67300 km
μ 86,000 ... 103,000 17000 00.14 08.5 · 10 −6 ? 00? at Mab , greatest brightness at 97,700 km

Remarks

  1. ↑ Forward scattered light is light that is deflected from the subject with a small scattering angle (close to 180 °), i. H. the light source is on the side just opposite the recording device.
  2. ↑ The optical depth τ of a ring is the ratio of the geometric cross section of the ring particles to the square of the area of ​​the ring. The value of τ can be between 0 and infinity. A value between 0 and 1 is referred to as an optically thin layer, while values ​​from 1 indicate an optically thick layer.
  3. ↑ The equivalent depth (ED) of a ring is defined as the integral of the optical depth over the ring cross-section. In other words: ED = ∫τdr, where r is the radius.
  4. ↑ Backward-scattered light is light that is deflected by the subjects with a very large scattering angle (scattering angle between 0 ° and 90 °), i. H. the light source is on the same side as the recording device.
  5. ↑ The radii of the rings 6,5,4, α, β, η, γ, δ, λ and the ε-ring originate from u. a. Esposito, 2002. The widths of the rings 6,5,4, α, β, η, γ, δ and the ε-ring are u. a. from Karkoshka, 2001. The radius and the width of the ζ and 1986U2R rings were u. a. taken from de Pater, 2006. The width of the λ-ring comes from Holberg, 1987. The radii and widths of the μ- and ν-rings were taken from Showalter, 2006.
  6. ↑ The equivalent depth of the 1986U2R ring is a product of its width and the optical depth. The equivalent depths of the rings 6,5,4, α, β, η, γ, δ and the ε-ring were taken from Karkoshka, 2001. The equivalent depths of the λ- and ζ-, the μ- and ν-rings were taken determined by the μEW values ​​from Pater 2006 and de Pater, 2006b. The μEW values ​​for these rings were multiplied by a factor of 20, which results from an assumed albedo of the ring particles of 5 percent.
  7. ↑ The optical depth of all rings with the exception of rings 1986U2R, μ and ν were calculated as the ratio of the equivalent depth to the width. The optical depth of the 1986U2R ring was taken from de Smith, 1986, those of the μ and ν rings are given peak values ​​from Showalter, 2006.
  8. ↑ The eccentricity and inclination of the rings were taken and a. from Stone, 1986 and French, 1989.

