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.
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 Mihály Horányi, 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,” Horányi told New Scientist.
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)