Volume: 04, Issue: 02 09/21/2005 
These images, taken over a span of 27 minutes, show a few faint, narrow spokes in the outer B ring. Images courtesy NASA/JPL.
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The bright, rough region on the left side of the image seems to be topographically high terrain cut by channels and bays. Image courtesy NASA/JPL.
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Canyonlands of Titan: Fluids have flowed and cut these deeply-incised channels into the icy surface of Titan as seen in this Synthetic Aperture Radar image. Image courtesy NASA/JPL.
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Other Articles in This Issue:
NASA Unveils Plans for Return to Moon
Durable Orbiter Tracks Mars Changes
Crash Claims Life of Dryden Chief Engineer Marta Bohn-Meyer
More Student Lessons on Hurricanes

Cassini Spots Ghostly Ring Spokes and Possible Titan Shoreline

Cassini has rediscovered the eerie radial markings in Saturnís rings first seen by NASAís Voyager spacecraft 25 years ago. A new sequence of images reveals faint, narrow spokes 2,200 miles long by 60 miles wide in Saturnís outer B ring.

Scientists previously believed the visibility of spokes depended on the elevation of the sun above the rings: the less sunlight, the more visible the spokes. For this reason, they weren't expecting to see spokes until later in the mission, when the sun angle will be low. But the new Cassini images were taken at very high sun angles, where small particles can brighten substantially, making them more visible.

"Cassini has found that the Saturn Kilometric Radiation period has changed since Voyager, which though hard to believe, may mean that the rotation of Saturnís interior has changed," said Dr. Carolyn Porco, Cassini imaging team leader at the Space Science Institute.

According to Porco, who was one of the first individuals to study spokes in Voyager images, the team will closely monitor the frequency of spoke activity to see if that has changed as well. Porco's analysis of spokes in the early 1980s found the narrow arrangements of small particles came and went with a period equal to that of the powerful bursts of radio waves called Saturn Kilometric Radiation. This indicated that spokes were a phenomenon involving electromagnetic effects due to Saturn's magnetic field.

No commonly accepted theory exists to explain the creation of spokes. Some suggest spokes result from meteoroid impacts onto the rings; others suggest they are created by instability in Saturn's magnetic field, which surrounds the planet near the rings. Whatever the cause, imaging team members will study the new spoke images and maintain their vigil for additional spoke sightings.

Cassini also completed a flyby of Saturn's moon Titan on Sept. 7, 2005, revealing what appears to be a large shoreline cutting across the smoggy moon's southern hemisphere. Images show the possible shoreline separating a distinct bright, possibly rough area from a very dark, smooth region.

"This [dark region] is the area where liquid or a wet surface has most likely been present, now or in the recent past,Ē said Steve Wall, radar deputy team leader from NASA's Jet Propulsion Laboratory. "Titan probably has episodic periods of rainfall or massive seepages of liquid from the ground."

The brightness patterns in the dark area indicate that it may once have been flooded with liquid that may now have partially receded. Bay-like features also lead scientists to speculate that the bright-dark boundary is most likely a shoreline.

Taken together with the two other radar passes in October 2004 and February 2005, these images have identified at least two distinct types of drainage and channel formation on Titan. Some channels in images from this pass are long and deep, with angular patterns and few tributaries, suggesting that fluids flow over great distances. By contrast, others show channels that form a denser network that might indicate rainfall.

Titan has an environment somewhat similar to that of Earth before biological activity forever altered the planetís atmospheric composition. The major differences on Titan, however, are the absence of liquid water and Titan's very low temperature. With a thick, nitrogen-rich atmosphere, Titan was until recently presumed to hold large seas or oceans of liquid methane. Cassini has been in orbit around Saturn for a year and has found no evidence for these large seas.

Cassini encountered an anomaly with one of two solid-state recorders during the Sept. 7 close flyby, resulting in some data not being recorded. Half of the data from the flyby was received, much to the delight of anxious scientists. The spacecraft team is troubleshooting the cause, and early indications point to a software problem that would be correctable with no long-term impacts.

This was Cassini's eighth out of 45 Titan flybys planned in the nominal four-year tour. The next radar pass will be Oct. 26, when the team will focus on the Huygens probe landing site close to the equator.

More information and images are available at the following websites:

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