Energetic Ion and Electron Irradiation of the Icy Galilean Satellites

Galileo Orbiter measurements of energetic ions (20 keV to 100 MeV) and electrons (20–700 keV) in Jupiter’s magnetosphere are used, in conjunction with the JPL electron model (<40 MeV), to compute irradiation effects in the surface layers of Europa, Ganymede, and Callisto. Significant elemental modifications are produced on unshielded surfaces to approximately centimeter depths in times of ≤106 years, whereas micrometer depths on Europa are fully processed in ∼10 years. Most observations of surface composition are limited to optical depths of∼1 mm, which are in direct contact with the space environment. Incident flux modeling includes Størmer deflection by the Ganymede dipole magnetic field, likely variable over that satellite’s irradiation history. Delivered energy flux of ∼8× 1010 keV (cm2-s)−1 at Europa is comparable to total internal heat flux in the same units from tidal and radiogenic sources, while exceeding that for solar UV energies (>6 eV) relevant to ice chemistry. Particle energy fluxes to Ganymede’s equator and Callisto are similar at ∼2–3× 108 keV (cm2-s)−1 with 5× 109 at Ganymede’s polar cap, the latter being comparable to radiogenic energy input. Rates of change in optical reflectance and molecular composition on Europa, and on Ganymede’s polar cap, are strongly driven by energy from irradiation, even in relatively young regions. Irradiation of nonice materials can produce SO2 and CO2, detected on Callisto and Europa, and simple to complex hydrocarbons. Iogenic neutral atoms and meteoroids deliver negligible energy∼104–5 keV (cm2-s)−1 but impacts of the latter are important for burial or removal of irradiation products. Downward transport of radiation produced oxidants and hydrocarbons could deliver significant chemical energy into the satellite interiors for astrobiological evolution in putative sub-surface oceans. c © 2001 Academic Press