6 April 2006 USA: New Ring Around Uranus is Blue, Scientists Find
By Maggie Fox, Health and Science Correspondent
WASHINGTON (Reuters) – The newly discovered outer ring of Uranus is bright blue, for the same reason the Earth's sky is blue – it is made up of tiny particles, astronomers said on Thursday.
It is "strikingly similar" to Saturn's outer ring, which astronomers last month confirmed was probably generated by one of the planet's moons, Enceladus.
Like Saturn's ring, the Uranus ring also has a small moon in it, called Mab. But Mab is too small and too cold to be spewing a geyser of ice that contributes to the ring as Enceladus is now believed to be doing.
"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 Mab's orbit," said Imke de Pater, a professor of astronomy at the University of California Berkeley, who helped lead the study.
"I think there is no chance that the blue ring is caused by geyser activity," added de Pater, whose report is published in Friday's issue of the journal Science.
"We don't know what the composition of the particles is."
Mab is probably covered with water ice, like the other moons of Uranus, but that has nothing to do with the ring's color, said de Pater.
"They are blue because they are tiny particles," De Pater said in a telephone interview.
"That is same reason why the sky is blue. You have little particles that scatter light as blue light."
Most other rings around planets in the solar system are red, because of the size of their particles. This is why Uranus's outer ring was missed for so long – scientists were looking for it in the infrared light spectrum.
The scientists believe that meteoroids hitting Mab's surface throw up debris. The larger pieces remain in the moon's orbit and eventually are swept up, but smaller ones drift around more and eventually make up the ring.
The blue ring was only seen after researchers compared notes from near-infrared observations by the Keck Telescope in Hawaii and visible-light photos taken by the orbiting Hubble Space Telescope.
Now they are looking for rings around other planets, including Mars. Rings are always easier to see when they are edge-on, de Pater said. "The interesting thing with Uranus is that in 2007 the rings will appear edge-on," she said.
"In 2007 the rings will be 100 times brighter."
A second paper, published in Geophysical Research Letters, shows that Saturn's prominent "A" ring contains more debris than once thought.
The Cassini-Huygens spacecraft now orbiting the planet has sent back ultraviolet images that show the ring is 35 times thicker than originally thought.
A team at the University of Colorado at Boulder said the particles making up the ring range in size from dust grains to chunks as big as buses, and orbit in long stringy clumps.
The same spacecraft helped planetary scientists discover the geyser on Enceladus, leading to speculation that the moon has a liquid water source under its frozen surface.