The Cassini spacecraft has snapped more images of strange, bright 'spokes' in Saturn's rings.
Spokey Saturn
24 Mar 2009
As Cassini sped around Saturn, the spacecraft turned to snap this image of bright 'spokes' giving chase around the B ring.
These radial markings are appearing more often as Saturn approaches equinox in August 2009
Spokes are ghostly features that stretch across Saturn's B ring, and they usually have a long, finger-like or wedge-shaped appearance. They can reach 16,000 kilometres long or more.
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.
Mysterious dark, wedge-shaped spokes in Saturn's B ring.
"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.
Answering the researchers' hopes, Cassini's first spoke sighting followed in September 2005. Cassini has observed many more spokes as the Sun's angle above the rings decreases with the approach of the 2009 equinox—when the Sun's illumination will transition from the southern side of the rings to the northern side, momentarily passing directly through the plane of the rings.
These three images, taken over a span of 27 minutes, show the movement of a few faint, narrow spokes in the outer B ring.
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.
This main image at the top of this page looks toward the sunlit side of the rings from about 31 degrees below the ringplane. The image was taken in visible light with the Cassini spacecraft wide-angle camera on February 2, 2009. The view was acquired at a distance of approximately 578,000 kilometres from Saturn, giving an image scale of 30 kilometres per pixel.
Adapted from information issued by NASA / JPL / Space Science Institute.
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