2009ApJ...690..486M

Ices of mixtures of carbon dioxide and methanol have been studied in a range of temperatures relevant for star-forming regions, comets, polar caps of planets and satellites, and other solar system bodies. We have performed temperature-programmed desorption measurements and recorded IR spectra of various types of samples. The presence of two slightly different structures of CO₂ is manifest. A distorted CO₂structure is characterized by bandshifts between 5/cm (ν₃) and 10/cm (ν₂) with respect to normal CO₂. If the samples are heated above 130 K, the distorted CO₂sublimates and only the normal structure remains. The latter can stay trapped until the sublimation of crystalline methanol (150 K). The desorption energy (E _d∼ 20 kJ/mol) of CO₂ from methanol ice, and the specific adsorption surface area (6 m2/g) of amorphous CH₃ OH ice, have been determined. CO₂ does not penetrate into crystalline ice. Whereas the desorption energy is similar to that of CO₂/H₂ O samples, the specific surface of methanol is much smaller than that of amorphous solid water (ASW). The interaction of CO₂ molecules with water and methanol is similar but ices of CH₃OH are much less porous than ASW. The inclusion of CO₂ into previously formed ices containing these two species would take place preferentially into ASW. However, in processes of simultaneous deposition, methanol ice can admit a larger amount of CO₂ than water ice. CO₂/CH₃OH ices formed by simultaneous deposition admit two orders of magnitude more CO₂ than sequentially deposited ices. These findings can have direct relevance to the interpretation of observations from protostellar environments (e.g., RAFGL7009S) and comet nuclei.