Surface charging of thick porous water ice layers relevant for ion sputtering experiments

Abstract

We use a laboratory facility to study the sputtering properties of centimeterthick porous water ice subjected to the bombardment of ions and electrons to better understand the formation of exospheres of the icy moons of Jupiter. Our ice samples are as similar as possible to the expected moon surfaces but surface charging of the samples during ion irradiation may distort the experimental results. We therefore monitor the time scales for charging and discharging of the samples when subjected to a beam of ions. These experiments allow us to derive an electric conductivity of deep porous ice layers. The results imply that electron irradiation and sputtering play a non-negligible role for certain plasma conditions at the icy moons of Jupiter. The observed ion sputtering yields from our ice samples are similar to previous experiments where compact ice films were sputtered off a micro-balance.

Sputtering of water ice films: A re-assessment with singly and doubly charged oxygen and argon ions, molecular oxygen, and electrons

“…The MEFISTO test facility for space instrument calibration consists of a vacuum chamber and an electron-cyclotron-resonance ion source (Marti et al, 2001). We also used this facility for the sputtering experiments with a deep porous ice sample described in (Galli et al, 2016). We did not insert thick ice samples into the chamber for the present study.…”

“…Depending on the specific moon to be studied, also salts, nonwater ices, silicates, and a frost layer may be present (Calvin et al, 1995;Domingue and Verbiscer, 1997;Grundy et al, 1999;Johnson et al, 2004;Shi et al, 2010). In Galli et al (2016) we presented experiments conducted with a 0.9 cm deep sample of icy regolith with a density of 0.3 g cm −3 . That approach allows us to conduct sputtering experiments with a thick porous sample.…”

“…Either the residual gas pressure in the vacuum chamber must be monitored to reveal a pressure rise (method used by Vidal et al (2005); Galli et al (2016)) or a cooled microbalance or any similar device must be mounted opposite the irradiated ice surface to collect a part of the emitted particles.…”

Water Ice Radiolytic O2, H2, and H2O2 Yields for Any Projectile Species, Energy, or Temperature: A Model for Icy Astrophysical Bodies

“…Since the early work of Brown et al () demonstrating that the sputtering yields of electrical insulators such as water ice exhibit a square dependence on the electronic stopping power (Baragiola et al, ), quantitative models of ice sputtering versus projectile species, energy, and ice temperature have been developed (Fama et al, ) and applied to estimate sputtering of Saturn's rings and icy satellites (Johnson et al, ), and of Europa (Cassidy et al, ; Plainaki et al, ) and Ganymede (Plainaki et al, ). However, while these models approximate the total amount of material ejected from the irradiated ice (e.g., (Galli et al, ), they do not quantify versus projectile species and energy the chemical composition of the ejecta, a major fraction of which can consist of H 2 and O 2 (Bar‐Nun et al, ; Brown et al, ; Johnson et al, ; Teolis et al, ). Various chemical models for the O 2 component have been proposed (e.g., Johnson et al, , ; Orlando & Sieger, ; Petrik et al, )), but these have not been applied to quantify the species and energy dependence of O 2 radiolysis at icy objects because they contain multiple unknown parameters.…”

First experimental data of sulphur ions sputtering water ice

“…This discussion only considered the effect of sulphur sputtering for the ejected particles and thus for Europa's atmosphere. The effects of sulphur ion irradiation on the chemical and physical properties of Europa's surface will be studied in future tests with chemically more complex ice films and with thick ice samples (Galli et al, 2016).…”