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Revisiting Electronic Sputtering Estimates at Mercury

Presentation #116.02 in the session Mercury (Poster)

Published onOct 23, 2023
Revisiting Electronic Sputtering Estimates at Mercury

We revisit ion-induced electronic sputtering yield calculations for Mercury’s exosphere. Ion-induced electronic sputtering occurs when incident ions produce electronic excitations along their path, resulting in the direct desorption of a surface atom or molecule, or initiate a cascade of collisions leading to ejection. Ion precipitation is predicted to contribute indirectly to temporal and spatial variabilities of the exosphere. However, correlating the microscopic physical properties of sputtering derived from experiments and theory to observations of the exosphere remains an important topic of investigation. Most sputtering investigations have focused on ion-impact, or knock-on sputtering: the direct momentum transfer from ions to atoms leading to ejection. Knock-on sputtering is suggested to produce a high energy population of Na atoms which may contribute to the variability of the exosphere at high altitudes. However, past work has shown that ion precipitation should contribute both ion-impact and electronically sputtered Na atoms to Mercury’s exosphere.

Recently, there have been updates on the Na surface fraction at Mercury, new measurements of the precipitating ion flux, and newly derived values for the surface binding energies of Na bound in silicate minerals. We re-estimated both the ion impact collisional and electronic sputtering source rates of Na and the resultant column of a Mercury-like exosphere. The electronic sputtering yield is too low to contribute significantly to the Na population at low altitudes, which is primarily driven by photon-stimulated desorption. The electronic sputtering yield has an ejecta energy spectrum too cold for sputtered atoms to contribute to the asymmetry observed in the extended tail and/or variability observed at high altitudes. However, ion sputtering experiments that have measured energy spectra from Na-bearing substrates suggest a wide range of surface binding energies and cannot distinguish the relative numbers of atoms ejected by knock-on collisions or electronic excitations, respectively. Understanding electronic sputtering is important for interpreting experimental sputter yields, but it is unlikely to play a significant role for Mercury’s exosphere.

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