Multi-wavelength Photometry of the Icy Saturnian Satellites: a First Look

Introduction: The light reflected from planetary surfaces and measured by remote sensing instruments is affected not only by the composition of the surface but by the physical structure of the surface. The structure of the surface is characterized by both endogenic and exogenic processes. We investigate the effects of these processes on the surfaces of the icy Saturnian satellites using photometric studies at multiple wavelengths in the UV-IR region. We model solar phase curves at many wavelengths using data from the Cassini Ultraviolet Imaging Spec-trograph (UVIS) and Visual Infrared Mapping Spectrometer (VIMS). This is a powerful dataset because the combination of IR and UV wavelengths allows us to distinguish between large and very small grains, and to probe the particle interiors and surfaces. The investigation will provide critical insight into the evolution of the moon regoliths and an understanding of their current environments. The full potential of using solar phase curves over a wide range of wavelengths to probe regolith structure has not yet been fully explored – this study addresses just that. Previous Voyager photometric studies generally included visible-wavelength filters; the VIMS coverage of 0.7 – 5 µm is a dramatic improvement and allows for important studies into structural characteristics of the regoliths that may be wavelength-dependent, such as the single-particle scattering function. This is the first opportunity to explore far-ultraviolet (FUV) solar phase curves of any solar system body. The physical modeling of FUV phase curves, especially in combination with phase curves from NIR wavelengths, promises to result in a wealth of information about the physical structure and evolution of the regoliths of these bodies, because the short FUV wavelengths are sensitive to very small grains (essentially invisible at larger wavelengths) and probe generally the outer surfaces of larger grains, making FUV wavelengths very sensitive to exogenic processes that may weather the outer portions of the grains themselves. Observations: The VIMS instrument is described by [1]. The VIMS disk-integrated phase curves are produced by summing the signal from each pixel and scaling all of the derived intensity measurements to account for spacecraft distance. The UVIS instrument is described by [2]. The far-UV channel of UVIS covers the 1115-1912 Å range. The detector format is 1024 spectral pixels by 64 spatial pixels. Each spectral pixel is 0.25 mrad and each spatial pixel is 1 mrad pro