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A gap but not a void: the most visible division in Saturn's rings is not devoid of material.

THE CASSINI DIVISION is by far the most prominent of the many thousands of gaps in Saturn's rings. Discovered in 1675 by Jean Dominique Cassini, it is a dark gap about 2,900 miles (4,700 kilometers) wide that separates the bright A and B rings. Although it's comparable in width to the Atlantic Ocean, the Cassini Division subtends only 0.7 arcsecond to Earth-bound observers even when Saturn is at its closest, corresponding to the apparent diameter of a penny as seen from a distance of just over 3 miles (5 km).

High-resolution images, as well as radio and stellar occultation data obtained during NASA's Voyager spacecraft flybys of the early 1980s and by the Cassini spacecraft currently orbiting Saturn, have revealed that the Cassini Division is not an empty void. The material within the gap is about five times sparser than the material in the adjacent A and B rings. Like the C ring, the Cassini Division is the site of a host of narrow concentric ringlets, some sharp-edged and opaque, others diffuse and translucent.

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The Cassini Division not only bears a strong resemblance to the C ring in structure and particle density, but also in composition and color as well. The icy particles in the A and B rings reflect 40% to 50% of the sunlight that strikes them, but the particles in the Cassini Division and the C ring reflect only 15% to 20%. While the principal constituent of all of the rings is nearly pure, colorless water ice, the traces of an unknown non-icy contaminant that weakly absorbs blue light are not as prevalent in the Cassini Division or in the C ring, which lack the exceedingly subtle warm hue of the A and B rings.

If the Cassini Division is almost identical to the C ring in apparent surface brightness, why does it appear so much darker through a telescope? After all, it isn't uncommon for visual observers to describe the appearance of the Cassini Division as "jet black" when they extol the optical performance of a telescope or the quality of the seeing during an observing session. A recently published guide to observing Saturn even advises readers that "any divergence from a completely black intensity for Cassini's Division is simply the result of poor viewing conditions, scattered light, or inadequate aperture."

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The source of the illusion lies in the stark contrast between the narrow Cassini Division and the comparatively brilliant adjacent edges of the A and B rings, which are the brightest regions of the entire ring system. More than a century ago, Austrian physicist Ernst Mach demonstrated that if two unequally bright extended surfaces are brought into contact with each other, the less brilliant of the two will appear even dimmer where it appears to touch the brighter surface. On the other hand, the gap between the inner edge of the C ring and the globe of Saturn is almost devoid of matter, providing a truly black border that accentuates rather than overwhelms the faint light reflected by the C ring.

A few visual observers, some equipped with surprisingly modest instruments, have managed to overcome these difficulties. Although the Voyager spacecraft are usually credited for discovering material in the Cassini Division, the historical record clearly indicates that the spacecraft merely confirmed suspicions that had been harbored for well over a century.

The description of the tenuous matter in the C ring in the first edition of Thomas William Webb's classic observer's handbook Celestial Objects for Common Telescopes, published in 1859, includes a remark that "a similar material may fill Cassini's Division." Webb cited observations by the Director of the Madras Observatory in India, Captain William Stephen Jacob, who in 1852 had noted that the Cassini Division did not appear black but had the color of slate through his 6.2-inch refractor. Four years later, Jacob reported that the shadow cast by Saturn's globe across the rings "could also be seen across the dark space between the two bright rings, which therefore cannot be a mere opening but must be filled with matter of some kind."

Thomas Gwyn Elger, a keen-eyed British amateur who is remembered today chiefly for his lunar studies and map of the Moon, commented in 1888 that the Cassini Division "has never impressed me as being perfectly black" through his 8.5-inch Newtonian reflector. In 1899 the French astronomer Camille Flammarion described the appearance of the Cassini Division through his 9.6-inch refractor as "dark gray, not black" and inferred that "there is probably some matter in it." Fifteen years later his countrymen Georges and Valentin Fournier reported that on the steadiest nights, the 19.7-inch refractor of the Jarry-Desloges Observatory in Algeria showed that the Cassini Division was "not devoid of particles."

As the rings of Saturn appear to gradually open in coming years, opportunities to duplicate these challenging observations will continue to improve. Carefully comparing the appearance of the Cassini Division with the jet black shadow cast by the globe across the rings is the best way to avoid being deceived by the detrimental effects of diffraction and atmospheric turbulence.

Sky & Telescope contributing editor Thomas Dobbins has observed the ringed planet for well over one Saturnian year.
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Title Annotation:Exploring the Solar System
Author:Dobbins, Thomas A.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Mar 1, 2011
Words:874
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