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TOPIC: Saturn's rings


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The Huygens Gap
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The sharp outer boundary of Saturn's B ring, which is the bright ring region seen to the right in this image, is maintained by a strong resonance with the moon Mimas. For every two orbits made by particles at this distance from Saturn, Mimas makes one orbit. The moon's repeated gravitational tugs force ring particles away from this region.

PIA08243
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The dark region is called the Huygens gap and it includes the bright, eccentric Huygens ringlet, also visible here near centre.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on July 23, 2006. The view was obtained from 15 degrees beneath the ringplane and at a distance of approximately 282,000 kilometres from Saturn. Image scale is 1 kilometre per pixel.

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Saturn Spoke
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This image from the Cassini spacecraft shows a ghostly white streak, called a spoke, in Saturn's B ring. This is the first sighting of a spoke in nearly a year, and the first spoke seen by Cassini on the sunlit side of the rings.

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It is also the first spoke seen at high phase angle -- that is, the angle formed between the sun, the rings and Cassini. In this geometry, the feature appears white (instead of black) against the rings because the very small particles comprising the spoke preferentially scatter light in the forward direction (i.e. toward Cassini), making the spoke brighter than the background rings.
The clear-filter image was taken in visible light with the Cassini spacecraft wide-angle camera on July 23, 2006, at a distance of approximately 692,000 kilometres from Saturn and at a sun-Saturn-spacecraft, or phase, angle of 115 degrees. Image scale on the sky at the distance of Saturn is 38 kilometres per pixel.

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RE: Saturn Rings
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This image of Saturn's Rings was taken by the Cassini spaceprobe on July 24, 2006, when it was approximately 472,721 kilometres away.

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The image was taken using the CL1 and CL2 filters.

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Over the last year, NASA's Cassini spacecraft has repeatedly spotted a debris pocket bulging out of one of Saturn's rings. This bright blob of material sits on the inner edge of the tenuous G ring, fattening one segment of it into a feature that planetary scientists call a ring arc the first ever seen around Saturn.

Ring arcs are rare, since orbital dynamics usually force particles to spread out into a nearly uniform disk. The Voyager 2 spacecraft imaged a set of arcs bulging from one of Neptune's rings, and mission scientists concluded that they were held there by the gravitational shepherding of the small moon Galatea.

ring
Credit NASA/JPL

Cassini scientists suspect that Saturn's newly discovered arc is confined by a similar process involving the moon Mimas. The small icy moon is in a 6:7 resonance with the arc, meaning it completes six orbits of Saturn in the time it takes the arc to do seven. Such a gravitational set-up creates pockets inside the ring where clumps of rock and ice can build up.
But like any unsightly bulge, Saturn's ring arc needed more than just a physical predisposition. It also needed material. Matthew Hedman (Cornell University), who is leading the work on the ring arc, speculates that a recent collision between two moonlets could have broken up large chunks of rock and ice, which were then scooped into the gravitational pocket.

"There are six spots around the ring where the arc could have formed. We do not know why only one of these regions contains the arc. We will have to make more observations and do some detailed modelling to know for sure. Fortunately, Cassini provides an unprecedented opportunity to study this system in detail" - Matthew Hedman.

Curiously, Saturn's ring arc might explain the strange appearance of the G ring, one of the planet's faintest, narrowest rings. The Cassini team speculates that tiny grains of dust and ice might be leaking away from the bright, thick ring arc, smearing out to form the diffuse ring.

"The G ring is the least understood ring in the Saturn system. Once we saw the arc, we at least may have a reason why the G ring is where it is and why it is narrow" - Matthew Hedman.

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The shadows of Saturn's C ring cover bright patches of clouds in the planet's atmosphere. The shadow-throwing rings stretch across the view at bottom. The dark inner edge of the B ring is visible at top.

PIA08191

The image was taken using a spectral filter sensitive to wavelengths of infrared light centred at 752 nanometers. The image was acquired with the Cassini spacecraft wide-angle camera on April 28, 2006 at a distance of approximately 340,000 kilometres from Saturn. Image scale is 17 kilometres per pixel.

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G ring
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This image of Saturn's G ring was taken by the Cassini space probe on May 26, 2006 when it was approximately 1,670,805 kilometres away.

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The image was taken using the CL1 and CL2 filters.

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L

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E ring
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This view of the faint E ring -- a ring feature now known to be created by Enceladus -- also shows two of Saturn's small moons that orbit within the ring, among a field of stars in the background.


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The image was taken in visible light with the Cassini spacecraft wide-angle camera on March 15, 2006, at a distance of approximately 2.4 million kilometres from Saturn. The image scale on the sky at the distance of Saturn is 142 kilometres per pixel.

The E ring extends from three to eight Saturn radii -- about 180,000 kilometres to 482,000 kilometres. Its full extent is not visible in this view.

