Electron-induced radiolysis and sputtering on the surface of icy moons: insights from laboratory experiments

Abstract

The surfaces of Jupiter's icy moons are continually irradiated by charged particles from the Jovian plasma environment. This irradiation triggers chemical reactions in the surface ice and also acts as an atmospheric release process. Remote observations, theoretical modelling, and laboratory experiments must be combined to understand
this plasma-ice interaction. Here, we present new experiment results concerning the chemistry of irradiated water ice samples relevant for icy moons and other icy objects in the solar system. 

Introduction

The University of Bern is developing the neutral gas mass spectrometer for ESA's Jupiter Icy moons Explorer (JUICE, Grasset et al. 2013), 
planned to reach the Jupiter system in 2029. We therefore strive to fill knowledge gaps about the basic physics of the surfaces and atmospheres of Jupiter’s icy moons before the arrival of JUICE. We combine the available facilities for developing and calibrating mass spectrometers and ion/electron spectrometers (Marti et al. 2001) with the sample preparation techniques and diagnostics of the Planetary Imaging Group (Pommerol et al. 2019).

Experiment setup

To study the effects of electrons irradiating water ice, we subjected a variety of ice samples (thin amorphous ice films and macroscopic samples of porous ice with customizable grain size) to an electron beam of energies between 200 eV and 10 keV at pressures and temperatures representative for the surfaces of Jupiter's icy moons. The physical and optical properties of these macroscopic ice samples make them realistic analogues for planetary surfaces beyond the ice line. The effect of chemical impurities in the water ice, such as NaCl, can also be investigated. The particles released from the ice were monitored with a newly designed time-of-flight (TOF) mass spectrometer and (in the case of the water ice film) with a microbalance. 

Preliminary results

Electron irradiation of pure water ice results mostly in the creation and release of H₂ and O₂ from H₂O in a stoichiometric 2:1 ratio, which is in agreement with the results based on an older quadrupole mass spectrometer (Galli et al. 2018). This seems to hold true for any type of water ice sample, independent of grain size and crystallinity. We also derive upper limits for rare radiolysis products (such as OH and H₂O₂) and the time scales for the build-up and release of radiolysis products. The O₂ release lags behind the immediate H₂ release by typically ~ 10 s and is reminiscent of the time-dependent sputtering yield of O₂ from water ice upon ion irradiation (Teolis et al. 2005). This delayed O₂ release has implications for the O₂/H₂O ratio observed at the surface of icy objects in the solar system, such as Ganymede, Europa, Callisto (Calvin et al. 1996, Spencer and Calvin 2002), and 67P/Churyumov-Gerasimenko (Bieler et al. 2015).