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 sputtering of wollastonite (CaSiO3) by solar wind-relevant ions has been investigated experimentally and the results are compared to the binary collision approximation (BCA) codes SDTrimSP and SRIM-2013. Absolute sputtering yields are presented for Ar projectiles as a function of ion impact energy, charge state and impact angle as well as for solar wind H projectiles as a function of impact angle. Erosion of wollastonite by singly charged Ar ions is dominated by kinetic sputtering. The absolute magnitude of the sputtering yield and its dependence on the projectile impact angle can be well described by SDTrimSP as long as the actual sample composition is used in the simulation. SRIM-2013 largely overestimates the yield especially at glancing impact angles. For higher Ar charge states, the measured yield is strongly enhanced due to potential sputtering. Sputtering yields under solar wind-relevant H + bombardment are smaller by two orders of magnitude compared to Ar. Our experimental yields also show a less pronounced angular dependence than predicted by both BCA programs, probably due to H implantation in the sample. Based on our experimental findings and extrapolations to other solar wind ions by using SDTrimSP we present a model for the complete solar wind sputtering of a flat wollastonite surface as a function of projectile ion impact angle, which predicts a sputtering yield of 1.29 atomic mass units per solar wind ion for normal impact. We find that mostly He and some heavier ions increase the sputtering yield by more than a factor of two as compared to H + bombardment only.
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.
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