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New Insights into the Surface Binding Energies of Elements in Plagioclase Feldspars

Presentation #116.04 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
New Insights into the Surface Binding Energies of Elements in Plagioclase Feldspars

The exospheres of Mercury and the Moon are formed, in part, by solar wind ion sputtering from the mineral surfaces of these airless bodies. Plagioclase feldspars are expected to be common minerals on these planetary bodies. The sputtering yield for the different elements in these minerals depends on the surface binding energy (SBE) for each mineral. Here, we consider here the effect of multiple binding sites in these minerals on the SBE and the resulting predicted sputtering behavior. Much of the data that inform sputtering processes comes from binary collision approximation (BCA) models. A fundamental parameter for all BCA models is the SBE of all element types in the impacted substrate. Most of the current research on sputtering assumes that the SBE is independent of the substrate composition or atomic arrangement (i.e., crystallinity). Recent research on Na sputtered from crystalline silicates has shown that this assumption is flawed and can drastically underpredict the actual mineral specific SBE, introducing significant errors in the sputtering yield and energy distribution. However, the SBE is not reliably known for other elements in key minerals commonly found on the surface of Mercury or the Moon. Mineral specific values are needed to move the field forward. To address this issue, we have used molecular dynamics simulations to predict the SBE of all element types from different albite and anorthite crystalline surfaces. Our results are the first to show that the SBE is highly dependent on the specific lattice position and on the surface orientation, meaning multiple SBEs are possible for a given element in a given crystal. For example, for a (001) albite surface the O SBE ranges from 7.5 – 16.8 eV, depending on the surface orientation. However, for a (010) surface the O SBE range is from 3.3 – 13.6 eV. Therefore, in contrast to previous results which commonly assume a fixed O SBE between 1-6.5 eV, several SBEs are likely, and the observed energy distribution is instead an overlapping of these different SBEs. We have incorporated these findings into SDTrimSP to show how incorporating multiple SBEs affects the predicted sputtering yield, energy distribution of the ejecta, and escape rates for the Moon and Mercury.

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