Volume: 04, Issue: 16 04/05/2006 
This collection of Cassini images provides context for understanding the location and scale of propeller-shaped features observed within Saturn's A ring. Image credit: 
NASA/JPL/Space Science Institute.
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This dramatic close-up of Tethys shows the large crater Penelope lying near center, overprinted by many smaller, younger impact sites. Image credit: NASA/JPL/Space Science Institute.
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Crater-scarred Rhea floats in the distance, peeking out from behind Saturn's partly shadowed rings. This view looks upward from just beneath the ringplane. The far side of the rings is masked by Saturn's shadow. Image credit: NASA/JPL.
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Other Articles in This Issue:
NASA’s Mars Reconnaissance Craft Begins Adjusting Orbit
Total Solar Eclipse
Venus within ESA Probe Reach
Professional Development Workshops and Institutes
 

Cassini Finds “Missing Link” Moonlet Evidence in Saturn’s Rings

Scientists with NASA’s Cassini mission have found evidence that a new class of small moonlets resides with 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 images as it slipped into Saturn’s orbit on July 1, 2004.

“These moonlets are likely to be chunks of the ancient body whose break-up produced Saturn’s glorious rings,” said Joseph Burns of Cornell University, Ithaca, New York. Burns is a co-author of the report.

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 one-half inch across to the size of a small house. Scientists knew about two larger embedded ring moons such as 19 mile-wide Pan and 4 mile-wide Daphnis. The latest findings mark the first evidence of objects of approximately 300 feet 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.

“The discovery of these intermediate-sized bodies tells us that Pan and Daphnis are probably just the largest members of the ring population, rather than interlopers from somewhere else,” said Matthew Tiscareno, an imaging team research associate at Cornell and lead author of the Nature paper.

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 centered 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.

“We acquired this spectacular, one-of-a-kind set of images immediately after getting into orbit for the express purpose of seeing fine details in the rings that we had not seen previously,” said Carolyn Porco, Cassini imaging team leader and co-author. “This will open up a new dimension in our exploration of Saturn’s rings and moons, their origin, and evolution.”

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,” said co-author Carl Murray, an imaging team member at Queen Mary, University of London. “Cassini is giving us a unique insight into the origin of planets.”

For more information about the Cassini-Huygens mission, please visit the following websites:
http://www.nasa.gov/mission_pages/cassini/main/index.html
http://www.nasa.gov/home/hqnews/2006/mar/HQ_06130_cassini_moonlets.html.

    
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