• Hedman, M. M.
• Nicholson, P. D.
• Sotin, C.
• Baines, K. H.
• Nelson, R.
• Buratti, B. J.
• Filacchione, Gianrico
• Brown, R. H.
• Cruikshank, D. P.
• Cuzzi, J. N.
• Hendrix, A.
• Pearson, N.
• Bradley, E. T.
• Perlman, Z. S.
• Clark, R. N.

Abstract

Saturn's icy satellites and ring particle surfaces have long been known to be composed mostly of frozen water. However, all surfaces show an absorption due to a non-water-ice component whose identity has not been well understood. In the near infrared, water ice has strong absorptions which limit detectability of other trace components. Similarly, at wavelengths less than about 0.18 microns, water is very absorbing. However, in the ~0.2 to ~1 micron range, water ice has low absorption and trace components are readily detected. Classical interpretations of the UV absorber and dark material on outer Solar System satellites have been varying amounts of tholins and carbon. However, tholins have spectral structure not seen in the icy spectra in the Saturn System. Many silicates also have UV spectral structure that reject them from contributing significantly to the observed spectral signatures. We have constructed a new UV spectrometer and a new environment chamber for studying the spectral properties of materials from 0.1 to 15 microns. In our survey of the spectral properties of materials so far, we find that small amounts of metallic iron and iron oxides in the icy surfaces are compatible with and can explain the UV, visible and near-infrared spectra of icy surfaces in the Saturn system (0.12 to 5.1 microns) using data from the Cassini UltraViolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS). The wide range of observed UV-NIR (0.1-5 micron) spectral signatures provide strong constraints on composition and grain size distribution, including grain sizes of the ice. Spectra of the Saturnian rings and icy satellites indicate they have a large range of ice grain sizes, from tens of microns to sub-micron. Sub-micron ice grains create unusual spectral properties, which are seen in the spectra of the rings and satellites of Saturn and on satellites further out in the Solar System. Clark et al. (2012, Icarus v218, p831) showed that VIMS spectra were explained by combinations ! <P />of water ice, CO2, nano-sized grains of metallic iron and iron oxide and trace amounts of other compounds. The new UV lab data are providing further evidence for this interpretation and placing further constraints on grain size distributions and abundances of the components. <P />...

Topics

• impedance spectroscopy
• surface
• compound
• Carbon
• grain
• grain size
• laser emission spectroscopy
• iron