Investigations of the thermal evolution of pure and mixed CH4 ices: application to interstellar and interplanetary environments*

In order to provide reliable and detailed laboratory data to interpret observations of spectral features of interstellar ices and of outer solar system frozen worlds, it is necessary to characterize the mid IR features of pure and mixed ices in realistic simulations of interstellar and planetary environments. We studied the characteristics and thermal evolution of thin films of pure CH₄ and CH₄ mixed with water and N₂. These are systems that have been observed in interstellar space (CH₄ and mixed with H₂O) and solar system objects such as Pluto and Triton (CH₄ mixed with N₂). We show how the mixing ratio, temperature, and the thickness affect the IR spectral features of the ice. The nuclear spin conversion in the ice was also studied using IR modes of methane. We found that the segregation and crystallization of CH₄:H₂O and CH₄:N₂ mixtures in response to heating can be clearly characterized by mid-IR features, thus providing a tool to help the interpretation of observations.