Abstract | Surface sputtering by solar wind (SW) ion irradiation is an important process for understanding Mercury’s exosphere. As SW ions impact the surface, they deposit energy, leading to sputtered atoms which can be detected in Mercury’s exosphere. The most common sputtering models use the binary collision approximation (BCA) and thus consider sputtering to be a result of binary collision cascades. These models can be used to predict the energy distribution and yield of sputtered atoms as a function of incoming ion type, energy, and angle, with only modest computational requirements. A fundamental physical parameter for BCA models is the surface binding energy (SBE) of the atoms being ejected. The SBE affects the overall sputtering yield, composition of the yield, and energy of the ejecta. However, despite the clear importance of the SBE, its actual value is not well understood for many species and substrates important for Mercury’s surface. For example, while Na is easily observed in the Hermean exosphere, there is still disagreement on the primary source of this exospheric Na. This is complicated by a large range of SBE values (0.27 – 2.6 eV) that have been used for SW sputtering simulations of Na. Similarly, while O is expected to be one of the most abundant elemental species found on the silicate-rich Hermean surface, it is less abundant in the exosphere. Again, there is a lack of consensus on what SBE value to use with previously suggested values ranging from 1 to 6.5 eV. Given that BCA methods rely on a user defined SBE, this can be a significant source of error for predicting the importance of SW induced sputtering from minerals.
We use molecular dynamics (MD) to provide the first accurate SBE data we are aware of for Na and O sputtered from albite (NaAlSi3O8)., which is expected to be an important plagioclase feldspar endmember. An iterative method was used to determine the minimum energy needed to remove one Na or O atom completely from the surface of the substrate. For both Na and O, the SBE from MD was significantly higher (7.9 and 7.4 eV respectively) compared to the commonly used monatomic cohesive energy approximation and other fitting values. SBE values for O were shown to also be dependent on the surface lattice position. Therefore, SBEs are both mineral and surface-position specific. After calculating the SBEs, we then used BCA models to determine the effect of SBE on the predicted yield and energy distributions of sputtered Na and O due to SW ions. As the SBE was increased, there was a significant decrease in the sputtering yield and an increase in the peak and broadness of the sputtered atom energy distribution. This shifted energy distribution also affected the proportion of atoms sputtered with an energy above the Mercury escape velocity. We then discuss the future of SBE-focused simulations for direct comparison to samples relevant to planetary science.
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