Investigating Saturn's Icy Moons using HST/STIS

The inner moons of Saturn - Mimas, Enceladus, Tethys, Dione and Rhea - exhibit remarkable large-scale albedo and color variations. These trends can be linked to a combination of the unique exogenic processes occurring the Saturn system, including E-ring grain bombardment and charged particle bombardment. One of the fascinating characteristics of the Saturn system is that the icy satellites, though their surface compositions are dominated by water ice, are spectrally red - they are absorbing in the ultraviolet-visible wavelength region (wavelengths <~550 nm) - a spectral feature not typical of pure water ice. In fact, the existing data show that in the UV, absorptions appear to be present superimposed on the overall red slope. Thus, though Cassini instruments have learned much about the surfaces of the icy moons, a basic question that remains is: What is their surface composition and what are the species or processes that cause these UV absorptions? Cassini's spectral coverage is lacking in precisely the near-UV wavelength regime in which the satellites appear to absorb most strongly. To resolve this issue and determine some understanding of the surface species present, we have obtained data using HST/STIS (Space Telescope Imaging Spectrograph). We have utilized the STIS G230L detector to obtain high SNR spectra in the 180-320 nm region along with short G430L exposures to obtain spectra in the 320-570 nm range, to completely fill in the Cassini gap in spectral coverage. Full-disk measurements have been made of the trailing and leading hemispheres of Mimas, Dione and Rhea; a spectrum of Enceladus was also obtained. We report on the results. In particular, we discuss implications for the presence of ammonia, ozone and organics.