From: baalke@zagami.jpl.nasa.gov   
      
   Office of News Services   
   University of Colorado-Boulder   
   Boulder, Colorado   
      
   Contact:   
   Larry Esposito, (303) 492-5990, Esposito@lasp.colorado.edu   
   Joshua Colwell, (303) 492-6805   
   Jim Scott, (303) 492-3114   
      
   Dec. 8, 2003   
      
   Rings Around The Planets: Recycling Of Material May Extend Ring Lifetimes   
      
   Although rings around planets like Jupiter, Saturn, Uranus and Neptune are   
   relatively short-lived, new evidence implies that the recycling of orbiting   
   debris can lengthen the lifetime of such rings, according to University of   
   Colorado researchers.   
      
   Strong evidence now implies small moons near the giant planets like Saturn and   
   Jupiter are essentially piles of rubble, said Larry Esposito, a professor at   
   CU-Boulder's Laboratory for Atmospheric and Space Physics. These re-constituted   
   small bodies are the source of material for planetary rings.   
      
   Previous calculations by Esposito and LASP Research Associate Joshua Colwell   
   showed the short lifetimes for such moons imply that the solar system is nearly   
   at the end of the age of rings. "These philosophically unappealing results may   
   not truly describe our solar system and the rings that may surround giant   
   extra-solar planets," said Esposito. "Our new calculations of models explain   
   how   
   inclusion of recycling can lengthen the lifetime of rings and moons."   
      
   The observations from the Voyager and Galileo space missions showed a variety   
   of   
   rings surrounding each of the giant planets, including Jupiter, Saturn, Uranus   
   and Neptune. The rings are mixed in each case with small moons.   
      
   "It is clear that the small moons not only sculpt the rings through their   
   gravity, but are also the parents of the ring material," said Esposito. "In   
   each   
   ring system, destructive processes like grinding, darkening and spreading are   
   acting so rapidly that the rings must be much younger than the planets they   
   circle."   
      
   Numerical models by Esposito and Colwell from the 1990's showed a "collisional   
   cascade," where a planet's moons are broken into smaller moons when struck by   
   asteroids or comets. The fragments then are shattered to form the particles in   
   new rings. The rings themselves are subsequently ground to dust, which is swept   
   away.   
      
   But according to Colwell, "Some of the fragments that make up the rings may be   
   re-accreted instead of being ground to dust. New evidence shows some debris has   
   accumulated into moons or moonlets rather than disappearing through collisional   
   erosion."   
      
   "This process has proceeded rapidly," said Esposito. "The typical ring is   
   younger than a few hundred million years, the blink of an eye compared to the   
   planets, which are 4.5 billion years old. The question naturally arises why   
   rings still exist, to be photographed in such glory by visiting human   
   spacecraft   
   that have arrived lately on the scene," he said.   
      
   "The answer now likely seems to be cosmic recycling," said Esposito. Each time   
   a   
   moon is destroyed by a cosmic impact, much of the material released is captured   
   by other nearby moons. These recycled moons are essentially collections of   
   rubble, but by recycling material through a series of small moons, the lifetime   
   of the ring system may be longer than we initially thought."   
      
   Esposito and former LASP Research Associate Robin Canup, now with the Southwest   
   Research Institute's Boulder branch, showed through computer modeling that   
   smaller fragments can be recaptured by other moons in the system. "Without this   
   recycling, the rings and moons are soon gone," said Esposito.   
      
   But with more recycling, the lifetime is longer, Esposito said. With most of   
   the   
   material recycled, as now appears to be the case in most rings, the lifetime is   
   extended by a large factor.   
      
   "Although the individual rings and moons we now see are ephemeral, the   
   phenomenon persists for billions of years around Saturn," said Esposito.   
   "Previous calculations ignored the collective effects of the other moons in   
   extending the persistence of rings by recapturing and recycling ring material."   
      
   Esposito, the principal investigator on a $12 million spectrograph on the   
   Cassini spacecraft slated to arrive at Saturn in July 2004, will look closely   
   at   
   the competing processes of destruction and re-capture in Saturn's F ring to   
   confirm and quantify this explanation. Esposito discovered the F Ring using   
   data   
   from NASA's Voyager 2 mission to the outer planets launched in 1978.   
      
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