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Saturn Spokes
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Weird Saturn Ring Spokes May Reappear In July, According To Colorado University-Boulder Study

Unusual spokes that appear fleetingly on the rings of Saturn only to disappear for years at a time may become visible again by July, according to a new study spearheaded by the University of Colorado at Boulder.

The spokes, which are up to 6,000 miles long and 1,500 miles in width, were first spotted 26 years ago by the Voyager spacecraft. But when the Cassini spacecraft arrived at Saturn in July of 2004, the striking radial features that cut across Saturn's ring plane were nowhere to be found -- an event that disappointed and puzzled many scientists.


Voyager 2 image of ring spokes.
Courtesy NASA/JPL


The Hubble Space Telescope occasionally observed the ring spokes in the late 1990s. But the spokes gradually faded, a result of Saturn's seasonal, orbital motion and its tilted axis of rotation that altered the light-scattering geometry.

"The spokes were switched off by the time Cassini arrived. We think it is a seasonal phenomena related to the sun rising and setting over the ring plane that changes the physical environment there, making it either friendly or hostile to their formation" - Professor Mihaly Horanyi of the CU-Boulder Laboratory for Atmospheric and Space Physics.

The spokes are made up of tiny dust particles less than a micron in width -- about 1/50th the width of a human hair -- that collect electrostatic charges in the plasma environment of the rings and become subject to electrical and magnetic forces. The right conditions cause them to gain an extra electron, allowing them to leap en masse from the surface of ring debris for brief periods, collectively forming the giant spokes that appear dark against the lit side of the rings and bright against the unlit side of the rings.
The researchers hypothesise that the conditions for the spokes to form are correlated to a decrease in the angle of the ring plane to the sun.

"Because the rings are more open to the sun now than when Voyager flew by, the charging environment above the rings has prevented the formation of the spokes until very recently," the researchers wrote in Science.

Cassini first imaged a "puny version" of Saturn's spoke rings from a distance of 98,000 miles in early September that were only about 2,200 miles in length and about 60 miles wide. The team believes the spoke sighting may have been an "early bird" event.
As the ring plane angle decreases when Saturn is near its two seasonal equinoxes, the conditions appear to become more suitable for the formation of the spokes. Although Cassini currently is orbiting too close to the ring plane to make observations, the researchers expect the spoke activity to have returned by the time the spacecraft increases its inclination in July 2006.
Once the spokes are visible again, the research team believes there will be spoke activity for about eight years, based on the fact that it takes Saturn about 30 Earth-years to complete one orbit around the sun. The eight-year period should be followed by about six-to-seven years of a spoke hiatus.
The dust grains levitated by plasma during spoke-forming periods are probably hovering less than 50 miles above the rings themselves and they scatter light from the sun differently than do the rings themselves.
But there are still many questions about the spokes.

"We don't know if they form by rapidly expanding, or if they form all at once. This is a weird phenomena; we don't have the full story on it yet" - Mihaly Horanyi .

During the Voyager mission, they were absent during one observation, but fully developed in a follow-up observation made just five minutes later.

A paper on the subject appears in the March 17 issue of Science magazine. The paper was authored by doctoral student Colin Mitchell and Horanyi of CU-Boulder's LASP, Ove Havnes of the University of Trosmo in Norway and Carolyn Porco of the Space Science Institute in Boulder.

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L

Posts: 131433
Date:
F_ring
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Structure in Saturn's narrow and complex F ring is seen here, including one of the faint strands (at the left) that Cassini has shown to curl around the planet in a tight, rotating spiral. Scientists think the spiral structure might be due to disturbance of micron-sized F-ring particles by a tiny moon (or moons).



This image was taken in visible light with the Cassini spacecraft narrow-angle camera on Jan. 19, 2006, at a distance of approximately 1.2 million kilometres from Saturn and from just above the ring plane. The image scale is 7 kilometres per pixel.

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L

Posts: 131433
Date:
Saturn Spokes
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Cassini has finally spotted the elusive spokes in Saturn's rings.
Spokes are the ghostly radial markings discovered in the rings by NASA's Voyager spacecraft 25 years ago. Since that time, spokes had been seen in images taken by NASA's Hubble Space Telescope but had not, until now, been seen by Cassini.


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These three images, taken over a span of 27 minutes, show a few faint, narrow spokes in the outer B ring. The spokes are about 3,500 kilometres long and about 100 kilometres-wide. The motion of the spokes here is from left to right. They are seen just prior to disappearing into the planet's shadow on the rings.

