Rings of Uranus
The planet Uranus is surrounded by a system of planetary rings, which although does not come up in its variation and complexity to the significantly larger area orbits of Saturn's rings, but can be arranged but the simpler structures of Jupiter and Neptune's rings. The first rings of Uranus were discovered on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink. Although 200 years previously reported, the astronomer William Herschel on the observation of rings, is doubted by modern astronomers that it might have been possible with the resources of that time and considering its dark and pale appearance, actually perceive the ring system. Two more rings were discovered in 1986 in images, which started the Voyager 2 spacecraft from the planet and an additional pair of rings were found between 2003 and 2005 in photos that created the Hubble Space Telescope from Uranus.
Since that is thus known by the ring system of Uranus, that it consists of a total of 13 independent rings. Classified by their distance from the planet, they are labeled 1986U2R / ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ provided. Your radii amounts to 38,000 km at 1986U2R/ζ-Ring and reach a length of up to 98,000 km at the μ - ring. Between the main dust rings additional mat strips and incomplete sheets were observed. The rings are extremely dark, so that the spherical albedo of the ring particles does not exceed 2%. They sat down together probably of frozen water, which has been associated 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 have a diameter between 0.2 and up to 20 m. Some of the rings are optically very small: So there are the extensive and matte rings 1986U2R / ζ, μ and ν of thin dust particles, while the narrow and too dull λ - ring composed of larger objects. The relative absence of dust within the ring system is due to the air resistance, which brings the extended exosphere of Uranus by his corona with it.
It is assumed that the rings of Uranus have an age of not more than 600 million years, and thus are relatively young. The ring system probably consists of the remains of a large number of moons, which originally had once orbited the planet before they collided with each other long ago. After the clashes, the moons were broken up into countless parts, which are then visible today than the narrow and optically dense rings survived and now surrounded the planet in strictly defined orbits.
The process, as the narrow rings are held in shape, is still not fully understood. Initially, it was assumed that each ring is narrow with a pair of related so-called shepherd moons in conjunction, which support their shape. However, Voyager 2 was only a shepherd pair ( Cordelia and Ophelia ) discover in their flyby in 1986, which exert an influence on the brightest ring ( ε ).
- 4.1 λ -rings
- 4.2 1986U2R/ζ-Ring
- 4.3 Other dust belt
Discovery
The first mention of a surrounding Uranus ring system dates from the 18th century and can be found in the records of William Herschel, in which he wrote down the findings from his observations of the planet. These contained the following passage: " February 22, 1789: A ring what Suspected ". ( German: " February 22, 1789. Suspected ring " ) Herschel drew a small diagram of the ring and was quoted to say that he was "a bit of tending to red out ". The Keck telescope in Hawaii was able to confirm with respect to the ν - ring it at least. Herschel's notes were published in 1797 in the Royal Society journal. Over the years, then serious doubts were raised as to whether Herschel could have ever seen anything like it, while hundreds of other astronomers had nothing like that can make. Nevertheless, there are legitimate objection, stating that Herschel a precise description of the dimensions of the ν ring which changes in the motion of Uranus could give to the sun as well as its color appearance, in fact, in relation to Uranus. In the next two centuries 1797-1977 were the rings of Uranus, if any, in research papers rarely mentions.
The undisputed discovery of Uranus rings may eventually be the astronomer James L. Elliot, Edward W. Dunham, and Douglas J. Mink awarded on 10 March 1977 which using the Kuiper Airborne Observatory succeeded the sighting of the Rings. This event, however, came into existence only by a random observation. Originally they planned to study the atmosphere of Uranus, by they intended to observe the occultation (coverage ) of the star SAO 158687 by the planet. When they analyzed their observations, they discovered that the star had been shown five times before and disappeared a short time after the passage of the planet. They concluded that the planet had to exist a system of narrow rings. The five of them observed occultation they identified in their papers with the Greek letters α, β, γ, δ and ε This designation is retained to this day as marking the end for the rings at. Later, they tracked down four more rings; a β between the rings and γ and three within the α - ring. The first they called η - ring, the latter received, according to the numbering of the occultation, the name ring 4, 5 and 6 after Saturn's rings, it was thus the second ring system, which had been discovered within our solar system.
