Energetic particle observations near Ganymede

This paper combines data from the Galileo spacecraft plasma and energetic particle instruments. Data are included from two Ganymede flybys when the spacecraft was at northern Ganymede latitudes. Electron intensities from the two instruments are in very good agreement. Fits to 60 electron energy spectra are presented. We find the power of precipitating 0.5–3.0 keV electrons into both polar caps of Ganymede is ∼3 × 10⁹ W. By assuming the instruments are intercalibrated, we infer that sulfur dominates the ion intensities at least in the tens of keV energy range. Furthermore, fits to the ion data indicate that the intensities are dominated by heavy ions below about 100 keV. Fits to these data are also used to estimate the sputtering rate of Ganymede's icy surface. It is found that ∼2 × 10²⁶ water molecules/s are sputtered from Ganymede's polar caps which, when not redeposited on the surface, give an erosion rate of ∼8 m/Gyr.