Study of H2O/CO2 Ices of Astrophysical and Planetary Interest

Ices of mixtures of H2O and CO2 can be found in different astrophysical environments, like cometary nuclei or making part of solar system bodies. Water and carbon dioxide can associate in different ways in the solid state, depending on the temperature, relative concentration and process of formation of the ice. Infrared spectroscopy provides a sensitive tool to study different structures of these systems. We have prepared ices of H2O and CO2 by admitting the corresponding gases into a high-vacuum, low-temperature chamber, where the solids are formed by deposition on a substrate. The samples are studied by transmission and reflection-absorption infrared spectroscopy, and the gas content in the chamber is monitored using mass spectrometry. The ices are formed with varying amounts of water and carbon dioxide, and their spectra are taken at a range of temperatures of astrophysical relevance. Four deposition schemes are used, according to whether the gases are admitted sequentially or simultaneously. Our study concentrates on the CO2 spectral features, whose changes reveal details of the sample characteristics and type of water-carbon dioxide association. A crystalline type of CO2 is dominant in sequentiallly prepared samples at low temperature and high carbon dioxide relative concentration, and is evaporated when heating at 105 K. On the ohter hand, a non crystalline CO2 which enters the amorphous water structure, remains embedded until the temperature of water crystallization, ~ 165 K, and may be the dominant phase in co-deposited samples of low carbon dioxide relative content.