Blue ring for Uranus
The recently discovered outermost ring of the planet Uranus is bright blue, report Imke de Peter of the University of California at Berkeley and her colleagues. So far only a ring of this color was known from Saturn. Both also contain a moon, which is why the researchers deduce a common formation mechanism.
The unusual color is created because the ring is made up of only correspondingly tiny particles in the micrometer range, which scatter the short-wave light. Normally, such rings also contain chunks up to several meters in size, which reflect longer-wave radiation and make the structures appear red. In the case of Saturn's blue ring, scientists previously concluded that the matter could have come from the still-active moon Enceladus, which lies in the ring. However, this cannot apply to the blue ring of Uranus, since the moon Mab orbiting here is too small for it and is also inactive - so it cannot deliver the particles.
De Pater and her colleagues now suspect that meteorite impacts on the moons release the dust particles. But while larger chunks were being recaptured, various forces caused by solar radiation, magnetic and gravitational fields ensured that the tiny particles were pulled or pushed out of the moon's sphere of influence and thus filled up the ring.
The scientists now also want to evaluate images in the visible light range of other planets and look for blue rings that are not revealed in infrared images. They will take a particularly close look at the blue ring around Uranus in 2007: Since it will then be facing the Earth with its edge, it will appear a hundred times brighter – the best conditions for a thorough investigation.