Electron-Induced Radiolysis of Water Ice and the Buildup of Oxygen

Abstract

Irradiation by energetic ions, electrons, and UV photons induces sputtering and chemical processes (radiolysis) in the surfaces of icy moons, comets, and icy grains. Laboratory experiments, both of ideal surfaces and of more complex and realistic analog samples, are crucial to understand the interaction of surfaces of icy moons and comets with their space environment. This study shows the first results of mass spectrometry measurements from porous water ice regolith samples irradiated with electrons as a representative analogy to water-ice rich surfaces in the solar system. Previous studies have shown that most electron-induced $H_2$O radiolysis products leave the ice as $H_2$ and $O_2$ and that $O_2$ can be trapped under certain conditions in the irradiated ice. Our new laboratory experiments confirm these findings. Moreover, they quantify residence times and saturation levels of $O_2$ in originally pure water ice. $H_2$O may also be released from the water ice by irradiation, but the quantification of the released $H_2$O is more difficult and the total amount is sensitive to the electron flux and energy.