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Photometric correction based on the Shkuratov method is applied to derive visible and infrared albedo maps of Rhea from disc-resolved Cassini VIMS data. Differently from I/F images, albedo maps offer an optimal disentanglement of composition and physical properties of the surface from illumination-viewing effects and to study spectral variations occurring at hemispherical and local scales. A similar methodology has been already applied to Dione’s and Tethys’s data sets returned by VIMS. Following the same scheme also for Rhea, spectral albedo is derived at 59 wavelengths between 0.35 and 5.047 µm. Equigonal albedo maps are rendered in cylindrical projection with a 0.5$^\circ \, \times$ 0.5° angular resolution in latitude and longitude, corresponding to a maximum spatial resolution of 6.7 km bin−1. Apart from albedo, 0.35–0.55 and 0.55–0.95 µm spectral slopes and the water ice 1.5–2.0 µm band depth maps are computed from photometric-corrected data with the specific scope to investigate the leading-trailing hemisphere colour-albedo dichotomy and to constrain spectral properties above different morphological units including fresh craters (Inktomi) and bright tectonics features (Wakonda-Avaiki Chasmata).

Near-infrared observations of the Pluto Charon system were captured with the Palomar High Angular Resolution Observer (PHARO) adaptive optics (AO) system on the 200-inch Hale telescope during the historically small solar phase angles in 2018–2019. Both objects exhibit large opposition surges of ∼30%–35% in the last half degree of solar phase angle, which is among the largest observed for icy moons and other Kuiper Belt Objects. In addition, Pluto's surge is exceptionally steep. Pluto's unusual phase curve may be due to an unusual surface texture caused by seasonal volatile transport and active geologic processes. These observations enable accurate determination of Pluto's geometric albedo in the JHK filter system, which we find to be 0.86 ± 0.04, 0.59 ± 0.05, and 0.39 ± 0.04 for Pluto, respectively, and 0.68 ± 0.06 for Charon in the J filter.

The New Horizons spacecraft observed Pluto and Charon at solar-phase angles between 16° and 169°. In this work, we use the Multispectral Visible Imaging Camera (MVIC) observations to construct multiwavelength phase curves of Pluto’s atmosphere, using the limb scatter technique. Observational artifacts and biases were removed using Charon as a representative airless body. The size and distribution of the haze particles were constrained using a Titan fractal aggregate phase function. We find that monodispersed and log-normal populations cannot simultaneously describe the observed steep forward scattering, indicative of wavelength-scale particles, and the non-negligible backscattering indicative of particles much smaller than the wavelength. Instead, we find it necessary to use bimodal or power-law distributions, especially below ∼200 km, to properly describe the MVIC observations. Above 200 km, where the atmosphere is isotropically scattering, a monodisperse, log-normal, or a bimodal/power law approximating a monodispersed population is able to fit the phase curves well. As compared to the results of previously published articles, we find that Pluto’s atmosphere must contain haze particle number densities an order of magnitude greater for small (∼10 nm) and large (∼1 μm) radii, and relatively fewer intermediate sizes (∼100 nm). These conclusions support a lower aggregate aerosol growth rate than that found by Gao et al., indicating a higher charge-to-radius ratio, upwards of 60e − μm−1. In order to generate large particles with a lower growth rate, the atmosphere must also have a lower sedimentation velocity (−1 at 200 km), which is possible with a fractal dimension of less than 2.

Observations of the opposition surges on the main moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus) during Cassini's prime and extended missions are reduced and analyzed. The main data set comes from the Visual Infrared Mapping Spectrometer (VIMS) with wavelength coverage in the visible and near-infrared out to 3.60 μm, covering 99% of the solar spectrum. Imaging Science Subsystem images and Ultraviolet Imaging Spectrometer data augment the VIMS data set. Hapke models are fit to Dione and Rhea, and Irvine's simpler shadowing model is fit to the sparser data sets of Enceladus, Dione, and Rhea. The high porosities (∼95% void space in the optically active portion of the regolith) and forward-scattering properties of the surfaces are similar to other icy moons and to Pluto. A change in the character of their opposition surge at 3.60 μm can be attributed largely to the noninteraction of long-wavelength photons with small particles on these moons’ surfaces. The opposition surge on the low- and high-albedo regions of Dione are similar. However, the low-albedo terrain of Iapetus exhibits a less robust surge than the high-albedo regions, which we attribute to differences in surface texture. The low-albedo hemisphere of Iapetus seems to be akin to the small number of bodies in the solar system that lack an appreciable opposition surge, possibly due to the accumulation of low-albedo dust. With observations over the range of the solar spectrum, we compute new values for the bolometric Bond albedos of these moons.

