  Electron impact ionization is critical in producing the ionospheres on many
planetary bodies and, as discussed here, is critical for interpreting
spacecraft and telescopic observations of the tenuous atmospheres of the icy
Galilean satellites of Jupiter (Europa, Ganymede, and Callisto), which form an
interesting planetary system. Fortunately, laboratory measurements,
extrapolated by theoretical models, were developed and published over a number
of years by K. Becker and colleagues (see Deutsch et al. 2009) to provide
accurate electron impact ionization cross sections for atoms and molecules,
which are crucial to correctly interpret these measurements. Because of their
relevance for the Jovian icy satellites we provide useful fits to the complex,
semi-empirical Deutsch-Mark formula for energy-dependent electron impact
ionization cross-sections of gas-phase water products (i.e., H2O, H2, O2, H,
O). These are then used with measurements of the thermal plasma in the Jovian
magnetosphere to produce ionization rates for comparison with solar
photo-ionization rates at the icy Galilean satellites.
