"The outer ring of Saturn is blue and has Enceledus right smack at its brightest spot, and Uranus is strikingly similar, with its blue ring right on top of Mab's orbit," said lead research author Imke de Pater of the University of California, Berkeley. "The blue color says that this ring is predominantly submicron-sized material, much smaller than the material in most other rings, which appear red."

Reporting in the April 7 issue of the journal Science, de Pater and colleagues said the blue color of the outer rings of the two gas-giant planets suggest a similar source - although moons such as Enceladus and Mab could not be that source.

Recent research has linked Saturn's blue E ring to the small dust, gas and ice particles spewed by Encedadus from the newly discovered plumes on its moon's surface. The problem is that Mab could not duplicate this phenomenon. It is a small, dead, rocky ball about 15 miles across, or only 5 percent of the diameter of Enceladus.

Instead, the astronomers wrote, both rings probably owe their blue color to subtle forces acting on dust in the rings that allow smaller particles to survive while larger ones are recaptured by the moon.

"We know now that there is at least one way to make a blue ring that doesn't involve plumes, because Mab is surely too small to be internally active," said Mark Showalter of the SETI Institute in Mountain View, Calif. He and astronomer Jack Lissauer of NASA's Ames Research Center, also in Mountain View, discovered Mab using Hubble Space Telescope images in 2003.

The likely scenario to explain Saturn's blue ring was proposed before plumes were discovered last November as the Cassini spacecraft flew by Enceladus. Astronomers have modeled meteoroid impacts on Enceladus scattering debris into its orbit, and creating the E ring.

The larger pieces of impact debris remain within the moon's orbit and eventually are swept up by Enceladus, smaller particles are subject to subtle forces that push them toward or away from Saturn out of the moon's orbit. These forces include pressure from sunlight, magnetic torques acting on charged dust particles and the influence of slight variations in gravity due to the equatorial bulge of Saturn.

The net result is a broad ring of smaller particles, most of them less than one-tenth of a micron (millionth of a meter) across - a thousandth the width of a human hair - that scatter and reflect predominantly blue light.

"This model can be transferred directly to what we now see in Uranus," de Pater said, although some details of the process remain unknown.

All other rings - those around Jupiter, Saturn, Uranus and Neptune - are reddish. Though they contain particles of many sizes that reflect many wavelengths of light, red dominates not only because larger particles - many microns to meters across - are abundant, but also because the material itself may be reddish, perhaps from iron.

�Arguing by analogy, the two outermost rings - the two rings that have satellites embedded in them - are both the blue rings,� Showalter said. �That can't be coincidental; there has to be a common thread of dynamics that is causing both of these phenomena.�

The astronomers discovered the blue ring after combining ground-based near-infrared observations by the Keck Telescope in Hawai`i and visible-light photos taken by the Hubble Space Telescope. They also have observed Uranus since 2000 with the second-generation NIRC2 infrared camera using the adaptive optics system on the Keck II telescope.

In August 2005, they acquired 30 new images of the planet in hopes of seeing new features as the ring plane moves edge-on to Earth, as well as numerous visible-light images of Uranus between 2003 and 2005 with Hubble's Advanced Camera for Surveys.

The new ring remained elusive until the team combined many images, allowing faint features to stand out from the background. Last December 2005, they reported finding two new rings - Uranus's 12th and 13th - and two new moons, Mab and Cupid, numbers 26 and 27. The blue ring peaks in brightness about 97,700 kilometers (67,575 miles) from the planet's center - exactly at Mab's orbit.

Further analysis showed the outer ring can be seen in visible light, but it is invisible in the near-infrared - meaning it must be blue. The analysis also showed Mab could not be seen in the infrared, so it probably is covered with water ice, like the other outer moons of Uranus - and it probably is Uranus's smallest moon.

"If Mab would have been similar to Uranus' other inner moons, we would have seen it," said co-author Seran Gibbard of Lawrence Livermore National Laboratory in California. This implies �that it probably is covered by water-ice, like Uranus' classical moons.�

The astronomers said they plan further observations of Uranus to discover more about the rings as well as about the planet's surface - particularly in 2007, when the rings will appear edge-on to Earth, and the fainter of the rings should become more visible.

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