Polar ''caps'' on Ganymede and Io revisited

The polar spectral features on Ganymede and Io, often called polar ''caps,'' have been attributed to magnetospheric-ion irradiation of these surfaces. The recent discovery of an intrinsic magnetic field and the mapping of the surface temperatures at Ganymede by the Galileo spacecraft re-enforce this hypothesis. In this note we describe how the spectrally very different polar features at Ganymede and Io are both signatures of plasma precipitation along open field lines to the polar surfaces of these objects. (C) 1997 Academic Press.