Far-ultraviolet photometric characteristics of Tethys and Dione

The icy moons of Saturn orbiting inside the E-ring experience weathering processes: E-ring grains as well as cold plasma ions, neutrals and energetic particles impact their surfaces. The Cassini UVIS instrument (UltraViolet Imaging Spectrograph), operating in the FUV wavelengths (118 - 190nm) probes the uppermost layers of the regolith and is uniquely suited to studying these exogenic processes, leading to a better understanding of the surfaces' evolution and the saturnian environment. We present an analysis of ultraviolet disk-integrated phase curves of Tethys and Dione. We display phase curves for the leading and trailing hemispheres as well as for the Saturn and anti-Saturn ones. As expected, we find that the leading hemisphere is brighter than the trailing and at large phase angles, it seems that we directly observe these E-ring grains in forward scattering. Our data set covers a wide range of phase angles, from 6.8 to 163.9 degrees for Tethys. Our analysis is completed by using a Hapke model to retrieve the photometric parameters of these surfaces, such as the single scattering parameters, the opposition effect parameters (where possible) and information on the roughness. The photometric behavior of Dione is compared and contrasted with that of Tethys, and implications for the exogenic processes affecting the surfaces are discussed.