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Mapping Vesta’ surface composition in the Sextilia region offers the unique possibility to investigate stratigraphic relationship of the surface compounds throughout the transition from Rheasilvia, Vesta’s prominent impact basin, to Vesta’s northern equatorial region. The VIR data point to an upper eucrite-dominated layer overlaying extended deposits of diogenite in the subsurface, which reaches the surface at Matronalia Rupes, the uplifted rim of the Rheasilvia impact structure. The dominance of diogenite in Matronalia Rupes suggests the existence of an extended diogenite-rich layer and supports the magma-ocean model as the most plausible formation model for Vesta’s interior. Local enrichment of diogenite outside of the Rheasilvia impact basin are interpreted as outcrops of diogenite-dominated breccia excavated during the Rheasilvia impact event and re-excavated by more recent impacts. On the contrary, the asymmetry in the distribution of diogenite-rich outcrops in the norther parts, i.e., the depletion of these deposits in the Veneneia region, is at odds with the equally distributed plutonic intrusions of diogenite-rich material as proposed in the serial magmatism model but could be explained by an uneven distribution of its ejecta on Vesta’s surface due to an oblique Rheasilvia impact. [ABSTRACT FROM AUTHOR]

Abstract: Since the arrival of the Cassini spacecraft at Saturn in June 2004, the Visual and Infrared Mapping Spectrometer has obtained new spectral data of the icy satellites of Saturn in the spectral range from 0.35 to 5.2μm. Numerous flybys were performed at Saturn’s second largest satellite Rhea, providing a nearly complete coverage with pixel-ground resolutions sufficient to analyze variations of spectral properties across Rhea’s surface in detail. We present an overview of the VIMS observations obtained so far, as well as the analysis of the spectral properties identified in the VIMS spectra and their variations across its surface compared with spatially highly resolved Cassini ISS images and digital elevation models. Spectral variations measured across Rhea’s surface are similar to the variations observed in the VIMS observations of its neighbor Dione, implying similar processes causing or at least inducing their occurrence. Thus, magnetospheric particles and dust impacting onto the trailing hemisphere appear to be responsible for the concentration of dark rocky/organic material and minor amounts of CO2 in the cratered terrain on the trailing hemisphere. Despite the prominent spectral signatures of Rhea’s fresh impact crater Inktomi, radiation effects were identified that also affect the H2O ice-rich cratered terrain of the leading hemisphere. The concentration of H2O ice in the vicinity of steep tectonic scarps near 270°W and geologically fresh impact craters implies that Rhea exhibits an icy crust at least in the upper few kilometers. Despite the evidence for past tectonic events, no indications of recent endogenically powered processes could be identified in the Cassini data. [Copyright &y& Elsevier]

Abstract: We present a detailed analysis of the variations in spectral properties across the surface of Saturn’s satellite Dione using Cassini/VIMS data and their relationships to geological and/or morphological characteristics as seen in the Cassini/ISS images. This analysis focuses on a local region on Dione’s anti-saturnian hemisphere that was observed by VIMS with high spatial resolution during orbit 16 in October 2005. The results are incorporated into a global context provided by VIMS data acquired within Cassini’s first 50 orbits. Our results show that Dione’s surface is dominated by at least one global process. Bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced CO2 absorption on the trailing hemisphere of Dione independent of the geology. Local regions within this terrain indicate a special kind of resurfacing that probably is related to large-scale impact process. In contrast, the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater Creusa (∼49°N/76°W). Although no geologically active regions could be identified, Dione’s tectonized regions observed with high spatial resolution partly show some clean H2O ice implying that tectonic processes could have continued into more recent times. [Copyright &y& Elsevier]