When the Voyager 2 spacecraft crossed the Uranus system in 1986, the first image documents that showed the rings in the top view emerged. There are two more matte rings were discovered, bringing the total number of rings increased to eleven. In 2003-2005, then another, as yet visible ring pair could be tracked by the Hubble Space Telescope, which now leads to the number of today's 13 known rings. Through the discovery of these outer rings of the until then known radius of the ring system increased in passing on the double. The images from Hubble took still two small satellites to the fore, with one of them, the moon Mab to its orbit with the newly discovered, notify the outermost ring.
Basic properties
As already mentioned, there is a ring system of Uranus according to current knowledge of thirteen clearly identifiable rings. Classified according to their distance from the planet, they are called 1986U2R / ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν, μ. They can thereby be divided into three groups: nine narrow main rings ( 6, 5, 4, α, β, η, γ, δ, ε ), two dust rings ( 1986U2R / ζ, λ ) and two outer rings ( μ, ν ). The rings of Uranus consist mainly of macroscopic particles that are mixed with some dust. How could while 1986U2R/ζ-, η, δ, λ, ν and dust is evidence in μ - ring. In addition to these well-known rings may exist between them quite numerous optically thin dust bands and more matte rings. However, such matte rings and dust bands can only exist temporarily, or be composed of a number of separate sheets, which can sometimes make up for Okkultationsbeobachtungen. Some of them were visible, for example in 2007 during a special astronomical event, in which the annular surfaces, viewed from Earth, repeatedly crossed. Also in the photos from Voyager 2, which were recorded at a geometric forward scattering [A 1], a number of dust bands could be identified between the rings. All rings of Uranus were still some brightness variations when observing them in an azimuthal angle.
The rings are made of extremely dark substances, respectively. The geometric albedo ring particles never exceeds a value of this 5-6%, while the albedo below, is even at about 2%. At a phase angle between the lines sun object and observation position object from near zero there is a clear increase in the albedo of the ring particles, whose value rises significantly here. This means that conversely its albedo is much lower when they have already been observed slightly outside the opposition area. The rings appear slightly red in the ultraviolet and visible part of the spectrum and gray in the near infrared. In this case, they have no apparent specific spectral characteristics. The chemical composition of the particles is not known to ring until now. It is certain, however, that they can not consist of pure ice as the rings of Saturn, because they are too dark for this purpose and appear even darker than the inner moons of Uranus. This suggests that they may consist of a mixture of ice and dark particles. Although the nature of these components is unclear, but it could be organic compounds which are considerably obscured by charged particles, which radiates the magnetosphere of Uranus. It is assumed that the ring particles are made of highly processed chunks, which initially have similarities to the nature of the inner moons.
Overall, the ring system of Uranus with either the matte dusty rings of Jupiter or with the broad and complex ring structure of Saturn is comparable, in some ring belts are made of very light material and chunks of ice. Nevertheless, there are certainly similarities to some parts of the latter ring system. For example, the ε - ring as well as the F- ring of Saturn both are narrow, relatively dark and are each guarded by a pair of moon. The newly discovered outer rings of Uranus in turn have matching properties to the outer G and E rings of Saturn. So, in the broad rings of Saturn as well as narrow stripes in the narrow rings of Uranus. In addition, dust bands between the main rings could be observed, as they also occur in the rings of Jupiter. In contrast to this is the ring system of Neptune, which although similar to that of Uranus, but is less complex, quite dark and dusty. In addition, the Neptune rings are positioned much further from their planet.
Narrow main rings
ε - ring
The ε ring is the brightest and densest portion of the Uranus ring system. It alone is responsible for two thirds of the light that is total reflected by the rings. During its orbit has the largest eccentricity of all Uranus rings and thus at least equivalent to a circle-like path, it has a negligible orbital inclination. Due to its eccentricity is varied during the course of its orbital path, the brightness with which it is perceived. The radiation intensity of the ring is close to the apse at the highest and near the periapsis at its lowest. The brightness ratio between maximum and minimum is between 2.5-3.0 mag. These fluctuations are associated with the change of ring width, ranging between 19.7 km at periapsis and 96.4 km at the apoapsis. As a result, reduces the shading between the particles at the points where the ring is expanding, why are all the more visible of them, which then leads to an increase in brightness in these sections. The deviations of the ring widths were measured on the basis of the Voyager 2 pictures that show the ε - ring ring was broken up by the cameras of the probe with one another. The observed in this kind of course suggests that the ring is not optically thin. Indeed, Okkultationsbeobachtungen carried out by both the ground as well as from the spacecraft, the optical depth of [A 2], varies between 0.5 and 2.5, wherein, near the periapsis having the greatest value. The equivalent depth [A 3] of the ε - ring is about 47 km and is almost constant along its orbit.
