Spectroscopy of some ices of astrophysical interest: SO2, N-2, and N-2:CH4 mixtures

Near infrared spectroscopic observations of icy surfaces provide powerful keys to identify specific molecules, and to derive information about the physical and chemical states of the surface ices. In particular, the high spectral resolution recently achievable in astronomical spectra, opens a new insight, but also implies that the complete analysis of these spectra requires careful spectroscopic studies, i.e. clean and systematic laboratory experiments associated with a rigorous interpretation of the spectra. Spectroscopic interpretation is focused on, taking into consideration the specific physical aspects of some ices (molecular solids). It is shown how this analysis allows specific astrophysical problems to be solved. At first, some relevant fundamentals of physics and spectroscopy of molecular solids are presented. The spectroscopy of these solids largely belongs to molecular physics, but also involves solid state effects (Davydov splitting, LO-TO splitting, etc.) which have to be considered to correctly assign spectra, as well as to understand the behaviour of the spectral profile of the bands as a function of various physical parameters. The specific treatment needed to explain the structure of combination and overtone bands occurring in the whole infrared range is particularly focused on. Those theoretical considerations are applied to two different problems concerning surface ices: the first one deals with the identification of two narrow SO2 bands on Io (Schmitt et al., Icarus 111, 70-105, 1994), the second one with the physical state of N-2 ice on Triton and Pluto. In a second step, the first results are presented of a systematic spectroscopic study in the near infrared on the two-phase system N-2:CH4 for CH4 concentrations ranging from 0.1 to 10%. This study was initiated with the view of investigating the question of the physical state of the surface of Pluto. It is shown that it is possible to investigate the N-2:CH4 phase diagram using the spectral profile of both the nu(1)+nu(4) and the nu(3)+nu(4) bands of CH4. Finally, the physical parameters (temperature, crystalline phase, etc.) that are expected to be extracted from a detailed analysis of near infrared observations of icy planetary surfaces are briefly reviewed. Copyright (C) 1996 Elsevier Science Ltd