[meteorite-list] Weird Saturn Ring Spokes May Reappear In July, According To CU-Boulder Study

[Ron Baalke]

Office of News Services
University of Colorado-Boulder
Boulder, Colorado

Contact:
Mihaly Horanyi, (303) 492-6903
Jim Scott, (303) 492-3114

March 16, 2006

Weird Saturn Ring Spokes May Reappear In July, According To CU-Boulder 
Study

Unusual spokes that appear fleetingly on the rings of Saturn only to 
disappear for years at a time may become visible again by July, according 
to a new study spearheaded by the University of Colorado at Boulder.

The spokes, which are up to 6,000 miles long and 1,500 miles in width, 
were first spotted 26 years ago by the Voyager spacecraft, said CU-Boulder 
Professor Mihaly Horanyi of the Laboratory for Atmospheric and Space 
Physics. But when the Cassini spacecraft arrived at Saturn in July of 
2004, the striking radial features that cut across Saturn's ring plane 
were nowhere to be found -- an event that disappointed and puzzled many 
scientists, he said.

The Hubble Space Telescope occasionally observed the ring spokes in the 
late 1990s, said Horanyi, a professor of physics at CU-Boulder. But the 
spokes gradually faded, a result of Saturn's seasonal, orbital motion and 
its tilted axis of rotation that altered the light-scattering geometry.

"The spokes were switched off by the time Cassini arrived," said Horanyi. 
"We think it is a seasonal phenomena related to the sun rising and setting 
over the ring plane that changes the physical environment there, making it 
either friendly or hostile to their formation."

A paper on the subject appears in the March 17 issue of Science magazine. 
The paper was authored by doctoral student Colin Mitchell and Horanyi of 
CU-Boulder's LASP, Ove Havnes of the University of Trosmo in Norway and 
Carolyn Porco of the Space Science Institute in Boulder.

The spokes are made up of tiny dust particles less than a micron in width 
-- about 1/50th the width of a human hair -- that collect electrostatic 
charges in the plasma environment of the rings and become subject to 
electrical and magnetic forces, said Horanyi. The right conditions cause 
them to gain an extra electron, allowing them to leap en masse from the 
surface of ring debris for brief periods, collectively forming the giant 
spokes that appear dark against the lit side of the rings and bright 
against the unlit side of the rings.

The researchers hypothesize that the conditions for the spokes to form are 
correlated to a decrease in the angle of the ring plane to the sun. 
"Because the rings are more open to the sun now than when Voyager flew by, 
the charging environment above the rings has prevented the formation of 
the spokes until very recently," the researchers wrote in Science.

Cassini first imaged a "puny version" of Saturn's spoke rings from a 
distance of 98,000 miles in early September that were only about 2,200 
miles in length and about 60 miles wide, said Horanyi. The team believes 
the spoke sighting may have been an "early bird" event.

As the ring plane angle decreases when Saturn is near its two seasonal 
equinoxes, the conditions appear to become more suitable for the formation 
of the eerie spokes, said Horanyi. Although Cassini currently is orbiting 
too close to the ring plane to make observations, the researchers expect 
the spoke activity to have returned by the time the spacecraft increases 
its inclination in July 2006.

Once the spokes are visible again, the research team believes there will 
be spoke activity for about eight years, based on the fact that it takes 
Saturn about 30 Earth-years to complete one orbit around the sun, said 
Horanyi. The eight-year period should be followed by about six-to-seven 
years of a spoke hiatus, he said.

The dust grains levitated by plasma during spoke-forming periods are 
probably hovering less than 50 miles above the rings themselves and they 
scatter light from the sun differently than do the rings themselves, he 
said.

But there are still many questions about the spokes, said Horanyi. "We 
don't know if they form by rapidly expanding, or if they form all at 
once," he said. During the Voyager mission, they were absent during one 
observation, but fully developed in a follow-up observation made just five 
minutes later, Horanyi said.

"This is a weird phenomena; we don't have the full story on it yet," he 
said.

IMAGE CAPTION:
[http://www.colorado.edu/news/releases/2006/images/102.jpg (10KB)]
Voyager 2 image of ring spokes. Courtesy NASA/JPL