Sputtering Behavior of Rough, Polycrystalline Mercury Analogs

Abstract

The solar wind continuously impacts on rocky bodies in space, eroding their surface, thereby contributing significantly to the exosphere formations. The BepiColombo mission to Mercury will investigate the Hermean exosphere, which makes an understanding of the precise formation processes crucial for evaluation of the acquired data. We therefore developed an experimental setup with two microbalances that allows us to compare the sputter behavior of deposited thin solid layers with that of real mineral samples in the form of pressed powder. In addition, this technique is used to study the angular distribution of the sputtered particles. Using 4 keV He+ and 2 keV Ar+ ions, the sputter behavior of pellets of the minerals enstatite (MgSiO3) and wollastonite (CaSiO3) is studied, because these minerals represent analogs for the surface of the planet Mercury or the Moon. Pellets of powdered enstatite show significantly lower sputter yields than thin amorphous enstatite films prepared by pulsed laser deposition. 3D simulations of sputtering based on surface topography data from atomic force microscopy show that the observed reduction can be explained by the much rougher pellet surface alone. We therefore conclude that sputter yields from amorphous thin films can be applied to surfaces of celestial bodies exposed to ion irradiation, provided the effects of surface roughness, as encountered in realistic materials in space, are adequately accounted for. This also implies that taking surface roughness into account is important for modeling of the interaction of the solar wind with the surface of Mercury.

New Compound and Hybrid Binding Energy Sputter Model for Modeling Purposes in Agreement with Experimental Data

“…This is not due to impacts realistically occurring at normal incidence in nature, but due to surface roughness leading to locally reduced incidence angles for shallow impinging ions and therefore flattened mass yield distributions. This is discussed in Biber et al (2022) for enstatite irradiation experiments and was previously shown for rough Bo and Be surfaces (Gauthier et al 1990;Roth et al 1991;Küstner et al 1999).…”

“…5. Biber et al (2022) used the in-house-built ray-tracing code SPRAY (Cupak et al 2021) with data from SDTrimSP and atomic force microscope images to discuss the effect of surface roughness on the sputter yield of a powder pellet and a flat, glassy thin film. They found that a rough pressed pellet surface reduces the yield, especially at shallow incident angles (above 45°r elative to surface normal).…”

“…Nevertheless, all the experimental data in Figure 4 show good agreement with the HB-C model close to normal incidence and up to at least 45°. This is relevant for approximating irradiation of realistic, rough surfaces because yield enhancements between a flat and rough surface are generally small for incidence angles below 45° ( Küstner et al 1998;Biber et al 2022). This is not due to impacts realistically occurring at normal incidence in nature, but due to surface roughness leading to locally reduced incidence angles for shallow impinging ions and therefore flattened mass yield distributions.…”

“…In recent years there were several efforts to better constrain the erosion of rocky planetary bodies exposed to highly energetic solar wind ions. This includes investigating the effect of surface roughness (Biber et al 2022) and porosity (Szabo et al 2022b), performing ion irradiation experiments with mass yield measurements (e.g., Hijazi et al 2017;Szabo et al 2018Szabo et al , 2020aBiber et al 2022), and introducing a new surface and bulk binding energy (BBE) model from theory (Hofsäss & Stegmaier 2022;Morrissey et al 2022). In this work, we discuss the parameter of density and its inclusion in SDTrimSP (Mutzke et al 2019), as well as a new hybrid binding energy model that reliably recreates experimental sputter yields completely without the requirement to adjust input parameters.…”

Establishing a Best Practice for SDTrimSP Simulations of Solar Wind Ion Sputtering

“…Previous research has shown that ion sputtering yields are highly dependent on the incidence angle (Behrisch & Eckstein 2007;Szabo et al 2020b), and the SW impact angle is often approximated as either normal (Nénon & Poppe 2020;Killen et al 2022) or 45°angle relative to a flat surface (Jäggi et al 2021). In addition, because the surfaces of airless bodies are not flat but composed of individual grains with varied size distributions, the incoming SW ions impact the surface at many different relative angles, further affecting the yield (Wehner et al 1963;Szabo et al 2018Szabo et al , 2022aBiber et al 2020Biber et al , 2022. This has been modeled as a parallel flow of incident ions, which is equivalent to a cosine distribution of impact angles onto a flat surface (see, e.g., Cassidy & Johnson 2005).…”