Another unexpected finding: a broad, dusty inner ring called zeta seems to have migrated further from the planet since Voyager's visit. Although the rings around Saturn and Neptune have also rearranged themselves in recent years, the report concludes that the Uranian rings are undergoing the most dramatic changes yet observed.
The new data can in principle answer a host of questions about the rings, says team member Imke de Pater, professor of astronomy at the University of California, Berkeley. "We can derive things like the thickness of the rings and how dense they are" as well as the concentration of dust, she says. Researchers also hope to discover new moons that may determine the width of the rings.
Uranus orbits the sun once every 84 years; twice during that time the planet is aligned so that the plane of the rings crosses between the sun and Earth. For about six months, dense groups of centimeter- to meter- (half inch- to yard-) size rocks become invisible from Earth as they cast one another in shadow. But sparser rocks and smaller micron-size (one hundred-thousandth of an inch) dust suddenly shine like smoke in a sunny room.
Seeking to capitalize on the first such ring plane-crossing of the century, which occurred May 3 (followed by a second August 16 and a third that will take place next February), astronomers trained the multifaceted, 10-meter-wide mirror of one of the twin telescopes at Hawaii's Keck Observatory to collect infrared light from the darkened rings. A May 28 snapshot caught the rings on top of one another, but researchers applied an image-processing technique to tease them apart.
Their analysis finds bright rings in places where Voyager detected no dust, such as a region 45,000 kilometers (28,000 miles) from the planet that is about half as bright as other dust rings. Even stranger, they say, the zeta ring discovered by Voyager at 37,000 kilometers out has moved out to about 40,000 kilometers.
The changes observed since the Voyager mission could result from the impact of microscopic meteorites, which produce dust that may linger for decades, de Pater says. Dusty rings may shift constantly, she says, citing changes in the pattern of arcs in Neptune's rings and the distribution of dust in Saturn's rings in recent decades.
"We've slowly come to the realization," she says, "that these impacts are very important to maintaining the rings, and these are things that happen all the time. Sometimes you get a large impact and that may change things completely."