The value of the geometric thickness of the ε - ring is not exactly known, although the ring can certainly be regarded as very thin. Some estimates suggest that its thickness is less than 150 m. Despite a still such an extremely low vertical diameter, it consists of several different layers of particles. The ε ring is actually a full on site to objects whose fill factor is estimated near the apoapsis of different sources to a value between 0.008 to 0.06, which means this is true, 0.8-6 % of the annular surface of solids. The average size of the ring is approximately from 0.2 to 20.0 m particles, wherein the average distance to each other is 4.5 times its radius. The ring is almost free of interstellar dust, which is probably due to the aerodynamic drag exerted by the outermost atmospheric corona of Uranus. Due to the razor- thin nature of the ε - ring it acts almost invisible when you look at his " edge ", which was the case in 2007 during the observation of a crossing of the ring planes.
During a radio occultation experiment, the spacecraft Voyager 2 received a strange signal came from the ε - ring. The signal appeared after a sharp rise of the feed-forward control that occurred at a wavelength of 3.6 cm near the apoapsis the ring. Such strong scattering angle point to the existence of a larger coherent structure. The fact that the ε - ring has such a fine structure was subsequently confirmed also at different following Okkultationsbeobachtungen. The ring appears to consist of a number of matte and visually dense stripes, of which probably put together some of incomplete arcs.
From the ε - ring is also known that he is both an internal as well as with an outer shepherd moon in conjunction, Cordelia and Ophelia. The inner border of the ring is to Cordelia in a 24:25 resonance, the outer edge, it has some orbital resonance from 14:13 to Ophelia. The masses of the moons must be at least three times the mass of the ring, so that it can be efficiently maintained in its limit. The mass of the ε - ring is estimated to be about 1016 kg.
δ - ring
The δ - ring has a circular shape and is slightly inclined. The sharp outer edge of the δ - ring case has a 23:22 resonance with Cordelia. In its optical depth and breadth to demonstrate significant, unexplained azimuthal variations, ie non-uniform values observed him along the horizontal plane. A possible explanation for this is that the ring in azimuth has a shaft -like structure which is caused by a small moon in its interior. In addition, there is the δ - ring consists of two components, a narrow and a broad optical component thin inner edge strip, which has only a small optical depth. The width of the narrow portion is 4.1 to 6.1 km and the depth is equivalent to approximately 2.2 km, which is comparable to an optical depth of 0.3-0.6. In contrast, the wide ring component has a width of about 10 -12 miles and its equivalent depth is nearly 0.3 km, suggesting an equally low optical depth of 3 × 10-2. However, these data are based only on data from Okkultationsbeobachtungen because the images from Voyager 2 ' the δ - ring were not able to resolve detailed enough. In observations by the spacecraft in the forward-scattered light of the δ - ring acts relatively light, indicating the presence of dust in its wide range. This wide range is also geometrically thicker than the narrow. This fact is supported by observations of the ring plane-crossing event from the year 2007, when the δ - ring increased in brightness, which corresponds to the behavior of the same geometric optical thick but thin ring.
γ - ring
The γ - ring can be described as narrow, optically deep and as slightly eccentric. Its orbital inclination is close to zero. The width of the ring varies between 3.6 to 4.7 km, although its equivalent depth is uniform at 3.3 km. The optical depth of the γ - ring is between 0.7 and 0.9. During a ring plane crossing in 2007 of the γ - ring, which leads to the realization that he be as thin as the ε - ring needs and to be as empty dust seems disappeared. The significant different azimuth values , as demonstrated by the width and the optical depth of the γ - ring, as well as the same properties of the ε - ring. The mechanism that holds such a narrow ring within its borders, has not yet explainable. Regardless was found that the sharp inner edge of the γ - ring is in a 6:5 orbital resonance to Ophelia.
