The Outer Dust Rings of Uranus in the Hubble Space Telescope

Between early May and mid-August 2007, the rings of Uranus were oriented nearly edge-on to Earth. This provided an exceedingly rare opportunity to obtain clear views of the system's faint, outer dust rings, μ and ν. We conducted regular imaging of the Uranian system throughout this period with the Wide Field/Planetary Camera on HST. Here we present our initial results on the outer rings. We confirm earlier reports that the μ ring is blue; previously this was based on comparisons between visual data and upper limits at 2.2 μm this could have been the result of an absorption band rather than a uniform trend. However, we find a factor-of-two drop in ring brightness between wavelengths of 0.6 to 0.8 μm this makes the μ ring as blue as Saturn's E ring, suggesting that sub-μm particles predominate. More surprisingly, the ring shows very large variations in brightness with longitude. Variations by a factor of several over scales of tens of degrees are common. No systematic pattern has yet been identified, but it is noted that one ansa of the ring is usually darkest when the other is brightest, suggesting a periodicity with an odd number of cycles. Further investigation is needed to determine the nature of these variations. Because the ring is very broad, one would expect such variations to shear out in a time scale of weeks to months, unless these variations are confined to a much narrower radial range than is the visible ring.