See also

Web links

Commons : Rings of Uranus  - Collection of images, videos and audio files

Individual evidence

  1. ^ Uranus rings 'were seen in 1700s'. In: BBC News. April 19, 2007, accessed April 19, 2007 .
  2. ↑ a b c d e f g h i j Imke dePater, Seran G. Gibbard, HB Hammel: Evolution of the dusty rings of Uranus . In: Icarus . tape 180 , 2006, p. 186–200 , doi : 10.1016 / j.icarus.2005.08.011 , bibcode : 2006Icar..180..186D .
  3. ↑ Did William Herschel Discover The Rings Of Uranus In The 18th Century? In: Physorg.com. 2007, accessed June 20, 2007 .
  4. ^ A b J. L. Elliot: The Occultation of SAO - 15 86687 by the Uranian Satellite Belt.
  5. ↑ JL Elliot, E. Dunham, D. Mink: The rings of Uranus . In: Nature . tape 267 , 1977, pp. 328-330 .
  6. ↑ PD Nicholson, SE Persson, K. Matthews et al: The Rings of Uranus: Results from April 10, 1978 Occultations . In: The Astronomical Journal . tape 83 , 1978, pp. 1240-1248 , doi : 10.1086 / 112318 , bibcode : 1978AJ ..... 83.1240N .
  7. ↑ RL Millis, LH Wasserman: The Occultation of BD -15 3969 by the Rings of Uranus . In: The Astronomical Journal . tape 83 , 1978, pp. 993-998 , bibcode : 1978AJ ..... 83..993M .
  8. ↑ a b c d e f g h i j k l m n L. W. Esposito: Planetary rings. In: Reports On Progress In Physics. 65, 2002, pp. 1741-1783. doi: 10.1088 / 0034-4885 / 65/12/201
  9. ↑ a b c d e f g h i j k l m n o p q r s t u v w x B. A. Smith, LA Soderblom, A. Beebe et al .: Voyager 2 in the Uranian System: Imaging Science Results. In: Science. 233, 1986, pp. 97-102. bibcode : 1986Sci ... 233 ... 43S . doi: 10.1126 / science.233.4759.43 . PMID 17812889 .
  10. ↑ a b c d e f g h i j k l Mark R. Showalter, Jack J. Lissauer: The Second Ring-Moon System of Uranus: Discovery and Dynamics. In: Science. 311, 2006, pp. 973-977. bibcode : 2006Sci ... 311..973S . doi: 10.1126 / science.1122882 . PMID 16373533
  11. ↑ a b c NASA's Hubble Discovers New Rings and Moons Around Uranus. In: Hubble site. 2005, accessed June 9, 2007 .
  12. ↑ a b c d e f g h i J. A. Burns, DP Hamilton, MR Showalter: Dusty Rings and Circumplanetary Dust: Observations and Simple Physics. In: E. Grun, BAS Gustafson, ST Dermott, H. Fechtig: Interplanetary Dust. Springer, Berlin 2001, S, pp. 641-725.
  13. ↑ a b Mark R. Showalter, JJ Lissauer, RG French et al: The Outer Dust Rings of Uranus in the Hubble Space Telescope . Ed .: American Astronomical Society. 2008, bibcode : 2008DDA .... 39.1602S .
  14. ↑ a b c d e f g h i j k l M. E. Ockert, JN Cuzzin, CC Porco, TV Johnson: Uranian ring photometry: Results from Voyager 2. In: J. of Geophys. Res. 92, 1987, pp. 14, 969-14, 978. bibcode : 1987JGR .... 9214969O .
  15. ↑ a b c d e f g h i j k l Arthur L. Lane, Charles W. Hord, Robert A. West et al .: Photometry from Voyager 2: Initial results from the uranian atmosphere, satellites and rings . In: Science . tape 233 , 1986, pp. 65-69 , bibcode : 1986 Sci ... 233 ... 65L .
  16. ↑ a b c d e f g h i j k l m Imke de Pater, HB Hammel, Mark R. Showalter, Marcos A. Van Dam: The Dark Side of the Rings of Uranus . In: Science . tape 317 , 2007, p. 1888–1890 , doi : 10.1126 / science.1148103 , PMID 17717152 , bibcode : 2007Sci ... 317.1888D .
  17. ↑ a b Erich Karkoshka: Rings and Satellites of Uranus: Colorful and Not So Dark . In: Icarus . tape 125 , 1997, pp. 348–363 , doi : 10.1006 / icar.1996.5631 , bibcode : 1997Icar..125..348K .
  18. ↑ a b c d Kevin H. Baines, Padmavati A. Yanamandra-Fisher, Larry A. Lebofsky and others: Near-Infrared Absolute Photometric Imaging of the Uranian System . In: Icarus . tape 132 , 1998, pp. 266–284 , doi : 10.1006 / icar.1998.5894 , bibcode : 1998Icar..132..266B .
  19. ↑ a b c d Imke dePater, Heidi B. Hammel, Seran G. Gibbard, Mark R. Showalter: New Dust Belts of Uranus: One Ring, Two Ring, Red Ring, Blue Ring . In: Science . tape 312 , 2006, pp. 92–94 , doi : 10.1126 / science.1125110 , PMID 16601188 , bibcode : 2006Sci ... 312 ... 92D .
  20. ↑ a b c d e f g h E. D. Miner, EC Stone: Voyager 2 encounter with the uranian system . In: Science . tape 233 , 1986, pp. 39-43 , bibcode : 1986 Sci ... 233 ... 39S .
  21. ↑ a b c d e f g h i j k l m n Erich Karkoshka: Photometric Modeling of the Epsilon Ring of Uranus and Its Spacing of Particles . In: Icarus . tape 151 , 2001, p. 78–83 , doi : 10.1006 / icar.2001.6598 , bibcode : 2001Icar..151 ... 78K .
  22. ↑ a b c J. L. Tyler, DN Sweetnam, JD Anderson et al.: Voyager 2 Radio Science Observations of the Uranian System: Atmosphere, Rings, and Satellites . In: Science . tape 233 , 1986, pp. 79-84 , doi : 10.1126 / science.233.4759.79 , PMID 17812893 , bibcode : 1986Sci ... 233 ... 79T .
  23. ↑ a b c d e f g h i j Joshua E. Colwell, LW Esposito: Creation of The Uranus Rings and Dust bands . In: Nature . tape 339 , 1989, pp. 605-607 , doi : 10.1038 / 339605a0 , bibcode : 1989Natur.339..605E .
  24. ^ A b c d Carolyn C. Porco, Peter Goldreich: Shepherding of the Uranian rings I: Kinematics . In: The Astronomical Journal . tape 93 , 1987, pp. 724-778 , doi : 10.1086 / 114354 , bibcode : 1987AJ ..... 93..724P .
  25. ^ LJ Horn AL Lane, PA Yanamandra-Fisher, LW Esposito: Physical properties of Uranian delta ring from a possible density wave . In: Icarus . tape 76 , 1988, pp. 485-492 , doi : 10.1016 / 0019-1035 (88) 90016-4 , bibcode : 1988Icar ... 76..485H .
  26. ↑ a b c d e J. B. Holberg, PD Nicholson, RG French, JL Elliot: Stellar Occultation probes of the Uranian Rings at 0.1 and 2.2 μm: A comparison of Voyager UVS and Earth based results . In: The Astronomical Journal . tape 94 , 1987, pp. 178-188 , doi : 10.1086 / 114462 , bibcode : 1987AJ ..... 94..178H .
  27. ^ Richard D. French, JL Elliot, Linda M. French et al .: Uranian Ring Orbits from Earth-based and Voyager Occultation Observations. In: Icarus. 73, 1988, pp. 349-478. bibcode : 1988Icar ... 73..349F . doi: 10.1016 / 0019-1035 (88) 90104-2 .
  28. ^ SG Gibbard, I. De Pater, HB Hammel: Near-infrared adaptive optics imaging of the satellites and individual rings of Uranus . In: Icarus . tape 174 , 2005, pp. 253–262 , doi : 10.1016 / j.icarus.2004.09.008 , bibcode : 2005Icar..174..253G .
  29. ↑ Eugene I. Chiang, Christopher J. Culter: Three-Dimensional Dynamics of Narrow Planetary Rings . In: The Astrophysical Journal . tape 599 , 2003, p. 675-685 , doi : 10.1086 / 379151 , bibcode : 2003ApJ ... 599..675C .
  30. ↑ Stephen Battersby: Blue ring of Uranus linked to sparkling ice. In: NewScientistSpace. 2006, accessed September 7, 2019 .
  31. ^ Robert Sanders: Blue ring discovered around Uranus. In: UC Berkeley News. April 6, 2006, accessed October 3, 2006 .
  32. ^ Peter Goldreich, Scott Tremaine : Towards a theory for the uranian rings . In: Nature . tape 277 , 1979, pp. 97-99 , doi : 10.1038 / 277097a0 .