Characterization of Carbon Dioxide (CO2) on Ganymede and Europa supported by experiments: The effect of temperature, porosity and mixing with water.

The surfaces of icy moons are primarily composed of water ice that can be mixed with other compounds, such as carbon dioxide. With the recent JWST observations (Bockelee-Morvan et al. 2024; Villanueva et al. 2023), infrared spectra of Ganymede and Europa’s terrains have been generated with unprecedented resolution, revealing hitherto undetectable variations of the spectral features with the location on the moon. With such high resolution’s data, not only can we identify the species present (in the present case CO₂) on the surface, but we can also link their subtle variations to the embedding of the molecule into a host material and/or to energetic processes occurring on the icy moon. 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 but also from one terrain to another on the moon itself. We investigated the cause of the observed shift in the CO₂ stretching absorption band experimentally. For this purpose, we explored the spectral behavior of CO₂ ice by varying the temperature and concentration with respect to water, focusing on the CO₂ stretching fundamental band. 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. To confirm the assignment of one of the CO₂ bands, we simulated IR spectra using density functional theory (DFT) methods, exploring the effect of porosity in CO₂ ice. Pure CO₂ and CO₂-water ice show distinct spectral changes and desorption behaviors at different temperatures, revealing intricate CO₂ and H₂O interactions. The number of discernible peaks increases from two in pure CO₂, referred to as  ν3,1 and ν3,2, and to three in CO₂-water mixtures, referred to as ν3,1, ν3,2 and ν3,3.

The different CO₂ bands were assigned to ν3,1 (2351 cm^-1, 4.25 µm) caused by CO₂ dangling bonds (CO₂ found in pores or cracks) and to  ν3,2 (2345 cm^-1, 4.26 µm) due to CO₂ segregated in water ice, whereas  ν3,3 (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 ν3,1 and ν3,3. For Europa, as shown in Fig.1, ν3,1 is located at lower wavelengths due to a lower temperature. The Ganymede data reveal latitudinal variations in CO₂ bands, with  ν3,3 dominating in the pole and  ν3,1 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   ν3,1 band throughout the day, possibly due to an increase in ice cracks or pores caused by large temperature fluctuations during a day on Ganymede.

 

References:

Bockelee-Morvan, D., Lellouch, E., Poch, O., et al. 2024, Astronomy & Astrophysics, 681, A27

Villanueva, G., Hammel, H., Milam, S., et al. 2023, Science, 381, 1305