[meteorite-list] Mystery of Saturn's Vanishing 'Spokes' Illuminated

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Mar 17 13:26:18 2006
Message-ID: <200603171646.k2HGkGR03421_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8860-mystery-of-saturns-vanishing-spokes-illuminated.html

Mystery of Saturn's vanishing 'spokes' illuminated
Kimm Groshong
New Scientist
16 March 2006

The mystery of the disappearing "spokes" in Saturn's rings may not be
because they are hard to see. New research suggests they may not be
there at all when the Sun is at a certain angle.

NASA's Voyager missions in 1980 and 1981 captured detailed images of the
peculiar radial structures, some of which stretched as far as 20,000
kilometres across Saturn's B ring. The Hubble Space Telescope has also
imaged the spokes. But the features disappeared in October 1998 and were
still nowhere to be seen when NASA's Cassini probe arrived at Saturn in
2004.

Some researchers argued the reason was that the viewing conditions were
not good enough and that Cassini would not see the spokes again until
2007, when Saturn's rings will lie nearly edge-on to the Sun. But in
September 2005, Cassini captured a series of images of the rings on the
dark side of the planet, featuring smaller, fainter spokes.

Scientists believe spokes are produced when micron-sized dust grains on
the surface of boulders in the main ring become charged and float above
the ring plane. But they do not agree about how the dust particles
become charged.

Background plasma

The most popular model says meteorites bombard the rings, producing a
transient cloud of dense plasma that charges the grains. Another
possible explanation is that high-energy electron beams from aurora on
Saturn create the temporary plasma cloud.

"We don't really know which model is correct," says Mih?ly Hor?nyi, at
the University of Colorado, Boulder, US. But he and his colleagues,
including spokes expert Carolyn Porco of the Space Science Institute
also in Boulder, say once they are triggered, the background plasma
environment above the rings plays an important role in determining how
long the grains will stay aloft. And the plasma density above the rings
is linked to the angle between the Sun and the rings.

"If that angle is too high, the particles will quickly fall back to the
ring and we won't have a chance to see a whole group of them as a
spoke," Hor?nyi told New Scientist.

Spoke cycles

The group argues that when the background plasma density is low, the
grains kicked up above the ring plane continue to be repelled by the
ring and can therefore create spokes. Such a low plasma density can be
produced when the Sun is at a low angle relative to the ring plane and
fewer photons shine down on the rings. If the plasma density is high,
the levitated grains will fall back down to the ring, the researchers say.

They suggest that, when the plasma density is relatively low, spoke
activity switches off when the angle between the rings and the Sun
exceeds 20?. In that case, they say, "we expect spoke activity for about
8 years at a time, followed by a period without spokes that lasts 6 to 7
years."

And although Cassini is too close to the ring plane to look for spokes,
the team expects that the spokes will have returned by July 2006, when
Cassini has a better opportunity for viewing.

Journal Reference: Science (vol 311, p 1587)
Received on Fri 17 Mar 2006 11:46:16 AM PST