New observations from the Cassini spacecraftnow at Saturn indicate the particles comprising one of its mostprominent rings are trapped in ever-changing clusters of debris thatare regularly torn apart and reassembled by gravitational forces fromthe planet.
According to University of Colorado at Boulder Professor LarryEsposito of the Laboratory for Atmospheric and Space Physics, particleclusters in the outermost main ring, the A ring, range from the size ofsedans to moving vans and are far too small to be photographed by thespacecraft cameras. The size and behavior of the clusters were deducedby a research team observing the flickering starlight as the ringpassed in front of several stars in a process known as stellaroccultation, he said.
This is the first time scientists have been able to measurethe size, orientation and spacing of these particle clumps in Saturn'srings, he said. Esposito is the science team leader for the UltraViolet Imaging Spectrograph, or UVIS, a $12.5 million instrumentdesigned and built at CU-Boulder that is riding on Cassini.
CU-Boulder planetary scientist Joshua Colwell, UVIS scienceteam member, said researchers believe Saturn's ring particles are madeup of ice, dust and rock, and range in size from dust grains tomountains. The new observations of the particle clusters indicate the Aring is primarily empty space.
"The spacing between the clumps as determined by UVIS data isgreater than the widths of the clumps themselves," Colwell said. "If wecould get close enough to the rings, these clumps would appear asshort, flattened strands of spiral arms with very few particles betweenthem."
Colwell participated in a press briefing on newCassini-Huygens observations at the 37th Annual Meeting of the Divisionfor Planetary Sciences meeting held Sept. 4 to Sept. 9 in Cambridge,England.
Bound to each other by their own gravity, the clumps areperiodically torn apart by the gravitational tides of Saturn, saidColwell. He likened the process to a handful of marbles placed in orbitaround a beach ball. The marbles closest to the ball would orbit morequickly and drift from the pack before reorganizing themselves intonew, orbiting clumps.
The individual clusters were largest near the middle of thering and became smaller toward the edges of the ring, the teamreported. The cluster cores range in size from two meters to 13 meters,or 7 feet to 43 feet. There are no indications yet that similar clumpsexist in Saturn's other rings, confirming predictions made by the teamfrom computer simulations.
The UVIS team also detected a tenuous atmosphere on Saturn'stiny moon Enceladus made of water vapor, said Esposito. The researchersdetected no free-floating hydrogen or oxygen atoms, implying the waterwas recently released -- perhaps from a local fissure near the moon'ssouth pole -- and was escaping from its surface. Enceladus is onlyabout 310 miles, or 500 kilometers, in diameter.
When combined with Cassini images and results from otherspectrometers onboard the spacecraft, the new Enceladus observationsindicate water and grains of ice are being spewed from the moon'ssurface much in the manner of gaseous jets that have been observederupting on the surface of comets, Esposito said. "The rate of waterreleased is sufficient to provide the neutral oxygen discovered by UVISaround Saturn last year and to re-supply Saturn's E ring."
In July, the UVIS team released new images from the missiondepicting emissions near Saturn's poles that resemble Earth's northernlights. The image can be viewed at: http://www.colorado.edu/news/releases/2005/290.html.
The Cassini-Huygens mission is a cooperative project of NASA,the European Space Agency and the Italian Space Agency. The JetPropulsion Laboratory, a division of the California Institute ofTechnology in Pasadena, manages the Cassini-Huygens mission for NASA'sScience Mission Directorate in Washington, D.C.
The Cassini orbiter was designed, developed and assembled atJPL. The ultraviolet imaging spectrograph was built, and the team isbased, at the University of Colorado at Boulder.
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