Scientists are celebrating the first Cassini spacecraft sighting ofspokes, the ghostly radial markings discovered in Saturn's rings byNASA's Voyager spacecraft 25 years ago.
A sequence of images taken on the side of the rings not illuminatedby the sun has captured a few faint, narrow spokes in the outer B ring,about 3,500 kilometers long and about 100 kilometers wide (2,200 milesby 60 miles).
Previously, scientists believed the visibility of spokes depended onthe elevation of the sun above the rings. The less sunlight, the morevisible the spokes. For this reason, they weren't expecting to seespokes until later in the mission when the sun angle will be low.
In Voyager images from 25 years ago, the spokes appeared dark whenseen at low sun angles and bright when seen at high sun angles. Thisbehavior indicated that they were comprised of extremely small icyparticles. Since Voyager days, spokes had been seen in images taken byNASA's Hubble Space Telescope. The new Cassini images were taken atvery high sun angles, where small particles can brighten substantially,making them more visible.
Determining the timing in the appearance of spokes will be ofintense interest and will require monitoring spoke activity from avariety of geometries over several years. "Cassini has found that theSaturn Kilometric Radiation period has changed since Voyager, whichthough hard to believe, may mean that the rotation of Saturn's interiorhas changed," said Dr. Carolyn Porco, Cassini imaging team leader atthe Space Science Institute in Boulder, Colo., and one of the firstindividuals to study spokes in Voyager images. "That would be a findingof enormous consequence, so we'll be looking very closely to see if thefrequency of spoke activity has changed too."
Porco's analysis of spokes in the early 1980s found that thesenarrow arrangements of small particles came and went with a periodequal to that of the powerful bursts of radio waves, called SaturnKilometric Radiation, discovered by Voyager and coming from Saturn'smagnetic field. This association indicated that spokes were aphenomenon involving electromagnetic effects due to Saturn's magneticfield.
There is no commonly accepted theory for the creation of spokes.Some ideas suggest that spokes result from meteoroid impacts onto therings; others suggest that they are created by instability in Saturn'smagnetic field, which surrounds the planet, near the rings. Whateverthe cause, imaging team members will study the new spoke images andmaintain their vigil for additional spoke sightings.
Cassini also completed a flyby of Saturn's moon Titan on Wednesday,Sept. 7. During that flyby, one of two solid-state recorders on boardthe spacecraft failed to record science data as planned. The spacecraftteam is troubleshooting the cause of the anomaly, and early indicationspoint to a software problem that would be correctable with no long-termimpacts. About half of the planned science data was received.
This was Cassini's eighth flyby out of 45 Titan flybys planned in the nominal four-year tour.
The Cassini-Huygens mission is a cooperative project of NASA, theEuropean Space Agency and the Italian Space Agency. The Jet PropulsionLaboratory, a division of the California Institute of Technology inPasadena, manages the Cassini-Huygens mission for NASA's ScienceMission Directorate, Washington. The Cassini orbiter and its twoonboard cameras were designed, developed and assembled at JPL. Theimaging operations center is based at the Space Science Institute inBoulder, Colo.
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