Compositional mapping of Saturn’s satellite Dione with Cassini VIMS and implications of dark material in the Saturn system

Cassini VIMS has obtained spatially resolved imaging spectroscopy data on numerous satellites of Saturn. A very close fly-by of Dione provided key information for solving the riddle of the origin of the dark material in the Saturn system. The Dione VIMS data show a pattern of bombardment of fine, sub-0.5-μm diameter particles impacting the satellite from the trailing side direction. Multiple lines of evidence point to an external origin for the dark material on Dione, including the global spatial pattern of dark material, local patterns including crater and cliff walls shielding implantation on slopes facing away from the trailing side, exposing clean ice, and slopes facing the trailing direction which show higher abundances of dark material. Multiple spectral features of the dark material match those seen on Phoebe, Iapetus, Hyperion, Epimetheus and the F-ring, implying the material has a common composition throughout the Saturn system. However, the exact composition of the dark material remains a mystery, except that bound water and, tentatively, ammonia are detected, and there is evidence both for and against cyanide compounds. Exact identification of composition requires additional laboratory work. A blue scattering peak with a strong UV–visible absorption is observed in spectra of all satellites which contain dark material, and the cause is Rayleigh scattering, again pointing to a common origin. The Rayleigh scattering effect is confirmed with laboratory experiments using ice and 0.2-μm diameter carbon grains when the carbon abundance is less than about 2% by weight. Rayleigh scattering in solids is also confirmed in naturally occurring terrestrial rocks, and in previously published reflectance studies. The spatial pattern, Rayleigh scattering effect, and spectral properties argue that the dark material is only a thin coating on Dione's surface, and by extension is only a thin coating on Phoebe, Hyperion, and Iapetus, although the dark material abundance appears higher on Iapetus, and may be locally thick. As previously concluded for Phoebe, the dark material appears to be external to the Saturn system and may be cometary in origin. We also report a possible detection of material around Dione which may indicate Dione is active and contributes material to the E-ring, but this observation must be confirmed

The surface composition of Iapetus: Mapping results from Cassini VIMS

Cassini VIMS has obtained spatially resolved imaging spectroscopy data on numerous satellites of Saturn. A very close fly-by of Iapetus on September 10, 2007 provided the best data on the spectral signature and spatial extent of dark material on Iapetus. This Cassini Rev 49 Iapetus fly-by provided spatially resolved imaging spectroscopy data of the dark material and the leading/trailing side transition from the dark material to visually bright ice on the trailing side. Compositional mapping and radiative transfer modeling shows that the dark material is composed of metallic iron, nano-size iron oxide (hematite), CO2, H2O ice, and possible signatures of ammonia, bound water, H2 or OH-bearing minerals, trace organics, and as yet unidentified materials. CO2 indicates a pattern of increasing CO2 strength from the leading side apex to the transition zone to the icy trailing side. A Rayleigh scattering peak in the visible part of the spectrum indicates the dark material has a large component of fine, sub-0.5-μm diameter particles consistent with nanophase hematite and nanophase iron. Spectral signatures of ice also indicate that sub-0.5-μm diameter particles are present in the icy regions. Multiple lines of evidence point to an external origin for the dark material on Iapetus, including the global spatial pattern of dark material, local patterns including crater and cliff walls shielding implantation on slopes facing away from the leading side, exposing clean ice, and slopes facing the leading direction which show higher abundances of dark material. Multiple spectral features and overall spectral shape of the dark material on Iapetus match those seen on Phoebe, Hyperion, Dione, Epimetheus, Saturn's rings Cassini Division, and the F-ring implying the material has a common composition throughout the Saturn system. The dark material appears to have significant components of nanophase metallic iron and nanophase hematite contributing to the observed UV absorption. The blue scattering peak with a strong UV-visible absorption is observed in spectra of all satellites that contain dark material, again pointing to a common origin of contamination by metallic iron that is partially oxidized

Cassini VIMS compositional mapping of Surfaces in the Saturn System and the role of water, cyanide compounds and carbon dioxide

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Compositional maps of Saturn's moon Phoebe from imaging spectroscopy

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini-Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin

The Surface Composition of Saturn's Moon Phoebe As seen by the Cassini Visual and Infrared Mapping Spectrometer

International audienc

Compositional maps of Saturn's moon Phoebe from imaging spectroscopy

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