Mixed ices of carbon dioxide (CO₂) and water (H₂O), prevalent in cometary nuclei, solar system satellites, and interstellar dust particles, have posed a persistent challenge in understanding the fundamental behaviors of carbon dioxide in these icy environments. Specifically, the intriguing shift in the CO₂ stretching fundamental band observed on icy moons like Europa and Ganymede has remained enigmatic. In this study, we undertake a comprehensive investigation to elucidate this phenomenon by characterizing carbon dioxide on Ganymede and Europa through temperature-dependent spectral analyses.

Laboratory experiments involve the study of pure CO₂ ice and its mixtures with water at varying concentrations, mimicking astrophysical conditions. Utilizing temperature-programmed desorption (TPD) and Fourier-transform infrared spectroscopy (FTIR) techniques, coupled with Gaussian deconvolution of obtained spectra, we explore the molecular interactions underlying the observed spectral shifts. Our experiments at 10 K under ultra-high vacuum conditions replicate interstellar medium conditions, providing crucial insights into CO₂ behavior in icy environments.

In the discussion, a detailed overview is presented of the positions and behaviors of the fitted Gaussians for the asymmetric stretching bands of CO₂ in pure form and when co-deposited with water. The temperature-dependent evolution of these bands reveals intricate dynamics, emphasizing the influence of CO₂ concentration on the shifting positions and shapes of the spectral features.

The conclusions highlight key findings, including the complex nature of CO₂ interactions within water ice, manifested in the emergence of multiple peaks at different concentrations. The thermal desorption analyses unveil distinct desorption behaviors, shedding light on the interactions during ice phase transitions. Importantly, our laboratory results align with observations from the James Webb Space Telescope (JWST), providing a synergistic approach to understanding CO₂ behavior on icy worlds. The variations observed in Ganymede’s spectra across latitudes and longitudes further corroborate the influence of temperature and UV irradiation on CO₂ dynamics within the ice.

This study contributes valuable insights into the molecular intricacies of CO₂ in mixed ices, bridging laboratory experiments with observational data. The synergy between these approaches promises a more profound understanding of CO₂ behavior in diverse astrophysical environments, advancing our knowledge of icy worlds in our Solar System and beyond.

Comets are celestial objects that are thought to have formed at the outer edges of our solar system. The ices they contain is thought to be preserved since their formation in the molecular cloud phase, before the formation of the sun. The formation of ices in the molecular cloud occur via the ice mantle accretion rocess, by condensation of molecules onto the cold dust surfaces. Because of the very cold temperatures, the water molecules accreting onto dust are expected to form highly porous ice, as it occurs in the laboratory simulations. The icy grains encountering each other will form larger bodies, such as comets, and the porous ices plus the coagulation of icy grains leads to a fluffy cometary ice structure. This study explores the feasibility of porosity measurements by means of ultrasonicmeasurements in these ices. A new, small liquid N2 cooled vacuum chamber was used to perform the experiments with different type of ices. strophysical ices were grown on a methacrylate substrate on which an ultrasonic transducer was mounted that sent and received ultrasonic signals. Experiments with H2O (water) and CH3OH (methanol) were done by deposition from a gas phase while also H2O (water) and C2H5OH (ethanol) were frozen on top of the methacrylate from their liquid phase. These two type of experiments required an adapted version of the setup that allowed for experiments that can freeze ice from its liquid state at atmospheric pressure after which the rest of the experiment could take place in the vacuum chamber. From this experiment, the speed of sound in water ice could be determined as a function of temperature. A linear trend for a decreasing temperature in water ice was found with slope 2.5494m/(s°C) and an offset at 0°C of 3616.86m/s for a temperature range from 0°C to -100°C. Results show that propagation of ultrasonic waves in vacuum chamber-grown ice encounters too much attenuation that prevented a distinct signal to be detectable by the transducer.