A Geoscientific Review on CO and CO2 Ices in the Outer Solar System

Laboratory Studies of the Clathrate Hydrate Formation in the Carbon Dioxide-Water Mixtures at Interstellar Conditions

This study investigates the formation of carbon dioxide clathrate hydrates under conditions simulating interstellar environments, a process of significant astrophysical and industrial relevance. Clathrate hydrates, where gas molecules are trapped within water ice cages, play an essential role in both carbon sequestration strategies and understanding of the behavior of ices in space. We employed a combination of Fourier Transform Infrared (FTIR) spectroscopy, mass spectrometry, temperature-programmed desorption (TPD), and Density Functional Theory (DFT) calculations to explore thin films of H2O:CO2 ice mixtures with varying CO2 concentrations (5-75%) prepared by vapor deposition at temperatures ranging between 11 and 180 K. The study revealed the influence of CO2 concentration and deposition temperature on the formation mechanism of diverse structures, including clathrate hydrates, polyaggregates, and segregated CO2 groups. Spectral features associated with CO2 encapsulation in clathrate hydrates were observed at 2335, 2349, and 3698 cm-1 in the 5 and 15% mixtures after deposition at 11 K and after warming at temperatures above 100 K. The observed increase in CO2 sublimation temperature to 145-155 K and co-condensation of CO2 molecules at 172 K with water molecules at a pressure of 0.5 μTorr can be attributed to the encapsulation of CO2 molecules within the robust hydrogen-bonded framework of the clathrate cages under specific conditions. These findings enhance our understanding of the intricate processes involved in clathrate and hydrate formation in CO2 and H2O mixtures, shedding light on their physical properties and the dependence of their specific characteristics on the formation method.

Ozone production in electron irradiated CO2:O2 ices

The detection of ozone (O₃) in the surface ices of Ganymede, Jupiter's largest moon, and of the Saturnian moons Rhea and Dione, has motivated several studies on the route of formation of this species. Previous studies have successfully quantified trends in the production of O₃ as a result of the irradiation of pure molecular ices using ultraviolet photons and charged particles (i.e., ions and electrons), such as the abundances of O₃ formed after irradiation at different temperatures or using different charged particles. In this study, we extend such results by quantifying the abundance of O₃ as a result of the 1 keV electron irradiation of a series of 14 stoichiometrically distinct CO₂:O₂ astrophysical ice analogues at 20 K. By using mid-infrared spectroscopy as our primary analytical tool, we have also been able to perform a spectral analysis of the asymmetric stretching mode of solid O₃ and the variation in its observed shape and profile among the investigated ice mixtures. Our results are important in the context of better understanding the surface composition and chemistry of icy outer Solar System objects, and may thus be of use to future interplanetary space missions such as the ESA Jupiter Icy Moons Explorer and the NASA Europa Clipper missions, as well as the recently launched NASA James Webb Space Telescope.