Context. The surfaces of icy moons are primarily composed of water ice that can be mixed with other compounds, such as carbon dioxide. The carbon dioxide (CO₂) stretching fundamental band observed on Europa and Ganymede appears to be a combination of several bands that are shifting location from one moon to another. Aims. We investigate the cause of the observed shift in the CO₂ stretching absorption band experimentally. We also explore the spectral behaviour of CO₂ ice by varying the temperature and concentration. Methods. We analyzed pure CO₂ ice and ice mixtures deposited at 10 K under ultra-high vacuum conditions using Fourier-transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) experiments. Laboratory ice spectra were compared to JWST observation of Europa's and Ganymede's leading hemispheres. The simulated IR spectra were calculated using density functional theory (DFT) methods, exploring the effect of porosity in CO₂ ice. Results. Pure CO₂ and CO₂-water ice show distinct spectral changes and desorption behaviours at different temperatures, revealing intricate CO₂ and H₂O interactions. The number of discernible peaks increases from two in pure CO₂ to three in CO₂-water mixtures. Conclusions. The different CO₂ bands were assigned to ν_3,1 (2351 cm^-1, 4.25 μm) caused by CO₂ dangling bonds (CO₂ found in pores or cracks) and ν_3,2 (2345 cm^-1, 4.26 μm) due to CO₂ segregated in water ice, whereas ν_3,3 (2341 cm^-1, 4.27 μm) is due to CO₂ molecules embedded in water ice. The JWST NIRSpec CO₂ spectra for Ganymede and for Europa can be fitted with two Gaussians attributed to ν_3,1 and ν_3,3. For Europa, ν_3,1 is located at lower wavelengths due to a lower temperature. The Ganymede data reveal latitudinal variations in CO₂ bands, with ν_3,3 dominating in the pole and ν_3,1 prevalent in other regions. This shows that CO₂ is embedded in water ice at the poles and it is present in pores or cracks in other regions. Ganymede longitudinal spectra reveal an increase of the CO₂ ν_3,1 band throughout the day, possibly due to ice cracks or pores caused by large temperature fluctuations.

Non-thermal desorption of interstellar and circumstellar ice mantles on dust grains, in particular ultraviolet photon-induced desorption, has gained importance in recent years. These processes may account for the observed gas phase abundances of molecules like CO toward cold interstellar clouds. Ice mantle growth results from gas molecules impinging on the dust from all directions and incidence angles. Nevertheless, the effect of the incident angle for deposition on ice photodesorption rate has not been studied. This work explores the impact on the accretion and photodesorption rates of the incidence angle of CO gas molecules with the cold surface during deposition of a CO ice layer. Infrared spectroscopy monitored CO ice upon deposition at different angles, ultraviolet irradiation, and subsequent warm-up. Vacuum ultraviolet spectroscopy and a Ni-mesh measured the emission of the ultraviolet lamp. Molecules ejected from the ice to the gas during irradiation or warm-up were characterized by a quadrupole mass spectrometer. The photodesorption rate of CO ice deposited at 11 K and different incident angles were rather stable between 0. and 45°. A maximum in the CO photodesorption rate appeared around 70° incidence deposition angle. The same deposition angle leads to the maximum surface area of water ice. Although this study of the surface area could not be performed for CO ice, the similar angle dependence in the photodesorption and the ice surface area suggests that they are closely related. Further evidence for a dependence of CO ice morphology on deposition angle is provided by thermal desorption of CO ice experiments.

In regions where stars form, variations in density and temperature can cause gas to freeze out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences, indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of A_V is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also more mobile, and this could increase the reactivity within interstellar ices.