At the bottom left corner of the left and centre images, the bright inner edge of the A ring is visible. Continuing radially inward (or toward Saturn) are several bands that lie within the Cassini Division, bounded by the bright outer edge of the B ring. The rounded shadow of Saturn cuts across the rings in the image at right.
Cassini's first sighting of spokes occurs on the unilluminated side of the rings, in the same region in which they were seen during the Voyager flybys. Although the most familiar Voyager images of spokes showed them on the sunlit side of the rings, spokes also were seen on the unilluminated side.

In Voyager images, when spokes were seen at low phase angles, they appeared dark; when seen at high phase angles, they appeared bright. The spokes seen here are viewed by Cassini at a very high phase angle, which is about 145 degrees at the centre of each image.
Imaging team members will be studying the new spoke images and will maintain their vigil for additional spoke sightings.

These images were taken using the clear filters on Cassini's wide-angle camera on Sept. 5, 2005, at a mean distance of 318,000 kilometres from Saturn. The radial scale on the rings (the image scale at the centre of each image) is about 17 kilometres per pixel.

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L

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RE: A-ring
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New observations from the Cassini spacecraft now at Saturn indicate the particles comprising one of its most prominent rings are trapped in ever-changing clusters of debris that are regularly torn apart and reassembled by gravitational forces from the planet.


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The ring is the bluest in the centre, where the gravitational clumps are the largest. The thickest black band in the ring is the Enke Gap, and the thin black band further to the right is the Keeler Gap.
The right image is a computer simulation about 150 meters across illustrating a clumpy region of particles in the A ring. The particles are moving counter clockwise, from bottom to top.


According to University of Colorado at Boulder Professor Larry Esposito of the Laboratory for Atmospheric and Space Physics, particle clusters in the outermost main ring, the A ring, range from the size of cars to juggernauts and are far too small to be photographed by the spacecraft cameras. The size and behaviour of the clusters were deduced by a research team observing the flickering starlight as the ring passed in front of several stars in a process known as stellar occultation, he said.

This is the first time scientists have been able to measure the size, orientation and spacing of these particle clumps in Saturn's rings. Esposito is the science team leader for the Ultra Violet Imaging Spectrograph, or UVIS, an instrument designed and built at Colorado University that is riding on Cassini.

Colorado University planetary scientist Joshua Colwell, UVIS science team member, said researchers believe Saturn's ring particles are made up of ice, dust and rock, and range in size from dust grains to mountains. The new observations of the particle clusters indicate the A ring is primarily empty space.

"The spacing between the clumps as determined by UVIS data is greater than the widths of the clumps themselves. If we could get close enough to the rings, these clumps would appear as short, flattened strands of spiral arms with very few particles between them" - Joshua Colwell.

Colwell participated in a press briefing on new Cassini-Huygens observations at the 37th Annual Meeting of the Division for Planetary Sciences meeting held September 4 to 9 in Cambridge, England.

Bound to each other by their own gravity, the clumps are periodically torn apart by the gravitational tides of Saturn, said Colwell. He likened the process to a handful of marbles placed in orbit around a beach ball. The marbles closest to the ball would orbit more quickly and drift from the pack before reorganizing themselves into new, orbiting clumps.

The individual clusters were largest near the middle of the ring and became smaller toward the edges of the ring, the team reported. The cluster cores range in size from 2 to 13 meters. There are no indications yet that similar clumps exist in Saturn's other rings, confirming predictions made by the team from computer simulations.

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L

Posts: 131433
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RE: F-ring
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This map of Saturn's F ring illustrates how the ghostly strands flanking the core of this contorted ring, when examined in detail, actually form a spiral structure wound like a spring around the planet.


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Two identical maps of the F ring have been joined, side-by-side, to show the nature of the spiral more clearly. The F ring has been mapped as if it were a circular feature, so that its eccentricity is not apparent here.

The spiral strand's path across the image begins about 350 kilometres inward of the F ring core at about 200 degrees longitude (bottom axis) on the right map, and moves closer to the ring core toward the left, wrapping over onto the map on the left.
The strand appears to cross the ring core around 100 degrees longitude, after which the distance between the strand and the ring core increases to the left and can be followed, moving even farther outward, wrapping around to the rightmost boundary of the right-hand map and continuing to the left.

