Ringparticle composition

No high-resolution ultraviolet, visible, or infrared spectra of the rings of Uranus were obtained by Voyager 2. The rings were too dark, too narrow, and too optically thin to permit the collection of such data. Color images shuttered at different wavelengths in the visible range of the spectrum could potentially provide some composition information. The rings of Uranus were uniformly dark at all colors. The only cosmically abundant material which matches both the low reflectivity and the uniformly gray color of the Uranus rings is carbon. The five largest satellites of Uranus also have neutral colors, but their reflection spectra indicate that there is water ice present on their surfaces. No indication of water ice is seen in Earth-based spectra of the rings, although the two recently discovered outer rings of Uranus (2003U1R and 2003U2R) appear to be blue and red, respectively [9]. The blueness of 2003U1R and the presence of a satellite (Mab) near its peak brightness bring to mind Saturn's E ring, the only other known ring with a bluish hue. By analogy, Mab could be the source of the ring's material, which is almost certainly dominated by tiny particles of water ice. The redness of 2003U2R is reminiscent of Saturn's G ring and may be indicative of a silicate composition.

The source of the carbon in the main rings of Uranus is very possibly the decomposition of methane (CH4) ice through bombardment by energetic protons [10]. Methane is known to be abundant in the outer solar system, and energetic proton fluxes are sufficiently high to blacken most exposed methane in the inner magnetosphere. It is also possible that part or all of the carbon is in the form of originally elemental carbon from the breakup of carbonaceous meteoroids or asteroids captured into orbit around Uranus.

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