Titan's Surface: Composition and Variability from the Near-Infrared Albedo

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We report observations of Titan covering the 1.075-, 1.28-, 1.6-, and 2.0-μm windows which show variations with the orbital phase, implying variability of the satellite's geometric albedo, as found earlier by other authors. The observations cover the region 4400 to 9600 cm-1 and were taken in June 1991, August 1993, and September 1994, using the Fourier transform spectrometer at the CFH Telescope (Hawaii). The resolving power was 10000, 1000, and 300 for the 1991, 1993, and 1994 spectra, respectively. The data sets cover Titan's Greatest Western Elongation and opposite Conjunctions. We find Titan's geometric albedo to be about the same (0.29) at 1.28 and 1.075 μm. (On the other hand, our observations of Uranus show the 1.28-μm albedo to be lower by 30% than that in the 1.075μm window.) The geometric albedos in the 1.6- and 2.0-μm methane windows on Titan are clearly lower (0.20 and 0.16, respectively). At the four wavelengths, a noticeable decrease in geometric albedo is observed near Greatest Western Elongation with respect to Inferior and Superior Conjunctions. The sum of the existing Titan data point to a bright leading side and a darker trailing one, with a maximum brightness near 120° (±20°) orbital longitude and a minimum near 230° (±20°). Despite important uncertainties in the value of the CH4 absorption coefficients in the window regions, modeling Titan's observed albedos strongly suggests the existence of a surface material with a comparable albedo at 1.075 and 1.28 μm (0.4-0.5) but much darker at 1.6 and 2.0 μm (about 0.2). Titan's surface spectrum appears consistent with that of Hyperion and we suggest that this behavior may be due to the dominant presence of water ice on Titan's surface. The inhomogeneity of the surface implies that some other material (e.g., hydrocarbon lakes) is mixed with water ice in variable proportions.