Methane and Nitrogen Ices on Pluto and Triton: a Combined Laboratory and Telescope Investigation.

In this dissertation, the surfaces of Pluto and Triton were investigated by means of spectroscopy over the visible and near infrared wavelength range from 0.5 to 2.5 mum. In this spectral region, the spectra of both bodies are dominated by solar continuum light reflected from their surfaces, interrupted by absorption bands due to the ices of methane, nitrogen, carbon monoxide, and carbon dioxide, in decreasing order of spectral significance. New spectroscopic observations of Pluto and Triton were made at the University of Arizona's 1.54 meter telescope on Mt. Bigelow and 2.3 meter telescope on Kitt Peak. These data were combined with earlier, unpublished observations and with data from the literature. In particular, the Pluto+Charon spectra span a dozen years from 1983 to 1994, the full 360^circ range of subsolar longitudes, and subsolar latitudes from -10 to +13^circ. Laboratory studies were undertaken to measure the spectral properties of nitrogen and methane ices at temperatures comparable to the surface temperatures of Pluto and Triton. These data were used as inputs to radiative transfer models in order to interpret the telescope spectra. The abundances of various ice species on Pluto and Triton were estimated under a wide range of model assumptions. The relative strengths of different methane absorption bands were examined, revealing an enhancement of weak bands relative to stronger bands. Various multiple scattering effects which might be responsible for the observed enhancement were simulated. The spectrum of Pluto's satellite Charon was shown to be consistent with a composition of dirty water ice. Finally, variations in the spectrum of Pluto with rotational phase were examined and used to constrain the geographic distributions of ices on Pluto's surface.