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  • Getting an Edgy View of the Rings of Uranus

Photonics Spectra
Nov 2007
Ground-based near-IR camera images dark side of rings for first time.

Hank Hogan

Like its neighbor and fellow gas giant Saturn, Uranus is ringed. Although not gaudy, its rings are full of surprises, say researchers.

At the University of California, Berkeley, at the Space Science Institute of Boulder, Colo., at the SETI Institute of Mountain View, Calif., and at the W.M. Keck Observatory in Kamuela, Hawaii, investigators took advantage of an infrequent celestial alignment to observe the dark side of the rings of Uranus. They found that the rings had significantly changed in brightness in comparison with images of the rings acquired by the passing Voyager spacecraft in 1986.


Because Uranus is tilted almost 90° with respect to its orbit (indicated by the compass rose), this is the first view of its rings edge-on since their discovery. Such infrared images, taken with adaptive optics and instrumentation at the Keck Observatory in Hawaii, revealed changes in relative ring brightness and, presumably, in the distribution of ring material. Reprinted with permission of Science.

“The observation that the features we saw did not correspond to anything in the Voyager data was a complete surprise,” said Berkeley professor of astronomy Imke de Pater.

Unlike other planets, Uranus rolls along on its side with its poles pointing at the sun. As a result, its rings can be seen edge-on from Earth only once every 42 years. This year is the first time such edge-on study has been possible since the rings were discovered in 1977.

The researchers used the NIRC2 near-infrared camera and the 10-m telescopes at the observatory. The camera has a 1024 × 1024 array of 27-μm InSb pixels and operates in the range of 0.9 to 5.3 μm. They used adaptive optics to compensate for turbulence in the atmosphere, thereby obtaining clearer images, and combined 30 exposures of 1-min duration to get their data.

In analyzing what they had captured, the scientists extracted the system profile from the edge-on view, which stacks all the rings atop one another. To do this, they used an “onion peel” technique in which they started with the outer edge and determined its intensity. They subtracted this intensity from the entire scan and repeated the process for the next zone inward, continuing until they had a complete profile. The method produced the same results with data taken at different times, de Pater said.

The researchers found that the relative brightness of the rings had changed since Voyager had observed them, with the outermost optically thick ring fading away. The inner rings had brightened considerably, with most increasing in brightness by a factor of 1.5 in the edge-on profile. They attributed these changes to the ring brightness being dominated by micron-scale dust. The orbit of small dust grains can change rapidly in response to sunlight, electromagnetic fields or other nongravitational forces. These results from Uranus indicate that ring systems can change radically over decades.

According to de Pater, when this planetary alignment comes again in several decades, telescopes measuring 30 or 100 m may be available. They may yield better results, but they are not the best way to study the rings.

“Sending a spacecraft would be optimal, of course — with appropriate instrumentation.”

Science, Sept. 28, 2007, pp. 1888-1890.

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