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Umbriel: Uranus' oddball satellite.

It seems only logical that the brightness of a full moon should equal that of four quarter moons. Yet the surge In brightness far exceeds that value as the moon comes into full view, in part because the shadows cast by its rocky surface vanish in full sun.

Astronomers have found that nearly every planetary satellite has a huge luminosity jump as its entire face becomes visible to an observer. But Umbriel, one of Uranus' five large moons, doesn't fit the pattern.

In studies concluded at Mt. Palomar Observatory near Escondido, Calif., researchers led by Bonnie Buratti of the Jet Propulsion Laboratory in Pasadena, Calif., found that Umbriel's brightness rises only slightly as it comes into full view.

Earlier this year, Buratti and her co-workers reported other evidence suggesting that Umbriel might be a Uranian oddity. Analyzing photographs taken through various color filters by the Voyager 2 spacecraft, they found that the ratio of green light to violet light varies widely over Umbriel's surface.

The gradual nature of the color variation seems to rule out the possibility that sharply distinct geologic regions, such as a clustering of craters on one part of the surface, could produce the variation, the researchers say. They propose instead that dar, red dust -- possibly the same primordial matter found in comets and asteroids -- has settled over large portions of the satellite, and that variations in the thickness of the dust layer could account for the color variations.

Astronomers have generally thought that satellites that don't brighten substantially in full sunlight most likely have smooth, compact surfaces. But Buratti says her group's new laboratory simulations indicate that compact surfaces do brighten significantly. However, surfaces composed of finely ground, fluffy dust particles brighten only slightly.

"The surface properties of Umbriel," she concludes, "are much different from the properties of other Uranian satellites and probably [any] other satellite In the solar system."
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Author:Cowen, Ron
Publication:Science News
Date:Nov 16, 1991
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