Characterization of carbon dioxide on Ganymede and Europa supported by experiments: Effects of temperature, porosity, and mixing with water

TL;DR

This study investigates how temperature, porosity, and water mixing affect the spectral features of CO2 ice on icy moons Ganymede and Europa, using laboratory experiments and spectral modeling to interpret observations.

Contribution

It provides new experimental data and spectral analysis on CO2 ice behavior under various conditions relevant to icy moon surfaces.

Findings

Distinct spectral changes observed in pure and mixed CO2 ice.

Number of spectral peaks increases with water mixing.

Desorption behaviors vary with temperature and composition.

Abstract

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 (CO$_2)$ 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. We investigate the cause of the observed shift in the CO$_2$ stretching absorption band experimentally. We also explore the spectral behaviour of CO$_2$ ice by varying the temperature and concentration.} %H$_2$O:CO$_2$ deposition ratios. We analyzed pure CO$_2$ 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 CO$_2$ ice. Pure CO$_2$ and CO$_2$-water ice show distinct spectral changes and desorption behaviours at different temperatures, revealing intricate CO$_2$ and H$_2$O interactions. The number of discernible peaks increases from two in pure CO$_2$ to three in CO$_2$-water mixtures.