IR band profile of CO2 combination modes in H2O ice mixtures

JUpiter ICy moons Explorer (JUICE) is a space mission that aims to make detailed observations of the planet Jupiter and its icy satellites Ganymede, Europa, and Callisto, also known as Galilean moons.

These three moons are continuously exposed to high-energy charged particles and thermal cycling, leading to complex physico-chemical modifications of their icy surface. Carbon dioxide has been widely detected on the surface of Galilean icy moons and represents a key species for understanding surface evolution processes. Recent observations by the James Webb Space Telescope (JWST) resolved for the first time the CO₂ ν₁+ν3 combination mode near 2.7 µm on Europa [1], highlighting the importance of laboratory investigations of CO₂ spectral features in icy environments.

In this work, we present a preliminary laboratory study of the infrared spectral behaviour of CO₂ in H₂O ice mixtures at temperatures relevant for icy moons. Experiments were performed between 50 and 150 K with pure CO₂ ice and H₂O:CO₂ mixtures with ratios of 20:1, 4:1, and 1:1. Pure CO₂ ice exhibits two characteristic features in the 2.7 µm spectral region, with components centred near 2.70 µm and 2.78 µm corresponding to the ν₁+ν₃ and 2ν₂+ν₃ combination bands, respectively [2]. We study how band profiles and the evolution of these bands vary with temperature and mixture composition.

Our results show that CO₂ segregation strongly depends on the H₂O:CO₂ mixing ratio and thermal evolution. In general, segregation processes begin around 60-70 K, producing significant variations in the band profiles. However, the highly diluted 20:1 mixture does not exhibit clear segregation signatures within the investigated temperature range, suggesting a stronger trapping of CO₂ molecules within the water ice matrix. We also observe distinct differences in the sublimation behaviour of pure and mixed CO₂ ice. Pure CO₂ sublimates at approximately 80 K, while in H₂O-rich mixtures, the sublimation occurs at significantly higher temperatures due to the interaction with the water ice matrix. These effects are reflected in the evolution of the infrared band shapes and relative intensities.

This work provides new laboratory constraints for the interpretation of infrared observations of icy moons and contributes to the spectroscopic characterization of CO₂-bearing icy surfaces relevant for current (e.g., JWST) and future (e.g., JUICE) observations of the Jovian system.

The Italian participation to JUICE mission is founded by the Italian Space Agency (ASI). This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0

 

[1] Goldberg, C., Trumbo, S., Brown, M., Davis, R., and Loeffler, M.: Investigating the Origin and State of Europa’s CO2 with Global Observations from JWST, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-434, https://doi.org/10.5194/epsc-dps2025-434, 2025.

[2] Sandford, S. A., and L. J. Allamandola. "The physical and infrared spectral properties of CO2 in astrophysical ice analogs." Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 355, May 20, 1990, p. 357-372. 355 (1990): 357-372.