Solar wind sputtering of small bodies: Exospheres of Phobos and Deimos

Tuesday, 16 December 2014: 9:15 AM
Micah J Schaible, University of Virginia Main Campus, Charlottesville, VA, United States, Robert E Johnson, Univ Virginia, Charlottesville, VA, United States, Pascal Lee, Mars Institute, Mountain View, CA, United States, Mehdi Benna, NASA - GSFC, Greenbelt, MD, United States and Richard C Elphic, NASA Ames Research Center, Moffett Field, CA, United States
Solar wind, magnetospheric ions and micrometeorites impact the surface of airless bodies in the solar system and deposit energy in the surface material. Excitation and momentum transfer processes lead to sputtering or desorption of molecules and atoms, thereby creating a dynamic exosphere about an otherwise airless body. Ion mass spectrometry of ejected materials provides a highly sensitive method for detecting sputter products and determining the surface composition [Johnson and Baragiola, 1991; Elphic et al., 1991]. Though most of the material is sputtered as neutral gas, UV photons can ionize ejected neutrals and a small fraction of the ejecta leaves the surface in an ionized state. However, ions are deflected by the variably-oriented solar wind magnetic field and thus relating their detection to a surface location can be problematic. Here we estimate the average ion density close to the surface of Phobos or Deimos to predict whether modern mass spectrometry instruments [Mahaffey et al. 2014] would be able to obtain sufficient compositional information to place constraints on their origin. The open source Monte Carlo program SRIM.SR was used to simulate the effect of ions incident onto a surface representing several different meteorite compositions and gave estimates of the damage and sputtering effects. As much of the empirical data supporting SRIM results comes from sputtering of metallic and organic molecular targets which can differ greatly from materials that make up planetary surfaces, measurements of cohesive energies and enthalpies of formation were used to estimate the surface binding energies for minerals, though these can vary significantly depending on the chemical composition. Since these properties affect the sputtering yield, comparisons of simulations with laboratory measurements were made to test the validity of our estimates. Using the validated results and a constant fraction to estimate ion yields, the density of ejected ions and neutrals vs. altitude from Phobos and Deimos was calculated using analytical equations. We use the results to make predictions on the expected signal for several meteorite classes.