Laboratory measurements were performed to study the spectral signature of H2O ice between 0.4 and 4.2 µm depending on varying temperatures between 70 and 220 K. Spectral parameters of samples with particle sizes up to ~1360 µm, particle size mixtures, and different particle shapes were analyzed. The band depth (BD) of the major H2O-ice absorptions at 1.04, 1.25, 1.5, and 2 µm offers an excellent indicator for varying particle sizes in pure H2O ice. The spectral changes due to temperature rather, but not exclusively, affect the H2O-ice absorptions located at 1.31, 1.57, and 1.65 µm and the Fresnel reflection peaks at 3.1 and 3.2 µm, which strongly weaken with increasing temperature. As the BDs of the H2O-ice absorptions at 1.31, 1.57, and 1.65 µm increase, the band centers (BCs) of the H2O-ice absorptions at 1.25 and 1.5 µm slightly shift to shorter wavelengths. However, the BCs of the strong H2O-ice absorptions can also be affected by saturation in the case of large particles. The collected spectra provide a useful spectral library for future investigations of icy satellites such as Ganymede and Callisto, the major targets of ESA's JUICE mission. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the near-infrared spectrum of the Saturn satellite Phoebe, a distant satellite observed before Cassini’s Saturn orbit insertion, using data from the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini orbiter. We have done a critical calibration of the dataset that involves careful correction of dark artifacts. We model areally mixed water ice and non-ice (assumed segregated because of the low ∼3% albedo of the non-ice material) for several high and medium resolution observations of Phoebe made near closest approach. Using a Hapke roughness factor of 15°, we find ice abundances from ∼0.1% to over 4%. The ice grain radii vary from 1 to 10μm. These are displayed on a projected map of Phoebe with about 50% coverage (about 33% coverage at better than 5km spatial resolution). Detailed looks at the water ice spectral fits shows that the weak 1.05 and 1.25-μm bands are missing in most of the spectra, implying that the ice endmember is not pure ice, but has a dark material mixed with it that lowers the albedo and suppresses these bands. We made a model of ice contaminated with Phoebe-like dark material showing that a few percent of dark material lowers the albedo to ∼50% and suppresses the bands. The dirty ice model produces better fits to the spectra and implies that the amount of dirty ice is about 1.5 times the amount of pure ice. We have also calculated the CO2 band depth for these same observations and projected the results. The CO2 band depth varies inversely with water ice abundance. [Copyright &y& Elsevier]

Abstract: The surface of Titan has been revealed by Cassini observations in the infrared and radar wavelength ranges as well as locally by the Huygens lander instruments. Sand seas, recently discovered lakes, distinct landscapes and dendritic erosion patterns indicate dynamic surface processes. This study focus on erosional and depositional features that can be used to constrain the amount of liquids involved in the erosional process as well as on the compositional characteristics of depositional areas. Fluvial erosion channels on Titan as identified at the Huygens landing site and in RADAR and Visible and Infrared Mapping Spectrometer (VIMS) observations have been compared to analogous channel widths on Earth yielding average discharges of up to 1600 m3/s for short recurrence intervals that are sufficient to move centimeter-sized sediment and significantly higher discharges for long intervals. With respect to the associated drainage areas, this roughly translates to 1–150 cm/day runoff production rates with 10 years recurrence intervals and by assuming precipitation this implies 0.6–60 mm/h rainfall rates. Thus the observed surface erosion fits with the methane convective storm models as well as with the rates needed to transport sediment. During Cassini''s T20 fly-by, the VIMS observed an extremely eroded area at 30° W, 7° S with resolutions of up to 500 m/pixel that extends over thousands of square kilometers. The spectral characteristics of this area change systematically, reflecting continuous compositional and/or particle size variations indicative of transported sediment settling out while flow capacities cease. To account for the estimated runoff production and widespread alluvial deposits of fine-grained material, release of area-dependent large fluid volumes are required. Only frequent storms with heavy rainfall or cryovolcanic induced melting can explain these erosional features. [Copyright &y& Elsevier]

The spectral position of the 3.6 µm continuum peak measured on Cassini-VIMS I/F spectra is used as a marker to infer the temperature of the regolith particles covering the surfaces of Saturn’s icy satellites. This feature is characterizing the crystalline water ice spectrum which is the dominant compositional endmember of the satellites’ surfaces. Laboratory measurements indicate that the position of the 3.6 μ m peak of pure water ice is temperature-dependent, shifting towards shorter wavelengths when the sample is cooled, from about 3.65 μ m at T =123 K to about 3.55 μ m at T =88 K. A similar method was already applied to VIMS Saturn’s rings mosaics to retrieve ring particles temperature (Filacchione, G., Ciarniello, M., Capaccioni, F., et al., 2014. Icarus , 241, 45-65). We report here about the daytime temperature variations observed on the icy satellites as derived from three different VIMS observation types: (a) a sample of 240 disk-integrated I/F observations of Saturn’s regular satellites collected by VIMS during years 2004–2011 with solar phase in the 20°–40° range, corresponding to late morning-early afternoon local times. This dataset is suitable to exploit the temperature variations at hemispherical scale, resulting in average temperature T <88 K for Mimas, T ≪88 K for Enceladus, T <88 K for Tethys, T =98–118 K for Dione, T =108–128 K for Rhea, T =118–128 K for Hyperion, T =128–148 and T > 168 K for Iapetus’ trailing and leading hemispheres, respectively. A typical ±5 K uncertainty is associated to the temperature retrieval. On Tethys and Dione, for which observations on both leading and trailing hemispheres are available, in average daytime temperatures higher of about 10 K on the trailing than on the leading hemisphere are inferred. (b) Satellites disk-resolved observations taken at 20–40 km pixel − 1 resolution are suitable to map daytime temperature variations across surfaces’ features, such as Enceladus’ tiger stripes and Tethys’ equatorial dark lens. These datasets allow to disentangle solar illumination conditions from temperature distribution when observing surface’s features with strong thermal contrast. (c) Daytime average maps covering large regions of the surfaces are used to compare the inferred temperature with geomorphological features (impact craters, chasmatae, equatorial radiation lenses and active areas) and albedo variations. Temperature maps are built by mining the complete VIMS dataset collected in years 2004–2009 (pre-equinox) and in 2009–2012 (post equinox) by selecting pixels with max 150 km pixel − 1 resolution. VIMS-derived temperature maps allow to identify thermal anomalies across the equatorial lens of Mimas and Tethys. A temperature T > 115K is measured above Enceladus’ Damascus and Alexandria sulci in the south pole region. VIMS has the sensitivity to follow seasonal temperature changes: on Tethys, Dione and Rhea higher temperature are measured above the south hemisphere during pre-equinox and above the north hemisphere during post-equinox epochs. The measured temperature distribution appears correlated with surface albedo features: in fact temperature increases on low albedo units located on Tethys, Dione and Rhea trailing hemispheres. The thermal anomaly region on Rhea’s Inktomi crater detected by CIRS (Howett, C. J. A., Spencer, J. R., Hurford, T., et al., 2014. Icarus , 241, 239–247) is confirmed by VIMS: this area appears colder with respect to surrounding terrains when observed at the same local solar time. [ABSTRACT FROM AUTHOR]

