Saturn gets the blues (and yellows)

Posted by Rich Talcott
on Friday, May 25, 2007
Saturn’s rings tell a story regardless of their color. This image,
created from Cassini spacecraft observations of stellar occultations,
reveals ring particles clump together far more than astronomers
previously thought. NASA/JPL/University of Colorado

Most of the time, Saturn's splendor appears to us in pastel shades of yellow and tan. But beauty knows no color boundaries. Even when converted to garish shades of blue and yellow, Saturn's rings retain their attraction — and tell a science story or two.

Scientists created the accompanying image of Saturn's rings from data returned by NASA's Cassini spacecraft. It doesn't come from the spacecraft's powerful camera, however, but from its Ultraviolet Imaging Spectrograph (UVIS). On a number of different occasions, UVIS observed stars as Cassini passed under the rings, recording each star's brightness as ring material intermittently occulted it. "Combining many of these occultations at different viewing geometries is like doing a CAT scan of the rings," says UVIS team member Josh Colwell of the University of Central Florida in Orlando.

Just as human CAT scans tell physicians about the human body, the UVIS ring scan tells planetary scientists about the rings' structure. In a surprise, researchers found the particles in Saturn's largest and densest ring — the middle B ring, which in this image encompasses all the yellow color and the blue interior to that — clump together and collide constantly. The scans let scientists map the distribution, shape, and orientation of the clumps. In the image, colors indicate the clumps' orientation, and brightness indicates the particle density.

Astronomers used to think the ring particles formed a uniform cloud. But the new observations confirm the particles' gravitational attraction creates clumps. The clumps form constantly and grow, then come apart once they reach about 100 to 160 feet (30 to 50 meters) across.

"At any given time, most particles are going to be in one of the clumps," says Colwell, "but the particles keep moving from clump to clump as the clumps are destroyed and new ones are formed." The classical cloud model predicted particles would collide twice per hour, on average. The new results show B-ring particles remain in almost continuous contact.

The B-ring clumps are broad and flat, roughly 10 to 50 times wider than they are thick. These clumps appear flatter and have smaller spaces between them than those found in the neighboring, outer A ring.

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