The findings made the planet's outermost ring only the second known blue ring in the solar system, with the first one belonging to Saturn. All other rings around planets Jupiter, Saturn, Uranus and Neptune are reddish.

Perhaps not coincidentally, both the blue rings are associated with small moons, the researchers said in the April 7 issue of the journal Science.

"The outer ring of Saturn is blue and has Enceladus right smack at its brightest spot, and Uranus is strikingly similar, with its blue ring right on top of(Uranus' moon) Mab's orbit," said Imke de Pater, professor of astronomy at 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." he added.

The similarity between these outer rings implies a similar explanation for the blue color, according to the researchers.

Many scientists ascribed Saturn's blue E ring to the small dust, gas and ice particles spewed into Enceladus's orbit by newly discovered plumes on that moon's surface.

But this is unlikely to be the case with Mab, a small, dead, rocky ball no more than 30 km across and one-twentieth the diameter of Enceladus.

Instead, the astronomers suspected both rings 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, a study co-author at NASA Ames Research Center.

The likely scenario to explain Saturn's blue E ring was proposed before plumes were discovered last November when the Cassini spacecraft flew by Enceladus.

As modeled for the E ring, meteoroid impacts on the surface of Enceladus scatter debris into its orbit, probably in a broad range of sizes. While the larger pieces remain within the moon's orbit and eventually are swept up by the moon, smaller particles are subject to subtle forces that push them toward or away from the planet 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 less than a tenth of a micron 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.

As for those red rings around solar planets, although they contain particles that reflect many wavelengths of light, red dominates not only because larger particles up to several meters in diameter are abundant, but also because the material itself maybe reddish, perhaps from iron, said the researchers.

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

The discovery of the blue ring of Uranus came last year after combining ground-based near-infrared observations by the Keck Telescope in Hawaii and visible-light photos taken by the Hubble Space Telescope, according to the researchers. The blue ring peaks in brightness about 97,700 km from the planet's center, exactly at Mab's orbit. Enditem