The Cassini-Huygens mission turned the moons of Saturn into tangible worlds. Although the discoveries from the spacecraft have been compiled in various review articles (e. g., Dougherty et al., 2009), there is no single publication that summarizes the remaining outstanding questions. Drawing on a workshop sponsored by the Cassini Project, we summarize the unanswered questions for the main icy moons of Saturn – Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe - for the disciplines of surface composition, geology, thermal properties, Enceladus's plume activity, interiors, and the interactions between Saturn's magnetosphere and the moons.

A search for temporal changes on Pluto and Charon was motivated by (1) the discovery of young surfaces in the Pluto system that imply ongoing or recent geologic activity, (2) the detection of active plumes on Triton during the Voyager 2 flyby, and (3) the abundant and detailed information that observing geologic processes in action provides about the processes. A thorough search for temporal changes using New Horizons images was completed. Images that covered the same region were blinked and manually inspected for any differences in appearance. The search included full-disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales. Changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters (especially geometry) or artifacts. No differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either Pluto or Charon. Limits on temporal changes as a function of spatial scale and temporal interval during the New Horizons encounter are determined. The longest time interval constraint is one Pluto/Charon rotation period (~6.4 Earth days). Contrast reversal and high-phase bright features that change in appearance with solar phase angle are identified. The change of appearance of these features is most likely due to the change in phase angle rather than a temporal change. Had active plumes analogous to the plumes discovered on Triton been present on the encounter hemispheres of either Pluto or Charon, they would have been detected. The absence of active plumes may be due to temporal variability (i.e., plumes do occur but none were active on the encounter hemispheres during the epoch of the New Horizons encounter ..., Accepted for publication in Icarus

The exploration of the Pluto-Charon system by the New Horizons spacecraft represents the first opportunity to understand the distribution of albedo and other photometric properties of the surfaces of objects in the Solar System's "Third Zone" of distant ice-rich bodies. Images of the entire illuminated surface of Pluto and Charon obtained by the Long Range Reconnaissance Imager (LORRI) camera provide a global map of Pluto that reveals surface albedo variegations larger than any other Solar System world except for Saturn's moon Iapetus. Normal reflectances on Pluto range from 0.08-1.0, and the low-albedo areas of Pluto are darker than any region of Charon. Charon exhibits a much blander surface with normal reflectances ranging from 0.20-0.73. Pluto's albedo features are well-correlated with geologic features, although some exogenous low-albedo dust may be responsible for features seen to the west of the area informally named Tombaugh Regio. The albedo patterns of both Pluto and Charon are latitudinally organized, with the exception of Tombaugh Regio, with darker regions concentrated at the Pluto's equator and Charon's northern pole The phase curve of Pluto is similar to that of Triton, the large moon of Neptune believed to be a captured Kuiper Belt Object (KBO), while Charon's is similar to that of the Moon. Preliminary Bond albedos are 0.25+/-0.03 for Charon and 0.72+/-0.07 for Pluto. Maps of an approximation to the Bond albedo for both Pluto and Charon are presented for the first time. Our work shows a connection between very high albedo (near unity) and planetary activity, a result that suggests the KBO Eris may be currently active., Comment: 32 pages, 3 tables, 8 figures Accepted for Publication in Icarus, 2016, v02

Cassini's last look at Saturn's rings During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

The Kuiper Belt is a distant region of the Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a Cold Classical Kuiper Belt Object, a class of objects that have never been heated by the Sun and are therefore well preserved since their formation. Here we describe initial results from these encounter observations. MU69 is a bi-lobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color and compositional heterogeneity. No evidence for satellites, ring or dust structures, gas coma, or solar wind interactions was detected. By origin MU69 appears consistent with pebble cloud collapse followed by a low velocity merger of its two lobes., 43 pages, 8 figure

International audience; We have analyzed the complete Visual and Infrared Mapping Spectrometer (VIMS) data archive of Titan. Our objective is to build global surface cartographic products, by combining all the data gathered during the 127 targeted flybys of Titan into synthetic global maps interpolated on a grid at 32 pixels per degree (∼1.4 km/pixel at the equator), in seven infrared spectral atmospheric windows. Multispectral summary images have been computed for each single VIMS cube in order to rapidly identify their scientific content and assess their quality. These summary images are made available to the community on a public website (vims.univ-nantes.fr). The global mapping work faced several challenges due to the strong absorbing and scattering effects of the atmosphere coupled to the changing observing conditions linked to the orbital tour of the Cassini mission. We determined a surface photometric function which accounts for variations in incidence, emergence and phase angles, and which is able to mitigate brightness variations linked to the viewing geometry of the flybys. The atmospheric contribution has been reduced using the subtraction of the methane absorption band wings, considered as proxies for atmospheric haze scattering. We present a new global three color composite map of band ratios (red: 1.59/1.27 µm; green: 2.03/1.27 µm; blue: 1.27/1.08 µm), which has also been empirically corrected from an airmass (the solar photon path length through the atmosphere) dependence. This map provides a detailed global color view of Titan's surface partially corrected from the atmosphere and gives a global insight of the spectral variability, with the equatorial dunes fields appearing in brownish tones, and several occurrences of bluish tones localized in areas such as Sinlap, Menvra and Selk craters. This kind of spectral map can serve as a basis for further regional studies and comparisons with radiative transfer outputs, such as surface albedos, and other additional data sets acquired by the Cassini Radar (RADAR) and Imaging Science Subsystem (ISS) instruments.

