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Considerations when Quantifying Mineral and Atomic-Arrangement-Specific Surface Binding Energies for Inputs into Global Exosphere Models

Presentation #207.02 in the session Surface Processes on Mercury: ’Tis but a Scratch.

Published onOct 20, 2022
Considerations when Quantifying Mineral and Atomic-Arrangement-Specific Surface Binding Energies for Inputs into Global Exosphere Models

Understanding the role solar wind (SW) ions play on surface sputtering of Mercury is critical to any exosphere model [1]. The most common sputtering models use the binary collision approximation (BCA) and thus consider sputtering to be a result of binary collision cascades. 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. 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. To address this issue, we have used a combined molecular dynamics (MD) and BCA simulation approach. First, we performed MD simulations to quantify the SBE of Na and O from single crystal albite. Albite was selected as it is a Na containing plagioclase feldspar endmember. For both Na and O, the SBE from MD was significantly higher compared to the commonly used approximation of the monatomic cohesive energy. After calculating the SBEs, we then use a recently developed BCA ‘best-practice’ methodology to determine the effect of the 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 [2]. This shifted energy distribution also affected the proportion of atoms sputtered with an energy above the Mercury escape velocity. Next, we will then investigate how the atomic arrangement, i.e. crystalline, amorphous, something in between and/or the level of damage, of the substrate affects the SBE. SBE’s from amorphous albite will be quantified along with the SBE of loosely absorbed Na and O. We will then discuss the future of SBE-focused simulations for direct comparison to samples relevant to planetary science.

[1] Killen, Rosemary M., et al. (2022) The Planetary Science Journal 3.6 (2022): 139. [2] L. S. Morrissey et al. (2022) The Astrophysical Journal Letters 925, no. 1 (2022): L6

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