Abstract
We present observations of the Uranian outer ring system at near-infrared and visible wavelengths. Observations with the Keck Telescope were taken in July-August 2007 at 2.12 and 1.63 mu m, when the ring plane was almost edge-on (ring opening angle -0.24). These data showed, for the first time, the mu ring at infrared wavelengths. NIRCam on the James Webb Space Telescope observed Uranus in 2023-2025 at wavelengths 1.4-4.8 mu m and 60-65. Hubble Space Telescope data were obtained between 2003 and 2013 at wavelengths 0.45-0.96 mu m and from to . We confirm that the mu ring is blue and the ring red. Both rings show strong absorption bands at 3 mu m; the mu ring also shows an emission peak at 3.6 mu m. Based upon a combination of the spectral slope and absorption/emission features, the mu ring must be composed of (sub-)micron-sized icy grains. The ring is a dusty ring rich in organics (similar to 10%-15% tholins). The radial profile of both rings is triangular, with an outward extension for the ring and inward for the mu ring. Both rings are optically thin; at 1.5 mu m for the mu ring and for the ring. Based upon the composition and radial extent of the rings we suggest the mu ring to originate via micrometeoroid impacts on the icy moon Mab, and the ring via collisions between, and micrometeoroid impacts on, parent non-icy bodies embedded within this ring.