Albedo, spectral slopes, and water ice band depths maps for the five midsized saturnian satellites Mimas, Enceladus, Tethys, Dione, and Rhea have been derived from Cassini-Visual and Infrared Mapping Spectrometer (VIMS) data. The maps are systematically built from photometric corrected data by applying the Kaasalainen-Shkuratov model (Kaasalainen et al., 2001; Shkuratov et al., 2011). In this work a quadratic function is used to fit phase curves built by filtering observations taken with incidence angle i ≤ 7 0 ∘ , emission angle e ≤ 7 0 ∘ , phase angle 1 0 ∘ ≤ g ≤ 12 0 ∘ , and Cassini-satellite distance D ≤ 100. 000 km. This procedure is systematically repeated for a subset of 65 VIMS visible and near-infrared wavelengths for each satellite. The average photometric parameters are used to compare satellites' properties and to study their variability with illumination conditions changes. We derive equigonal albedo, extrapolated at g = 0 ∘ , not including the opposition effect, equal to 0.63 ± 0.02 for Mimas, 0.89 ± 0.03 for Enceladus, 0.74 ± 0.03 for Tethys, 0.65 ± 0.03 for Dione, 0.60 ± 0.05 for Rhea at 0. 55 μ m. The knowledge of photometric spectral response allows to correct individual VIMS spectra used to build maps through geolocation. Maps are rendered at a fixed resolution corresponding to a 0. 5 ∘ × 0. 5 ∘ bin on a longitude by latitude grid resulting in spatial resolutions of 1.7 km/bin for Mimas, 2.2 km/bin for Enceladus; 4.7 km/bin for Tethys; 4.5 km/bin for Dione; 6.7 km/bin for Rhea. These spectral maps allow establishing relationships with morphological features and with endogenic and exogenic processes capable to alter satellites' surface properties through several mechanisms. The hemispheric dichotomies in albedo and spectral indicators between leading and trailing hemispheres are common properties of all midsized satellites: the accumulation of fine E ring grains is responsible for the higher albedo measured across the leading hemispheres of Tethys, Dione, Rhea, and the trailing side of Mimas. Conversely, the dark and red-colored material visible across the trailing hemispheres of Tethys, Dione, and Rhea is associated with the implantation of cold plasma particles. The thermal anomaly lenses located on the leading equatorial regions of Mimas and Tethys have been resolved and mapped. VIMS data evidence that the distribution of water ice band depths on Mimas lens is biased by the presence of the large Herschel impact crater pointing to a different regolith size distribution with respect to Tethys. Maps show local changes of albedo and spectral indicators in correspondence of recent impact craters (Inktomi on Rhea, Creusa on Dione) and on Dione's wispy terrains are caused by the exposure of pristine water ice. Enceladus' tiger stripes, the active sources of plumes in the southern polar region, despite being partially resolved on VIMS maps allow measuring an exceedingly high band depths in comparison with the rest of the satellite's surface. Moreover, Enceladus' smooth terrains located on the leading hemisphere around (l o n , l a t) = (9 0 ∘ , 3 0 ∘) show peculiar properties (low infrared albedo, positive 0.35- 0. 55 μ m slope and maximum band depth) possibly associated with the presence of a buried diapir in this area. The variability of average spectral albedos and indicators induced by phase angle is investigated and exploited to establish a comparison among satellites' properties as a function of orbital distance from Saturn. • Computation of photometric parameters of Mimas, Enceladus, Tethys, Dione, and Rhea satellites. • Rendering of equigonal albedo and spectral indicators maps at 0.5 deg resolution. • Study of average albedo and spectral indicators variability with phase angle. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]