Electron-induced radiolysis of water ice and the buildup of O2

Tinner, Chantal; Galli, André; Bär, Fiona; Pommerol, Antoine; Rubin, Martin; Vorburger, Audrey; Wurz, Peter (2023). Electron-induced radiolysis of water ice and the buildup of O2 (In Press). In: EGU General Assembly 2023. Wien. 23.-28.04.2023. 10.5194/egusphere-egu23-7540

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Irradiation by energetic ions, electrons, and UV photons induces sputtering and chemical processes (radiolysis) on the surfaces of icy moons and comets. Radiolysis of water ice has important implications for the chemistry and evolution of these celestial bodies. In this work, we carried out a series of laboratory experiments to investigate the products of radiolysis and the retention of O2 in porous water ice samples when irradiated with high-energy electrons.

To conduct our experiments, we irradiated two types of water ice samples with high energy electrons (0.5 keV to 5 keV ) and measured the resulting chemical species using time of flight mass spectrometry. The experiments were performed under conditions replicating the icy moons’ surface conditions ( K and -7 mbar). Our results showed production of H2 and O2 radicals, but other predicted radiolysis species, such as H2O2 and O3, were not detected so far; their abundances remain below 0.005 by number compared to the release of O2. This is in contrast to previous studies, which have reported the production of OH and H2O2 through the radiolysis of water ice.

We also studied the retention of oxygen in the ice. By computing the timescales of rise for the O2 signal upon irradiation, we observed that it rises faster for non-pristine (follow-up) ice irradiations. This suggests that O2 (or an O2 precursor) produced during the first irradiation can be retained in the ice.

For some irradiations, the electron energy and current were chosen higher to provoke the water ice's sublimation. The water release showed different properties depending on the porosity and grain size of the irradiated ice.

Overall, our results contribute to our understanding of the radiolysis of water ice and its role in the chemistry and evolution of ice-covered bodies in the solar system. Further studies will be needed to fully understand the factors that influence the production and retention of different chemical species during the radiolysis of water ice.

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