An observational program was carried out to investigate the spectrum of Pluto at various points on its lightcurve. Spectrophotometry of Pluto in the wavelength range of 5600 to 10500 Angstroms was obtained on four nights covering lightcurve phases of 0.18, 0.35, 0.49, and 0.98. The four phases included minimum light (0.98) and one near maximum light (0.49). The spectra reveal variations in the absorption depths of the methane bands at 6200, 7200, 7900, 8400, 8600, 8900, and 10000 Angstroms. The minimum amount of absorption was found to occur at minimum light.
A model for the surface and atmosphere of Pluto was constructed in an attempt to explain the phase variation observed. The model is based upon a previous photometric two-spot model which was contructed to explain the variations in the lightcurve from 1950 to 1982. Two dark circular spots (46deg and 28deg in radius, both at latitude -23deg, separated by 134deg in longitude) were used to constrain the surface distribution of methane frost on the surface of Pluto. The reflectance properties of the two terrains were modelled with a theory by B. Hapke (J.G.R., v. 86, p. 3039, 1981) which includes the effects of multiple scattering in the surface frost. The particle size and continuum optical depth of the frost particles were allowed to vary between the dark regions inside the spot boundaries and the brighter regions surrounding the spots. The transmission of the atmosphere was calculated using the Mayer-Goody band model.
The model fit to the spectrum required the presence of a frost with the particle sizes on the order of 1-20 mm in order to explain the observed phase dependence of the methane bands. Using only the atmosphere and no surface frost implies a variation in column abundance of 30% within three days. From energy balance considerations this variation in column abundance is not possible. By including the absorption of methane frost on the surface a range of model solutions was obtained. This range yields an approximate limit of 5.5 m-amagats to the amount of gas that can be present and still achieve a good fit to the phase variation of the 7200 Angstrom band. If the atmosphere is removed from the model an equally good fit to the 7200 Angstrom band is obtained.
A major problem with the model is its failure to reproduce the relative absoprtion band depths. The gaseous atmospheric calculation on the other hand can fit the spectrum quite well. Possible explanations include a particle size distribution within a given terrain.
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