Calypso (22 kilometres across) and Helene (32 kilometres across) orbit within the E ring's expanse. Helene skirts the outer parts of the E ring, but here it is projected in front of a region deeper within the ring.
Calypso and Helene are trojan satellites, or moons that orbit 60 degrees in front or behind a larger moon. Calypso is a Tethys trojan and Helene is a trojan of Dione.
An interesting feature of note in this image is the double-banded appearance of the E-ring, which is created because the ring is somewhat fainter in the ringplane than it is 500-1,000 kilometres above and below the ringplane. This appearance implies that the particles in this part of the ring have nonzero inclinations (a similar affect is seen in Jupiter's gossamer ring). An object with a nonzero inclination does not orbit exactly at Saturn's ringplane. Instead, its orbit takes it above and below the ringplane. Scientists are not entirely sure why the particles should have such inclinations, but they are fairly certain that the reason involves Enceladus.
One possible explanation is that all the E ring particles come from the plume of icy material that is shooting due south out of the moon's pole. This means all of the particles are created with a certain velocity out of the ringplane, and then they orbit above and below that plane.
Another possible explanation is that Enceladus produces particles with a range of speeds, but the moon gravitationally scatters any particles that lie very close to the ringplane, giving them nonzero inclinations.
Stray light within the camera system is responsible for the broad, faint "Y" shape across the image.

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Posts: 131433
Date:
A ring
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Views of Saturn's stunning ring system from above by the Cassini-Huygens spacecraft now orbiting the planet indicate the prominent A ring contains more debris than once thought, according to a new University of Colorado at Boulder study.

Previous observations with the Voyager spacecraft in the early 1980s found the ring was more transparent, indicating less material, said Joshua Colwell of CU-Boulder's Laboratory for Atmospheric and Space Physics. But new calculations based on May 2005 observations with Cassini's Ultraviolet Imaging Spectrograph, or UVIS, indicates the opacity of the ring is up to 35 percent higher than previously reported.


The image is a false-colour ultraviolet view of Saturn's B ring (centre) and A ring (right), separated by a large gap known as the Cassini Division and showing a bright horizontal streak created by a series of time-lapse images involving the star, 26 Taurus.
Image by NASA/JPL/University of Colorado.


Because of the uneven distribution of the ring particles - which range in size from dust grains to school buses - the transparency of the rings depends on the angle from which they are viewed. The particles are arranged essentially parallel in long stringy clumps as large as 18 metres across, 5 metres thick and 49 metres long, according to models produced from observation data.

A paper on the subject by Colwell, Larry Esposito and Miodrag Sremcevic of CU-Boulder's LASP appears in the April 1 issue of Geophysical Research Letters, or GRL. Esposito is science team leader for UVIS, a $12.5 million instrument designed and built at CU-Boulder by LASP that is riding on the Cassini spacecraft.

A new image released by the team in conjunction with the GRL paper shows the distribution of the ring material. The opaque B ring has more material than the A ring, located just outside it, and the A ring is densest near its inner edge, according to the team. The new clumps observed by Cassini mean a larger amount of material overall .
The particles are trapped in ever-changing clusters of debris that are regularly torn apart and reassembled by gravitational forces from the planet. The size and behaviour of the clusters were deduced by observing flickering light as the ring passed in front of a star in a process known as stellar occultation.

"The flickers are like a time-lapse movie of a car's headlights taken from the other side of a picket fence. The flickering would provide us details about the pickets" - Joshua Colwell, LASP research associate and UVIS science team member.

The observations of the particle clusters indicate the A ring is primarily empty space. A close-up view of the rings would show as "short, flattened strands of spiral arms with very few particles between them".

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RE: Saturn Rings
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The Cassini mission has found evidence that a new class of small moonlets resides within Saturn's rings. There may be as many as 10 million of these objects within one of Saturn's rings alone.

The moonlets' existence could help answer the question of whether Saturn's rings were formed through the break-up of a larger body or are the remnants of the disk of material from which Saturn and its moons formed.
Careful analysis of high-resolution images taken by Cassini's cameras revealed four faint, propeller-shaped double streaks. These features were found in an otherwise bland part of the mid-A Ring, a bright section in Saturn's main rings. Cassini imaging scientists reporting in this week's edition of the journal Nature believe the "propellers" provide the first direct observation of how moonlets of this size affect nearby particles. Cassini took the images as it slipped into Saturn orbit on July 1, 2004.

Previous measurements, including those made by NASA's Voyager spacecraft in the early 1980s, have shown that Saturn's rings contain mostly small water-ice particles ranging from less than 1 centimetre across to the size of a small house. Scientists knew about two larger embedded ring moons such as 30-kilometer-wide Pan and 7-kilometer-wide Daphnis. The latest findings mark the first evidence of objects of about 100 meters in diameter. From the number of moonlets spotted in the very small fraction of the A ring seen in the images, scientists estimated the total number of moonlets to be about 10 million.

Moons as large as Pan and Daphnis clear large gaps in the ring particles as they orbit Saturn. In contrast, smaller moonlets are not strong enough to clear out the ring, resulting in a partial gap centred on the moonlet and shaped like an airplane propeller. Such features created by moonlets were predicted by computer models, which give scientists confidence in their latest findings.


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The propeller moonlets represent a hitherto unseen size-class of particles orbiting within the rings. This image provides broad context within the rings, and shows the B ring, Cassini Division, A ring and F ring.

The detection of moonlets embedded in a ring of smaller particles may provide an opportunity to observe the processes by which planets form in disks of material around young stars, including our own early solar system.

"The structures we observe with Cassini are strikingly similar to those seen in many numerical models of the early stages of planetary formation, even though the scales are dramatically different. Cassini is giving us a unique insight into the origin of planets" - Carl Murray, imaging team member at Queen Mary, University of London.

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This image was taken by the Cassini spaceprobe on March 20, 2006 and received on Earth March 20, 2006.


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The camera was pointing toward the moon Daphnis.
The image was taken using the CL1 and CL2 filters.

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