Energy and Temperature Dependencies for Electron-induced Sputtering from H2O Ice: Implications for the Icy Galilean Moons

To better assess the role that electrons play in exosphere production on icy bodies, we measured the total and O-2 sputtering yields from H2O ice for electrons with energies between 0.75 and 10 keV and temperatures between 15 and 124.5 K. We find that both total and O-2 yields increase with decreasing energy over our studied range, that they increase rapidly at temperatures above 60 K, and that the relative amount of H2O in the sputtered flux decreases quickly with increasing energy. Combining our data with other electron data in the literature, we show that the accuracy of a widely used sputtering model can be improved significantly for electrons by adjusting some of the intrinsic parameter values. Applying our results to Europa, we estimate that the contribution of electrons to the production of the O-2 exosphere is equal to the combined contribution of all ions. In contrast, sputtering of O-2 from Ganymede and Callisto appears to be dominated by irradiating ions, though electrons still likely contribute a nonnegligible amount. While our estimates could be further refined by examining the importance of spatial variations in electron flux, we conclude that, at the very least, electrons seem to be important for exosphere production on icy surfaces and should be included in future modeling efforts.