Other spiralling structures seen in the main rings of Saturn, the density and bending waves, are initiated by the gravitational influence of an orbiting moon.
Density and bending waves move across the rings because of the way that relatively massive ring particles exert a gravitational influence on each other and can all move together.
In contrast, the F ring spiral structure contains very little mass and appears to originate from material somehow episodically ejected from the core of the F ring and then sheared out due to the different orbital speeds followed by the constituent particles.

Scientists have speculated that the spiral may be a consequence of moons crossing the F ring and spreading its particles around.

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RE: Saturn Rings
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Saturn's rings make up an enormous, complex structure. From edge-to-edge, the ring system would not even fit in the distance between Earth and the Moon. The seven main rings are labelled in the order in which they were discovered. From the planet outward, they are D, C, B, A, F, G and E.

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Credit: NASA/JPL

The D ring is very faint and closest to Saturn. The main rings are A, B and C. The outermost ring, easily seen with Earth-based telescopes, is the A ring. The Cassini Division is the largest gap in the rings and separates the B ring from the A ring.
Just outside the A ring is the narrow F ring, shepherded by tiny moons, Pandora and Prometheus. Beyond that are two much fainter rings named G and E. Saturn's diffuse E ring is the largest planetary ring in our solar system, extending from Mimas orbit to Titans orbit, about 1 million kilometres.

The particles in Saturn's rings are composed primarily of water ice and range in size from microns to tens of meters. The rings show a tremendous amount of structure on all scales; some of this structure is related to gravitational interactions with Saturn's many moons, but much of it remains unexplained.
One moonlet, Pan, actually orbits inside the A ring in a 330-kilometer-wide gap called the Encke Gap. The main rings (A, B and C) are less than 100 meters thick in most places, compared to their radial extent of 62,120 kilometres. The main rings are much younger than the age of the solar system, perhaps only a few hundred million years old. They may have formed from the breakup of one of Saturns moons or from a comet or meteor that was torn apart by Saturns gravity.

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This view shows Saturn's Encke Gap (325 kilometres wide) whose centre is 133,590 kilometres from Saturn. This division in the rings is home to the small moon called Pan (20 kilometres across).


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The four bright bands - two on either side of the gap - are density waves generated by gravitational resonances with Prometheus and Pandora. The rest of the ring structures seen here are "wakes."

Like a placid lake surface disturbed by a boat, ring particles near the gap have their orbits perturbed by Pan's gravity, organizing themselves into wakes that stream away from the moon. The spacing of these wakes (their wavelength) increases with distance from the gap, as can be seen here.

Unlike most waves in the rings, wakes do not propagate or sustain themselves; rather, they preserve the memory of a single event (a passing of Pan in its orbit). These ripples are surprisingly persistent.
In this image, for example, Pan is 120 degrees farther around the planet from this location, and the wakes here were generated as long as four months before the image was taken.
Scientists are working to revise models of how ring wakes evolve using data from Cassini.
The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 20, 2005, at a distance of approximately 332,000 kilometres from Saturn.
The image scale is 2 kilometres per pixel.




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Posts: 131433
Date:
Saturn's rings
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Specially designed Cassini orbits place Earth and Cassini on opposite sides of Saturn's rings, a geometry known as occultation. Cassini conducted the first radio occultation observation of Saturn's rings on May 3, 2005.
Three simultaneous radio signals of 0.94, 3.6, and 13 centimetre wavelength (Ka-, X-, and S-bands) were sent from Cassini through the rings to Earth.
The observed change of each signal as Cassini moved behind the rings provided a profile of the distribution of ring material as a function of distance from Saturn, or an optical depth profile.


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This simulated image was constructed from the measured optical depth profiles. It depicts the observed ring structure at about 10 kilometres in resolution. Colour is used to represent information about ring particle sizes in different regions based on the measured effects of the three radio signals.
Purple colour indicates regions where there is a lack of particles of size less than 5 centimetres. Green and blue shades indicate regions where there are particles smaller than 5 centimetres and 1 centimetre. The saturated broad white band near the middle of ring B is the densest region of ring B, over which two of the three radio signals were blocked at 10-kilometer resolution, preventing accurate colour representation over this band. From other evidence in the radio observations, all ring regions appear to be populated by a broad range particle size distribution that extends to boulder sizes (several to many meters across).
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.
The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The radio science team is based at JPL. For more information about the radio science team visit http://saturn.jpl.nasa.gov/spacecraft/instruments-cassini-rss.cfm.




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