H2O-ice particle size variations across Ganymede's and Callisto's surface

Kurzfassung

The detailed investigation of the variations in the H2O-ice spectral signatures across Ganymede's and Callisto's surfaces derived from the NIMS (Near Infrared Mapping Spectrometer) data of the Galileo mission have shown that in the case of areal mixing of H2O ice and dark components the associated band depth ratios (BDRs) of the individual H2O-ice absorptions can be reliably used to qualitatively infer variations in the H2O ice particle size and even to semi-quantitatively estimate particle size ranges on the satellites' surfaces when largely unaffected by hydrated non-ice materials or varying illumination conditions. This modeling approach implies H2O ice particle radii ranging between ~1 μm and ~1 mm on Ganymede and up to ~2 mm on Callisto Our analyses point to a continuous decrease of H2O - ice particle sizes toward the poles on both satellites. These variations may be caused by similar surface processes. The smooth latitudinal trend may be related to the surface temperatures and possible thermal migration of H2O vapor to colder region in higher latitudes (Spencer, 1987) and particle welding at lower latitudes. Larger particles in the equatorial region of Callisto compared to Ganymede could be due to its higher maximum surface temperature and a longer Callistoan day resulting in more extensive particle welding.