About Saturn & Its Moons
Rings - Spokes
Spokes in Saturn's Rings
Spokes are short-lived features: they form in minutes and last an hour or two before disappearing again. They seem to form just after the outer portions of the B ring exit the shadow of Saturn and enter sunlight. They seem be generally radial, meaning they stretch directly outward across the ring – like spokes on a wheel – when formed, but then tilt increasingly as they travel around the planet. The tilting occurs because particles in the rings that are closer to the planet orbit faster, meaning they outpace the material orbiting farther out.
Spokes were first seen in images taken by NASA's Voyager spacecraft as they flew past Saturn in 1980 and 1981, near the time of Saturn's equinox. Researchers developed theories that Saturn's electromagnetic field was lifting the tiniest ring particles out of the B ring, forming the spokes.
"The spokes are most prominent at a point in the rings where the ring particles are moving at the same speed as Saturn's electromagnetic field," says Brad Wallis, Cassini rings discipline scientist. "That idea and variations of it are still the most prominent theories about the spokes."
But these mysterious features are not present all the time. Much to scientists' surprise, spokes were absent from the rings when Cassini arrived at Saturn in 2004. The Voyagers had visited Saturn during a time of Saturn's year (equal to about 29 Earth years) when the sun shone down from only a few degrees above the plane of the rings. Scientists speculated that spokes do not form when the rings are receiving lots of sunlight – in other words, when the sun is high above the rings, from about 17-24 degrees above the ringplane. They surmised that spokes might become a common sight as the sun's angle above the rings slowly changed with the seasons.
Cassini scientists hope to capture image sequences of spokes forming so they can measure how fast the process occurs and exactly how these features are associated with Saturn's magnetic field. As the sun sets on the southern side of the rings near equinox, Cassini will watch for any indication of how high the spokes float above the surface of the rings. These and other observations may explain just how these curious and beautiful phantoms of the rings are created.