Using the variation in charged particle bombardment to study satellite surfaces

The Galileo Energetic Particles Detector (EPD) measures electrons and individual and total ions in the energy range from tens of keV to tens of MeV. During the many close passes of Europa, Ganymede, and Callisto, EPD made high time resolution measurements of these particles. EPD data analysis has shown that radiation dose varies greatly with the mechanisms by which particles gain access to the surface. We will focus on how particle flux differs across the surface, such as into leading/trailing hemispheres and polar/equatorial latitudes, and what implications these asymmetries have for surface weathering. Near Europa, EPD data analysis revealed a strong interaction between energetic charged particles and the satellite surface as evidenced by strong flux depletions in the wake distribution. We showed that among the energetic charged particles, electron bombardment of the trailing hemisphere gives the highest dose of radiation to the optical layer of the satellite. Furthermore, we pointed out an important asymmetry in the electron bombardment pattern that was then correlated with concentrations of frozen, hydrated sulfuric acid, inferred from Galileo NIMS observations. At Ganymede, we showed that particle signatures at low Ganymede latitudes reveal the presence of closed magnetic field lines, i.e. having both endpoints in the surface. We measured radiation belts associated with Ganymede much like Earth's, with precipitating particles at higher energies than expected based on the surrounding plasma. We also pointed out that polar field lines on Ganymede are refilled by an energy-dependent pitch angle diffusion. This indicated to us that there exists large polar cap/low latitude differences in the intensity of precipitating particles. Prior to Galileo, it would have been difficult, if not impossible, to predict the bombardment pattern or radiation levels to the satellite surfaces. We suggest that these empirical studies of the energetic particle flux near the satellites have illuminated the importance of high time resolution studies of the near-satellite environments. To the extent that optical properties are used to infer the satellite history, it is critical to understand the role of radiation in modifying the surface features.