Electron‐Induced Radiolysis of Water Ice and the Buildup of Oxygen.

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 H2 ${\mathrm{H}}_{2}$O radiolysis products leave the ice as H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$ and that O2 ${\mathrm{O}}_{2}$ 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 O2 ${\mathrm{O}}_{2}$ in originally pure water ice. H2 ${\mathrm{H}}_{2}$O may also be released from the water ice by irradiation, but the quantification of the released H2 ${\mathrm{H}}_{2}$O is more difficult and the total amount is sensitive to the electron flux and energy. Plain Language Summary: The surface of any airless body exposed to space is irradiated by charged and neutral particles. This irradiation can alter the surface and trigger chemical reactions (so‐called radiolysis) and is an important factor in the context of icy satellites and their environment in the solar system. This paper presents one approach to better understand these irradiation processes by irradiating porous water ice samples with electrons at laboratory conditions representative of the icy moons of Jupiter. We find that most of the radiolysis products of water leave the ice as H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$, while some of the produced O2 ${\mathrm{O}}_{2}$ remains trapped in the irradiated ice. This could help explain remote measurements of the surfaces of Europa and Ganymede. Water may also be released from the irradiated ice. Key Points: Laboratory irradiation experiments are crucial to understand the surfaces of icy bodies in spaceFirst mass spectrometer results from water ice regolith samples irradiated with electrons are shownThe dominant radiolysis products released from the ice samples are H2 ${\mathrm{H}}_{2}$ and O2 ${\mathrm{O}}_{2}$, with a fraction of the O2 ${\mathrm{O}}_{2}$ being retained in the ice [ABSTRACT FROM AUTHOR]