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Simulating Solar-Wind-Ion Sputtering of Sodium from Mercury’s Surface: Constraining the Surface Binding Energy of Sodium from a Range of Silicates

Presentation #502.03 in the session “Solar Wind & Exospheres”.

Published onOct 03, 2021
Simulating Solar-Wind-Ion Sputtering of Sodium from Mercury’s Surface: Constraining the Surface Binding Energy of Sodium from a Range of Silicates

Na and He are the two most abundant confirmed neutral species in Mercury’s exosphere. Whereas the source of He is from the solar wind (SW) and planetary radiogenesis, the source of the Na is potentially due to sputtering from silicates on the surface. As SW ions impact the surface, they deposit energy, leading to sputtered atoms. The sputtering yield and angular /energy distributions of the sputtered atoms depends on the energy of the incoming SW ions and the composition of the surface. Understanding the role SW ions play on surface sputtering of Mercury is critical to any exosphere model. 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. A fundamental physical parameter for BCA models is the surface binding energy (SBE) of atoms in the substrate. Despite the clear importance of the SBE in simulating sputtering, its actual value is not well understood for many substrates. Specific to Na, there is a large range of reported values that have been used for SW sputtering simulations from the surfaces of airless planetary bodies. Given that BCA methods rely on a user defined SBE, this can be a significant source of error for sputtering predictions from complex substrates. To address this issue, we have performed molecular dynamics simulations to better constrain the SBE of Na from silicates. We then consider the effect that these modified inputs have on predicted yield and energy distributions of sputtered Na due to SW ions. An iterative method was used to determine the minimum energy needed to remove one Na atom completely from the substrate. BCA models were then used to determine how this modified SBE value affected the predicted yield and energy distribution of sputtered Na. Results from MD simulations determined SBEs between 2.6-7.9 eV for Na from various silicate surfaces. Binding energies appear to be correlated to the coordination of the Na atoms. In contrast, the individual cohesive energy of pure Na is only 1.1 eV. Therefore, SBEs from a compound can be drastically different than their atomistic cohesive energies. The SBE of a specific atom is instead a function of the compound in which the atom is bound. The newly predicted Na SBE value was then used to determine the sputtering yield and energy distribution of the sputtered atoms using SDTrimSP and the Thompson energy distribution. We find that increasing the SBE had a significant effect on predicted Na energy distribution and sputtering yield.

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