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Aims. This article aims to provide an alternative method of measuring theporosity of multi-phase composite ices from their refractive indices and ofcharacterising how the abundance of a premixed contaminant (e.g., CO2) affectsthe porosity of water-rich ice mixtures during omni-directional deposition.Methods. We combine optical laser interference and extended effective mediumapproximations (EMAs) to measure the porosity of three astrophysically relevantice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as abenchmarking test of this new laboratory-based method. Results. Byindependently monitoring the O-H dangling modes of the different water-rich icemixtures, we confirm the porosities predicted by the extended EMAs. We alsodemonstrate that CO2 premixed with water in the gas phase does notsignificantly affect the ice morphology during omni-directional deposition, aslong as the physical conditions favourable to segregation are not reached. Wepropose a mechanism in which CO2 molecules diffuse on the surface of thegrowing ice sample prior to being incorporated into the bulk and then fill thepores partly or completely, depending on the relative abundance and the growthtemperature.

Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations