Electron-stimulated desorption of silicates: A potential source for ions in Mercury's space environment

摘要

The potential role of electron-stimulated desorption (ESD) in the formation of Mercury's exosphere has been examined. Experimental results involving electron irradiation of Na- and K-bearing silicate glasses yielded direct desorption of H~+, H_2~+,O~+, H_3O~+, Na~+, K~+, and O_2~+. A simulation has also been performed to calculate electron precipitation fluxes and energies that may be used in the interpretation of measurements made by MESSENGER (Mercury Surface, Space Environment,Geochemistry, and Ranging) spacecraft instruments and to better understand the formation of the ion and neutral exosphere at Mercury. The electron fluxes are estimated to be ～10~(10) cm~(-2) s~(-1) with average energies up to 500 eV. The threshold energies for production/release of ions have been measured to be 25 ± 2 eV for H~+; 30 ± 2 eV for O~+, Na~+, and K~+; 40 ± 2 eV for H_2~+ and H_3O~+; and 90 ± 2 eV for O_2~+.The 25-30 eV thresholds correlate with deep valence holes in the O 2s levels that undergo Auger decay. The thresholds for H_2~+ and H_3O~+ correspond to two-hole states in chemisorbed water, which produce energetic protons that undergo reactive scattering on the surface. A significant increase in ESD yield of all ions is observed above a substrate temperature of 350 K. The estimated total cross section for ESD at a surface temperature of 400 K is ≥10~(-19) cm~2, much larger than neutral production followed by electron impact ionization or photoionization in the gas phase. These results indicate that ESD may contribute to the production and release of regolith constituents, particularly in ionic form, directly into the exosphere of Mercury.