Knowledge Bank

Frozen CO$2$ is, after water, one of the main components of cometary nuclei. The structure of CO$2$ ice in interaction with water ice depends on several factors, like deposition scheme, temperature or density. We present an investigation on ice mixtures of water and CO$2$ and their variation with temperature, based on temperature programmed desorption (TPD) and infrared spectroscopy measurements. Two slightly different CO$2$ structures are shown to exist, and their relative concentration is found to depend critically on the deposition sequence and temperature of the sample.