Stimulated Desorption of Silicates: A Potential Source for Ions in Mercury's Space
Environment
Thomas M. Orlando1, Jason L. McLain1, Ann L. Sprague2, Gregory A. Grieves1, David Schriver3
and Pavel Travinicek3,4
1
Georgia Institute of Technology, Atlanta, GA, USA
University of Arizona, Lunar and Planetary Laboratory, , Tucson, AZ, USA
3
Institute of Geophysics and Planetary Physics, Univ. of California at Los Angeles, CA, USA
4
Astronomical Institute, ASCR, Prague, Czech Republic
2
The potential role of photon- and electron-stimulated desorption (PSD and ESD,
respectively) in the formation of Mercury’s exosphere will be discussed. A general overview of
PSD and ESD will be followed by a detailed discussion of recent experimental results involving
electron irradiation of Na- and K-bearing silicate glasses. These electron-beam induced
processes yield direct desorption of H+, H 2 +, O+, H 3 O+, Na+, K+ and O 2 +. A simulation has also
been carried out to calculate electron precipitation fluxes and energies that may be used in the
interpretation of measurements made by MESSENGER spacecraft instruments and to better
understand the formation of the ion and neutral exosphere at Mercury. The electron fluxes are
estimated to be ~1010 cm-2s-1 with average energies up to 500 eV. The threshold energies for
production/release of ions have been measured. The thresholds correlate with deep valence holes
that undergo Auger decay or the production of energetic protons that undergo reactive scattering
on the surface. The estimated total cross-section for ESD at a surface temperature of 400 K is
≥10-19 cm2; much larger than neutral production followed by electron impact ionization or
photoionization in the gas-phase. These results indicate that ESD may contribute significantly to
the production and release of regolith constituents, particularly in ionic form, directly into the
exosphere of Mercury. If time permits, a discussion of the potential role of electron and proton
bombardment in the formation of molecular water from regolith material will also be discussed.