η - ring
The η ring has an orbital eccentricity and inclination which is practically zero. As the δ - ring, it can be divided into two sections, a narrow, optically dense component and a wide band of outer low optical depth. The width of the narrow component is 1.9 to 2.7 km and the equivalent depth has a size of about 0.42 km, which is consistent with an optical depth of about 0.16 to 0.25. The wide range of vastness has an extent of about 40 km and its equivalent depth is close at 0.85 km, indicating a low optical depth of 2 × 10-2. This could be resolved in this way also to images by Voyager 2. In forward-scattered light from the η - ring appears bright, resulting in the presence of a significant amount of dust in this ring can be derived, which is probably mainly be found in the broad component. This part of the ring is geometrically considered much thicker than the narrow component. This conclusion is supported by observations during the ring plane crossing from the year 2007, when an increase in the brightness at the η - ring and showed that briefly became the second brightest part of the ring system. This is in agreement with the behavior of a geometric thick but at the same optically thin ring. Like the majority of other rings shows the η - ring as azimuthal variations in the observation of the optical depth and width. In some places, it even happens that the narrow component disappears completely.
α - and β -rings
Ε after the α - ring and β -ring, the next brightest of all Uranus rings. Like the ε ring, they have a smooth transition to their brightness and width. Your brightest and widest section is at a distance of about 30 ° to the apoapsis, while the darkest and narrowest part is 30 ° away from periapsis. The α - and β - rings have a significant eccentricity of their orbits and a not negligible inclination. Their widths are between 4:8-10 km or 6.1 to 11.4 km. The equivalent depths are 3.29 km and 2.14 km, resulting in an optical depth of 0.3-0.7 and 0.2-0.35 can be derived. During a ring plane crossing in 2007, the rings, which suggests that they are like the ε ring extremely thin and empty dust disappeared. At the same event a dense and optically thin strips were discovered just outside the β - ring, which had been previously seen on the images of Voyager 2. The masses of the α - and β -rings are respectively estimated to be about 5 x 1015 kg, which corresponds to about half the weight of the ε - ring.
Rings 6, 5 and 4
The rings 6, 5 and 4, the innermost and the narrow darkest Uranus rings. They are also the rings with the greatest slope. Also the expression of its orbital eccentricity is surpassed only by that of the ε - ring. Your inclinations (0.06 °, 0.05 ° and 0.03 ° ) were for Voyager 2 large enough to resolve their individual layers, spreading to 24-46 km above the equatorial plane of Uranus can. The rings 6, 5 and 4 are also the smallest rings of the planets and have thicknesses of 1.6 to 2.2 km, 1.9 to 4.9 km, and 2.4 to 4.4 km. Your equivalent depths are 0.41 km, 0.91 km and 0.71 km, what values for the optical depth of 0.18-0.25, 0.18 to 0.48 and from 0.16 to 0.3 corresponds. Since they are very narrow and dusty empty, they were during the ring plane crossing not visible at all in 2007.
Dust rings
λ -rings
The λ - ring was one of the two rings, which discovered the Voyager 2 spacecraft in 1986. It is a narrow, dull ring that is positioned within the ε - ring between the inner edge and the shepherd moon Cordelia. Here, the moon ensures that a dust -free strip is created within the λ - ring. With a view to backscatter light [A 4], the λ - ring appears extremely narrow, between 1-2 km and its equivalent depth is between 0.1-0.2 km at a wavelength of 2.2 microns, while the optical depth of a value of 0.1-0.2 achieved. It shows a strong dependence on the wavelength, which is atypical of the ring system of the planet. The equivalent depth in the ultraviolet part of the spectrum is higher than 0.36 km, which explains why the λ - ring was originally detected by Voyager 2 only at a stellar occultation in the UV range. However, by means of a stellar occultation he could be tracked only in 1996, this also at a wavelength of 2.2 microns.
