Abstract
THE discovery of rings surrounding Uranus1–4 has prompted a careful re-examination of telescopic observations made nearly a year ago with a new type of solid-state area detector called a charge-coupled device or CCD. The extreme narrowness of the individual Uranus rings explains why they had not previously been detected either by visual or conventional photographic methods, for even if they were composed of highly reflecting, ice-coated particles, the rings would likely be hidden in the glare of reflected light from Uranus itself. The average reflectivity (averaged over the disk) of Uranus in the blue and green regions of the spectrum is very high, ∼ 0.55–0.60. At longer wavelengths, however, methane in the planet's atmosphere begins to absorb much of the incident solar radiation, hence the bluish-green appearance of Uranus in the telescope. In the deep methane absorption band at 886 nm the reflectivity of the planet drops to only 0.012, and Uranus would thus appear very dark to any detector which was sensitive to this near-infrared region of the spectrum. It is of particular significance to the ring detection problem that such imaging would suppress the light reflected from the planet relative to that reflected from any nearby solid bodies, such as satellites or ring particles, by a factor of nearly 50. Unfortunately, most detectors have lost nearly all of their detectability at wavelengths longer than 800 nm.
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SMITH, B. Uranus rings: an optical search. Nature 268, 32 (1977). https://doi.org/10.1038/268032a0
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DOI: https://doi.org/10.1038/268032a0
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