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The sunlit side of the rings of Uranus (two left images), captured in the infrared by the Keck II telescope in 2004 and 2006 as Uranus approached the point where its rings appear edge-on from Earth. On May 28, less than a month after the first ring-plane crossing on May 3, Keck II captured the unlit side of the rings for the first time (right). The dotted lines show the position of the epsilon (upper line) and zeta (lower line) rings, with other rings indicated. (Courtesy Imke de Pater, UC Berkeley; Heidi B. Hammel, SSI, Boulder; and the W. M. Keck Observatory)

Keck, Hubble provide new view of Uranus' rings
It's a once-every-42-years event as astronomers worldwide peer at the distant planet's rings, which are swinging edge-on to Earth for the first time since their discovery in 1977

| 29 August 2007

As the rings of Uranus swing edge-on to Earth - a short-lived view we get only once every 42 years - astronomers observing the event are getting an unprecedented, glare-free view of the rings and the fine dust that permeates them.

Unlike the rings of Saturn, which were first observed by Galileo nearly 500 years ago, Uranus' rings were discovered in 1977, so this is the first opportunity astronomers have had to observe a Uranus ring crossing, during which the rings can be viewed edge-on instead of from above or below, and it becomes possible to look at the rings from the shadowed side. From that vantage, the normally bright outer rings grow fainter because their centimeter- to meter-size rocks obscure one another, while the dim inner rings get brighter as their material merges into a thin band along the line of sight.

While the Keck II telescope in Hawaii and the Hubble Space Telescope have been looking at the planet for years in anticipation of this event, ground-based telescopes in Chile and Southern California have targeted the planet during the actual ring crossing.

Based on the Keck observations, a team of astronomers led by Berkeley's Imke de Pater reported last week that the rings of micron-sized dust have changed significantly since the Voyager 2 spacecraft photographed the Uranus system 21 years ago. (That report appears in Science Express, the online edition of Science magazine.).

The inner rings are much more prominent than expected, revealing material in otherwise empty regions of the system of rings.

"People tend to think of the rings as unchanging, but our observations show that not to be the case," says de Pater, a Berkeley professor of astronomy. "There are a lot of forces acting on small dust grains, so it is not that crazy to find that the arrangement of rings has changed."

Using the near infrared camera (NIRC2) and adaptive optics on the Keck II telescope on May 28, the team took striking images of the nearly edge-on ring appearing as a bright line bisecting a dim Uranus, which appears dark in the infrared. The observations were conducted during an engineering run by Marcos van Dam, adaptive-optics scientist at the W. M. Keck Observatory, after the installation of a new wavefront sensor.

"The improvements to the adaptive optics systems allowed us to capture unbelievably crisp images of Uranus; it was as if the Keck telescope was orbiting in space," says van Dam.

Trying to spot the blue ring

On Aug. 14, the Hubble Space Telescope also imaged the planet very near the moment when the rings were perfectly aligned with Earth, showing similar features but also including some recently discovered outer rings.

"The outermost ring is not visible in our infrared images," says de Pater's co-author, Heidi Hammel of the Space Science Institute in Boulder, Colo.
"This ring is very blue, and therefore harder to see in the infrared. We may detect it when the rings are fully edge-on and we can observe it for several hours."

With further analysis of the Hubble data, astronomer Mark Showalter of the SETI Institute hopes to detect some of the small moons, and perhaps some not seen before, that shepherd the debris into distinct rings.

"Two little satellites called Cordelia and Ophelia straddle the brightest ring, the epsilon ring, and keep it in place, but people have always assumed there must be a bunch more of these satellites that are confining the nine other narrow rings," Showalter says. "This is the unique viewing geometry that comes along only once in 42 years, when we have a chance of imaging these tiny satellites, because normally they are lost in the glare of the rings. Now the rings are essentially invisible."

Astronomers at the Very Large Telescope (VLT) in Chile, run by the European Southern Observatory (ESO), and at the Palomar Observatory in Southern California, operated by the California Institute of Technology, also observed Uranus during the current crossing. "The VLT took data at the precise moment when the rings were edge-on to Earth," says de Pater.

Until the Voyager 2 spacecraft flew by in January 1986, the rings were known only from the way they temporarily blocked the light of stars passing behind Uranus.

Earth-based images have been too blurry until recently, with the advent of Keck adaptive optics and the Advanced Camera for Surveys of the Hubble telescope. Nevertheless, when the sunlit side of Uranus' rings are in full view of Earth, the densely packed rings reflect so much light that their glare completely dominates the fainter glow from micron-sized dust.

Earth's orbit around the sun permits three opportunities to view the rings edge-on: Uranus made its first ring crossing as seen from Earth on May 3; it made its second crossing on Aug. 16; and will cross for the third and last time on Feb 20, 2008. Though the last ring crossing relative to Earth will be hidden behind the sun, most of Earth's premier telescopes, including Keck, Hubble, VLT, and Palomar, plan to focus on the planet again in the days following Dec. 7, the Uranian equinox. At that point the rings are perfectly edge-on to the sun; a brief period follows during which astronomers will view the dark side of the rings . following which they become illuminated again, and hence not viewable for another 42 years.

The dust belts that Voyager saw differ radically from today's dust distribution, according to co-authors de Pater and Showalter. Most interesting is a broad, inner ring called zeta, whose position today is several thousand kilometers farther from the planet than when it was discovered by Voyager.

"The ring may have moved, or it may be an entirely new ring," notes Showalter.

Similar, dramatic changes in dust distribution have also been observed recently in Saturn's and Neptune's rings. This is not surprising, because gravity keeps the larger ring particles in orbit, but other smaller forces can nudge the tiny dust grains around, de Pater says. These forces include pressure from sunlight, drag produced as the dust plows through ionized plasma around Uranus, and even drag from the planet's magnetic field.

"Impacts into the larger bodies in the system also could knock dust off and create new rings," de Pater says.

"With further observations, the time scales over which these variations occur should provide new insight into the physical processes at work," the authors conclude.

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