However, the appearance of the λ ring changed drastically when he was observed in 1986 in forward-scattered light. In this light the ring to the brightest part of the Uranus ring system was and even outshone the ε - ring. These observations together with the wavelength depend on the optical depth and suggest that the λ ring contains a significant amount of small dust particles of a few microns in size. The optical depth of the dust varies between 10-4 and 10-3. Further observations by the Keck telescope confirmed during a plane-crossing of the Rings in 2007, this conclusion because the λ - ring it again became one of the brightest parts of the ring system.
Detailed analyzes of the images from Voyager 2 also have revealed azimuthal variations in the brightness of the λ - ring. These variations appear to be periodic, similar to a standing wave. The origin of such a delicate structure within the λ - ring, however, can not be explained and therefore remains to this day a mystery.
1986U2R/ζ-Ring
In 1986, Voyager 2 took a broad and dull location of bodies inside of ring 6 to the fore. This ring was identified as an independently temporarily the name 1986U2R. He had an optical depth of 10-3 or less and appeared extremely dull. In fact, he was visible only on a single shot from the Voyager 2 photos. The distance of the ring from Uranus midpoint was expressed on this recording between 37,000 km and 39,500 km, or in other words, he was only 12,000 km on the clouds. Up to 2003-2004, no further observations have been made, then the Keck telescope discovered just inside of ring 6 again a wide and matte layer of bodies. This ring was identified as an independent given the name ζ ring. The position of the ζ ring differs significantly from what the scientists observed in 1986. He now owns as opposed to the then discovered ring 1986U2R a distance from Uranus midpoint between 37,850 km and 41,350 km. In addition, an inside, gradually fading expansion is evident that extends up to a distance of 32,600 km line.
The ζ - ring was again under surveillance during the ring plane crossing in 2007, when he became the brightest part of the ring system and shone brighter than all the other rings together. The equivalent depth of this ring is given with almost 1 km ( 0.6 km for the inside dimension), while the optical depth should in turn 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 at which you studied their geometry. Thus, the rings were observed in the observations between 2003 and 2007 in the backward scattering geometry, 1986, however, in the lateral scattering geometry. Nor can it be ruled out that changes have occurred during the past 20 years, which were reflected in the expansion of the ring -controlling dust.
More dust belt
In addition to the rings 1986U2R / ζ and λ, there are other extremely dull dust bands within the Uranus ring system. During an occultation observation ( occultation ) and are invisible because they have a negligible optical depth, although they appear bright in forward-scattered light. The photographs from Voyager 2, which were taken in forward-scattered light, so disclosed the presence of bright dust lanes 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 tapes were rediscovered during observations with the Keck telescope in 2003-2004 and during the ring plane crossing in 2007 in rückwärtsgestreutem light, with their exact positions and relative brightnesses of the earlier observations distinguished by Voyager. The optical depth of the dust bands is 10-5 or less. The grain size distribution of the dust particles fit after power law with p = 2.5 ± 0.5.
The outer ring system
Between 2003 and 2005, the Hubble Space Telescope detected a pair of previously unknown rings, which are now referred to as the outer ring system and the number of known rings of the planet increased to 13. These rings were subsequently provided with the names of μ and ν rings. The μ ring is the outer of the two. He is twice as far away from the planet, such as the bright η - ring. The outer rings differ in many aspects from the inner narrow rings. They are broad and very weak in extent 17,000 km and 3800 km. The highest values of the optical depths of 8.5 × 10-6 and 5.4 × 10-6 The resulting equivalent depths are measured at 0.14 km and 0.012 km. The rings are also characterized by a triangular radiating brightness profile.
The maximum brightness of the μ ring lies almost exactly on the orbit of the small moon Mab Uranus, which is presumably the source of the ring particles. The ν - ring is positioned between the moons Portia and Rosalind, but has not itself own moons in its orbit space. An examination of the photos, which had taken in forward-scattered light Voyager 2, also revealed here clearly the μ and ν rings. In this view, the rings appear much wider, which suggests that they are composed of many microscopic dust particles. The outer rings of Uranus are very similar to the G and E rings of Saturn. So the G- ring is missing also of observable parent bodies during the E ring is extremely broad and receives dust from his Beimond Enceladus.
