Spectrophotometric observations at ultraviolet, visible, and near-infrared wavelengths of five of Saturn's satellites-Tethys, Dione, Rhea, Titan, and lapetus-and the equatorial band of Saturn were made during the 1968 and 1969 apparitions. Spectral resolution, achieved by using a series of narrow-band interference filters, ranged from 0.02 in the ultraviolet to 0.05 in the infrared. The 60-inch (152 cm) and 100-inch (254 cm) telescopes on Mount Wilson were used. All satellites except Titan show evidence of synchronous rotation. Tethys, Dione, and Rhea all show some variation in brightness with orbital phase, their leading sides being 1() 20 percent brighter than their trailing sides. No change in Titan's brightness with phase was observed. The large change in Iapetus's brightness with orbital phase was confirmed. lapetus shows significant variation in its spectrum with orbital phase, with its darker side "redder" than the brighter side by at least 15 percent between 0.4 and 0.8 . All of the satellites except Titan have similar reflection spectra-relatively constant from 0.4 to 0.8 and then decreasing from 0.8 to 1.1 M Dione shows the largest decrease in the infrared with a decrease of about 15 percent from 0.8 to 1.0 . The reflection spectra of these four satellites are consistent with what is known of water and ammonia frost spectra in the spectral range 0.4-1.1 . The satellite spectra are not similar to spectra for most ferrosilicates; nor are they similar to spectra of the Moon, Mars, or the Galilean satellites. More surprisingly, the satellite spectra in this spectral region are quite different from the spectrum of Saturn's rings, where water ice has been identified in infrared spectra. This difference implies the presence of some materials in the rings which are absent on the satellite surfaces. Titan's spectrum is also different from the spectra of the other satellites, but it is similar to the spectrum of Saturn's equatorial belt. This suggests that material present in the optical surface of Saturn's belt is similar to materials on Titan, perhaps on Titan's surface. Methane absorptions affect both the belt spectrum and Titan's spectrum beyond 0.6 . The Saturn and Titan spectra differ in the ultraviolet. Rayleigh scattering in gases above an ultraviolet haze layer (about 10 km-atm H2) can account for the ultraviolet turnup in Saturn's spectrum. There is much less Rayleigh scattering on Titan, which suggests a thin ( m-atm) atmosphere, through which the surface may be visible.
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McCord et al. (1971) studied this question.