Irradiation of water ice samples in the laboratory: Implications for surface processes on icy moons and comets

The surfaces of icy bodies in the solar system are continually irradiated by charged particles from planetary magnetospheres or from the solar wind. This irradiation triggers chemical reactions in the surface ice and releases particles to form an atmosphere. Remote observations, theoretical modelling, and laboratory experiments must be combined to understand this plasma-ice interaction. This presentation takes a closer look at the latter: we present new results from irradiated water ice samples and discuss their relevance for icy moons and comets. We concentrate on the chemical and physical alterations in water ice samples upon irradiation with electrons. The physical and optical properties of these macroscopic ice samples make them realistic analogues for planetary surfaces. We monitor the irradiation-induced alterations in the ice samples with spectral cameras in the visible and near-infrared wavelength range and with a dedicated new time-of-flight mass spectrometer.

Previous results obtained with an old quadrupole mass spectrometer (Galli et al. 2018, Planetary and Space Sciences) indicated that most water escaping the ice sample upon electron irradiation does so in the form of the radiolysis products H₂ and O₂. The freshly produced H₂ appeared to leave the porous water ice sample immediately whereas the O₂ escape slowly increased before reaching a steady-state ratio of 1:2 compared to H₂. With the new mass spectrometer, we investigate the release and storage of radiolysis products at a higher temporal resolution and sensitivity for a variety of ice sample porosities and thicknesses. We pay special attention to less abundant radiolysis products such as H₂O₂ and to the O₂/H₂O ratio in the irradiated water ice layer.