The μ - ring is probably almost completely to include from dust without larger particles. This hypothesis is supported by observations by the Keck, through the μ - ring in the near infrared region of 2.2 microns, in contrast to the ν - ring, was not clear. This lack means that the μ ring is its color appears blue, which suggests that it is dominated mainly by very small ( a few micrometers ) dust particles. The dust is itself probably consists of ice. In contrast, the ν ring is shown in red.
Movement and origin
A still outstanding problem relates to physical influence, which acts on the narrow Uranus rings and fixed in their boundaries. Without a mechanism that holds the ring particles, the rings should spread very rapidly in all directions and spread in space. The lifetime of the Uranus rings without such a mechanism may extend to no more than 1 million years. The most widely cited model for such a limitation that has been proposed by Goldreich and Tremaine, assumes that a pair of obvious moons, outer or inner shepherding satellites, with their gravity is in an interaction into a ring and reinforcing for one or weakening of the angular momentum in the ring particles provides. The shepherd moons thus keep the particles in place while she slowly but surely remove yourself from the rings. In order to be effective, the mass of the Beimonde must exceed the mass of the Rings by at least a factor of two or even three. The operation of this mechanism in the case of ε - ring observable effect on the Cordelia and Ophelia as inner and outer shepherd moons. In addition, Cordelia is an outer shepherd moon for the δ - ring, while Ophelia takes as an external Beimond influence on the γ - ring. Nevertheless, no moon could be detected in the vicinity of other rings, which would be greater than 10 km. The instantaneous distance of Cordelia and Ophelia to the ε - ring can be taken as a guide to estimate the age of the rings thereof. The calculations show that the ε - ring it can not be older than 600 million years.
Since the Uranus rings seem to be very young, it is necessary that they should be replaced by fragments constantly, which result from collisions of larger chunks. The estimation of the lifetime shows that they can hardly be the collision remains of a single moon with a size of Puck, whose life lasts a few billion years. The lifetime of a satellite, however, is smaller or much smaller. This would require all current inner moons and rings to be the end product of the destruction of various satellites that had possessed the size of Puck in and about during the last four and a half billion years were broken apart. Any such break apart would thus triggering a collision cascade, which almost every major body would quickly crumbles into much smaller particles including dust. Under certain circumstances, they lost the majority of their mass and it only remained those particles in position, which could be stabilized by mutual resonances and shepherding satellites. The final product of this decay would eventually explain the formation of a narrow ring system, as it presents us with the Uranus. A few small moons must be embedded within the still rings today. The maximum size of these tiny moons here is probably not more than 10 km.
The origin of the dust bands is, however, less difficult to explain. The dust has a very short lifetime of 100-1000 years, but renews itself continuously by collisions between larger ring particles, small moons and meteoroids from outside the Uranus system. The belt from the original small moons and particles themselves are invisible, as they only have a low optical depth, while the dust is revealed only in forward-scattered light. In the narrow main rings and the belts of small moons that give rise to the bands of dust, it is assumed that they are distributed in different particle sizes. The main rings consist mainly of some centimeters in size and to a lesser extent, from meter-sized bodies. Such spread expands the material interspersed with the rings and surrounding area and results in a large optical density, which can then be observed in rückwärtsgestreutem light. In contrast, the dust bands consist of relatively few larger particles, which in turn results in its low optical depth.
Research
When the Voyager 2 spacecraft flew by Uranus in January 1986 on, began the most thorough investigation of the ring system. It was discovered two new rings, λ and 1986U2R, which increased the total number of previously known Uranus rings on eleven. The rings were doing based on the analysis of radiometric data, ultraviolet and optical occultation studiert.Voyager 2 photographed the rings in different positions relative to the Sun, created images in the back-scattered, forward-scattered light and seitwärtsgestreutem. The analysis of these images permitted discharges of the total phase function, as well as the geometric and bonding of ring albedo particles. The pictures were also subsequently two more rings, ε and η, are identified, which brings the complex and delicate structure of the ring system even more clearly. Further analysis of the Voyager photos led to the discovery of 10 inner Uranian moons, including the two shepherd moons of the ε - ring, Cordelia and Ophelia.
List of properties
The following table lists the properties of the rings of Uranus on: