Porosity and thermal collapse measurements of H2O, CH3OH, CO2, and H2O:CO2 ices

Abstract

The majority of astronomical and laboratory based studies of interstellar ices have been focusing on ice constituents. Ice structure is a much less studied topic. Particularly the amount of porosity is an ongoing point of discussion. A porous ice offers more surface area than a compact ice, for reactions that are fully surface driven. In this paper we discuss the amount of compaction for four different ices – H₂O, CH₃OH, CO₂ and mixed H₂O : CO₂ = 2 : 1 – upon heating over an astronomically relevant temperature regime. Laser interference and Fourier transform infrared spectroscopy are used to confirm that for amorphous solid water the full signal loss of dangling OH bonds is not a proof for full compaction. These data are compared with the first compaction results for pure CH₃OH, pure CO₂ and mixed H₂O : CO₂ = 2 : 1 ice. Here we find that thermal segregation benefits from a higher degree of porosity.