Near-infrared spectroscopy of simple hydrocarbons and carbon oxides diluted in solid N-2 and as pure ices: Implications for Triton and Pluto

In this paper are presented near-infrared laboratory spectra of pure ices of CH4, C2H4, C2H6, CO, and CO2, as well as a systematic study of changes in their spectral behavior when isolated in a matrix of nitrogen ice. These studies were prompted by recent low-noise and high-spectral-resolution infrared observations of the surfaces of Triton and Pluto (e.g., D. P. Cruikshank ef al., 1993, Science 261, 742-745; T. C. Owen et al., Science 261, 745-745), The data in this paper permit a sophisticated analysis of the published Pluto and Triton spectra, and will be useful in interpreting future observations as well, Two different techniques were employed for preparing our ice samples: (1) condensation of thin films on a cold window, and (2) growth of crystals from the liquid phase in a closed cryogenic cell, An important result we obtained is that spectra strongly depend on the technique used. With a closed cell, samples are formed under conditions of thermodynamical equilibrium and experiments are perfectly reproducible, Additionally, the samples formed from mixtures of N-2 and CH4, in the closed cell, show characteristics consistent with the N-2:CH4 phase diagram obtained by A, I. Prokhvatilov and L. D. Yantscvich (1982, Sov. J. Low Temp Phys, 9, 94-98). On the other hand, it appeared that this is not at all the case with thin films, Assuming that the surfaces of Triton and Pluto are in thermodynamical equilibrium, the closed cell technique is more appropriate. Finally, the measurements conducted with different closed cryogenic cells show that the peak frequencies of the bands of the CH4 molecule isolated in solid N-2 are shifted with respect to pure CH4 ice and are also dependent on the temperature and crystal phase of solid N-2 (alpha vs beta). These dependencies have been precisely measured, and it is shown how they could be used to determine the CH4 dilution state and the temperature at the surfaces of Triton and Pluto to higher precision than has been previously achieved. (C) 1997 Academic Press.