Photon Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury’s Exosphere - Data Files

Abstract

Mercury has a relatively high sulfur content on its surface, and a signal consistent with S+ was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance enhanced multiphoton ionization (REMPI) and time-of-flight (TOF) mass spectrometry. Though the PSD process is inherently non-thermal, the measured velocity distributions were fit using flux weighted Maxwellian distributions with translation energies ˂E> expressed as translational “temperatures” Tt = ˂E>/µkB. A bi-modal distribution consisting of both thermal (Tt = 300 K) and supra-thermal (Tt >1000 K) components in roughly a 2:1 ratio was found to best fit the data. The experimental PSD cross-section, ~4×10-22 cm2, and integrated velocity distributions were used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely a primary source to S0 in Mercury’s exosphere at low (<1000 km) altitudes.