Pyroxenes ((Ca, Mg, Fe, Mn) 2 Si 2 O 6 ) belong to the most abundant rockforming minerals that make up the surface of rocky planets and moons. Therefore sputtering of pyroxenes by solar wind ions has to be considered as a very important process for modifying the surface of planetary bodies. In order to quantify this effect, sputtering of wollastonite (CaSiO 3 ) by He 2+ ions, which are seen as a very prominent contribution to solar wind potential sputtering, was investigated. Thin films of CaSiO 3 deposited on a quartz crystal microbalance were irradiated allowing precise in-situ real time sputtering yield measurements. Experimental results were compared with simulations with the code SDTrimSP, which were improved by adapting the used surface binding energy.On a freshly prepared surface He 2+ ions show a significant increase in sputtering compared to equally fast He + ions. The yield, however, decreases exponentially with fluence, reaching steady state at considerably lower values after sputtering of the first few monolayers.Experiments using Ar 8+ ions show a similar behavior and are qualitatively explained by a preferential depletion of surface oxygen due to potential sputtering. A corresponding quantitative model is applied, which is able to reproduce the observed potential sputtering behavior of both He and Ar very well. The results of these calculations support the assumption that mainly O atoms are affected by potential sputtering. We conclude that the defect-mediated model of potential sputtering is also well-suited for CaSiO 3 .

The surfaces of airless bodies are covered by a porous regolith, a loose ensemble of rocks and dust grains, due to a multitude of erosion and impact processes over billions of years (McKay et al., 1991). Its upper layers determine how those planetary bodies are observed as their surface morphology strongly affects optical properties (Hapke, 2008;Vernazza et al., 2012). The porous structure of stacked grains will also influence the interaction of any planet, moon, or asteroid with its environment or precipitating radiation. Especially the effect of porosity on thermal conductivity has been of recent interest (Ryan et al., 2022;Wood, 2020). The porosity of the upper regolith is also connected to the mechanical properties of the grain stacking (Kiuchi & Nakamura, 2014) as well as grain transport processes across a planetary surface (Schwan et al., 2017;Vernazza et al., 2012). While a large number of studies of lunar regolith have been performed, the porosity of the pristine upper regolith, defined as the ratio of voids to the total volume in the region near the surface that is accessible for precipitating radiation, is difficult to deduce from returned samples and requires non-invasive methods (Ohtake et al., 2010). Early investigations estimated a porosity value between 0.8 and 0.9 from reflectance measurements (Hapke & van Horn, 1963). Similarly, Ohtake et al. ( 2010) found a high porosity for the Apollo 16 sample site, which was confirmed by Hapke and Sato (2016), determining a porosity of 0.83 ± 0.03 for the upper lunar regolith at this specific site. This value for the upper regolith differs from the result of studies with returned samples of 0.52 ± 0.02 for the upper 15 cm of the lunar soil (Carrier III et al., 1991). Impacting particles, such as photons, ions, or electrons, however, have much smaller interaction regions on the order of millimeters (Hapke & Sato, 2016). It is thus questionable how applicable measurements of the porosity from returned samples are

The surface of Mercury is continuously exposed to impinging solar wind ions. To improve the understanding of space weathering and exosphere formation, a detailed investigation of the ion-surface interaction is necessary. Magnesium and iron rich pyroxene (Ca,Mg,Fe) 2 [Si 2 O 6 ] samples were used as analogues for Mercury's surface and irradiated with He + ions at solar wind energies of 4 keV. Several regimes of implantation and sputtering were observed there. The total estimated mass of implanted He coincides with the mass decrease due to He outgassing during subsequent Thermal Desorption Spectroscopy measurements. Comparison to established modeling efforts and SDTrimSP simulations show that a He saturation concentration of 10 at.% has to be assumed. A complete removal of He is observed by heating to 530 K. On the surface of Mercury, temperatures between about 100 K and 700 K are expected. This temperature will therefore influence the implantation and release of He into Mercury's exosphere.

The Moon and Mercury are airless bodies, thus they are directly exposed to the ambient plasma (ions and electrons), to photons mostly from the Sun from infrared range all the way to X-rays, and to meteoroid fluxes. Direct exposure to these exogenic sources has important consequences for the formation and evolution of planetary surfaces, including altering their chemical makeup and optical properties, and generating neutral gas exosphere. The formation of a thin atmosphere, more specifically a surface bound exosphere, the relevant physical processes for the particle release, particle loss, and the drivers behind these processes are discussed in this review.

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.

Key Points: • Experiments with Phobos analogs were carried out to evaluate the sputtering by O + , C + , O2 + and CO2 + ions from the Martian atmosphere. • The sputter yield from O ions is lower than previously assumed and no yield increases due to molecular effects were observed. • Previous predictions that planetary O ions will dominate sputtering in the Martian tail region are supported by the measurements presented.

Rocky planets and moons experiencing solar wind sputtering are continuously supplying their enveloping exosphere with ejected neutral atoms. To understand the quantity and properties of the ejecta, well-established binary collision approximation Monte Carlo codes like TRIM with default settings are used predominantly. Improved models such as SDTrimSP have come forward, and together with new experimental data, the underlying assumptions have been challenged. We introduce a hybrid model, combining the previous surface binding approach with a new bulk binding model akin to Hofsäss & Stegmaier. In addition, we expand the model implementation by distinguishing between free and bound components sourced from mineral compounds such as oxides or sulfides. The use of oxides and sulfides also enables the correct setting of the mass densities of minerals, which was previously limited to the manual setting of individual atomic densities of elements. All of the energies and densities used are thereby based on tabulated data, so that only minimal user input and no fitting of parameters are required. We found unprecedented agreement between the newly implemented hybrid model and previously published sputter yields for incidence angles up to 45° from surface normal. Good agreement is found for the angular distribution of mass sputtered from enstatite MgSiO3 compared to the latest experimental data. Energy distributions recreate trends of experimental data of oxidized metals. Similar trends are to be expected from future mineral experimental data. The model thus serves its purpose of widespread applicability and ease of use for modelers of rocky body exospheres.

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