It’s Spoke Season in Saturn’s Rings
One of the most intriguing features of Saturn’s rings is the occasional appearance of ghostly “spokes”. Gone for several years, we got a glimpse of them from Cassini only just before the spacecraft went into orbits in Saturn’s equatorial plane from which the rings are all but invisible. Now that Cassini is able to see the rings again, the Cassini cameras have seen spokes again.
These features are so named because they cut radially across the rings, like the spokes of a wheel. Unlike bicycle spokes, however, they are ephemeral, appearing in a matter of only a few minutes (or less), and rarely (if ever) lasting more than one orbit around the planet (around 10 hours). (That orbital time period is pretty amazing by itself: it takes the Earth’s Moon about 27 days to orbit the Earth along a path that is only about four times longer than the orbits of Saturn’s ring particles. These particles are really zipping around Saturn, though their speeds relative to each other are literally at a snail’s pace.)
The spokes, first observed by the Voyager spacecraft in the early 1980s, are made up of particles roughly the same size of smoke particles. Particles this small are easily pushed around by electric fields. This is why your computer screen, even though it is vertical, is probably coated with a significant amount of dust right now. Saturn’s spinning magnetic field produces an electric field that can explain the radial nature of the spokes. But what makes them come and go? And why did they disappear for years? The spokes may be caused by meteoroid impacts onto the rings. Some theories connect the spokes to thunderstorms on Saturn. Their disappearance may be due to the changing seasons of Saturn. Direct sunlight on the rings in summer and winter may make the environment near the rings inhospitable to spokes. Now that spring is approaching on Saturn (Saturn’s year is 29 years long, so seasons run over 7 years on Saturn), the environment appears to be favorable for spokes again. Keep your eyes on the Cassini imaging team’s site and the Saturn web site for spectacular images and movies in the months ahead.