"Spokes" on Saturn's ring
The inner bright ring of the planet Saturn, also referred to by astronomers as the B ring, is intermittently interspersed with dark radial "spokes" that disappear without a trace over periods of several years. Scientists from the Cassini team led by Mihaly Horányi from the University of Colorado in Boulder now believe they have found the cause of this game of hide-and-seek: Depending on the position of the ring in relation to the incoming sunlight, the electrical conditions in the area of the rings change and tiny dust particles are redistributed.
The spokes of Saturn's ring were discovered in the 1980s when the Voyager probes passed the planet. They were later seen by the Hubble Space Telescope, but by the time the Cassini spacecraft arrived at the ringed planet, there was no sign of them. It wasn't until September 2005 that the spokes reappeared. They consist of dust particles a few thousandths of a millimeter in diameter and extend over 10,000 kilometers in length and 2,000 kilometers in width. They lead a dynamic life of their own and presumably emerge in a short time only to fade hours later.
An older hypothesis has it that the spokes are initiated by small meteorite impacts, which electrically charge the larger ring bodies and some of the dust, causing the dust to be ejected and levitate above the ring plane. The researchers' new calculation model shows that no spokes can form if the density of the background plasma, which is present anyway, is too high. The critical threshold is exceeded when the sun hits the ring relatively broadly. If it is at a very slanted angle, the plasma density decreases and the chance for spokes increases.
Based on their simulation, the scientists predict the next spoke season for July 2006 – then their model will have to pass the test.
