Blue ring of Uranus linked to sparkling ice

[FLO]

Uranus is circled by a blue ring - a colour that makes it almost unique among planetary rings, which are usually red.

The rare blue hue is probably created by a small moon called Mab that orbits just inside the ring, according to a team of astronomers led by Imke de Pater at the University of California in Berkeley, US.

The ring and the moon were only discovered in 2005 by the Hubble Space Telescope. Now de Pater's team have reported their observations using the Keck telescope on Hawaii. At visible wavelengths the ring shows up clearly enough, but it is invisible to a detector sensitive to infrared light.

The team's explanation is that the outer ring is made of very fine water ice particles, mostly less than a tenth of a micrometre across. They are too small to scatter long-wavelength infrared light, but visible wavelengths can bounce off the particles - especially at the shorter blue end of the spectrum.

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