Characterization of the Outer Uranian Rings in the Visual and Near‐IR Using Keck, JWST, and HST Observations

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 μm, when the ring plane was almost edge‐on (ring opening angle –0.24). These data showed, for the first time, the μ ring at infrared wavelengths. NIRCam on the James Webb Space Telescope observed Uranus in 2023–2025 at wavelengths 1.4–4.8 μm and 60–65. Hubble Space Telescope data were obtained between 2003 and 2013 at wavelengths 0.45–0.96 μm and from to . We confirm that the μ ring is blue and the ring red. Both rings show strong absorption bands at 3 μm; the μ ring also shows an emission peak at 3.6 μm. Based upon a combination of the spectral slope and absorption/emission features, the μ ring must be composed of (sub‐)micron‐sized icy grains. The ring is a dusty ring rich in organics (∼10%–15% tholins). The radial profile of both rings is triangular, with an outward extension for the ring and inward for the μ ring. Both rings are optically thin; at 1.5 μm for the μ ring and for the ring. Based upon the composition and radial extent of the rings we suggest the μ 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.