Observations of Pluto from New Horizons have been combined with previous ground-based observations and fit to a radiative transfer model based on Chandrasekhar’s planetary problem and Hapke theory to simultaneously derive the physical properties of the dwarf planet’s surface and atmosphere. We derive the macroscopic roughness, single-scattering albedo, and directional scattering properties of the surface, and the single-scattering albedo, optical depth, and single-scattering phase function of Pluto’s haze. The haze particles are small, with best-fit sizes in the range of ∼0.41–1.14 μm. We find that Pluto’s haze is more similar to that of Titan, rich in organic compounds and highly forward scattering, than that of Triton. With organic compounds and a likely subsurface water ocean, Pluto may harbor sustainable habitable environments. Our model, which includes the coherent backscatter effect, fits the anomalously large opposition surge recently discovered on Pluto.

Saturn’s moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan’s atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan’s northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan’s surface, our findings suggest that dust can be suspended in Titan’s atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan’s dune fields are actively evolving by aeolian processes. Saturn’s moon Titan may have an active dust cycle in equatorial regions driven by storm winds, Cassini observations consistent with dust suspension in Titan’s atmosphere suggest.

We have analyzed the complete archive of the Visual and Infrared Mapping Spectrometer (VIMS) data in order to monitor and analyze the evolution of the clouds and haze coverage at both poles of Titan during the entire Cassini mission. Our objective is to give a cartographic synopsis from a VIMS perspective, to provide a global view of the seasonal evolution of Titan's atmosphere over the poles. We leave the detailed comparison with the Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) data sets to further studies. We have computed global hyperspectral mosaics for each of the 127 targeted flybys of Titan to produce synthetic color maps emphasizing the main atmospheric features. The north pole appears fully covered by a huge cloud as soon as the first observations in 2004 and up to the equinox in 2009 (Le Mou\'elic et al. 2012). The northern skies then became progressively clearer, after the circulation turnover in 2009, revealing the underlying lakes and seas to the optical instruments up to 2017. The reverse situation is observed over the south pole, which was mostly clear of such a high obscuring cloud during the first years of the mission, but started to develop a polar cloud in 2012. This feature grew up month after month until the end of the mission in 2017, with a poleward latitudinal extent of 75$^\circ$S in 2013 up to 58$^\circ$S in April 2017. Thanks to the spectral capabilities of VIMS, we have detected HCN spectral signatures over the north pole in almost all flybys between 2004 and 2008. These HCN signatures started then to show up over the south pole in almost all flybys between 2012 and 2017, so perfectly matching the timing and spatial extent of the northern and southern polar atmospheric features.

We report about the derivation of visible (VIS) and infrared (IR) albedo maps and spectral indicators of Saturn's satellite Tethys from the complete Cassini‐Visual and Infrared Mapping Spectrometer (VIMS) data set. The application of a photometric correction is necessary to remove illumination and viewing effects from the I/F spectra, to compute spectral albedo and to correctly associate spectral variations to changes in composition or physical properties of the surface. In this work we are adopting the photometric correction proposed by Shkuratov et al. (2011, https://doi.org/10.1016/j.pss.2011.06.011) to derive albedo maps of Tethys from disk‐resolved Cassini‐VIMS data. After having applied a similar methodology to Dione's data (Filacchione et al., 2018, https://doi.org/10.1002/2017GL076869), we present here the results achieved for Tethys: surface albedo maps and photometric parameters are computed at five visible (0.35, 0.44, 0.55, 0.70, and 0.95 μm) and five infrared (1.046, 1.540, 1.822, 2.050, and 2.200 μm) wavelengths and rendered in cylindrical projection with a 0.5° × 0.5° angular resolution in latitude and longitude, corresponding to a highest spatial resolution of 4.7 km/bin. The 0.35‐ to 0.55‐ and 0.55‐ to 0.95‐μm spectral slopes and the water ice 2.050‐μm band depth maps are computed after having applied the photometric correction, in order to trace the leading‐trailing hemisphere dichotomy, to constrain the shape of the equatorial lens generated by the bombardment of high‐energy magnetospheric electrons on the leading hemisphere, and to observe the stronger water ice band depth and reddening within the floors of Odysseus and Penelope impact craters.

[1] The Framing Camera (FC) on the Dawn spacecraft provided the first view of 4 Vesta at sufficiently high spatial resolution to enable a detailed correlation of the asteroid's spectral properties with geologic features and with the vestoid (V-type) asteroids and the Howardite-Eucrite-Diogenite (HED) class of meteorites, both of which are believed to originate on Vesta. We combine a spectral analysis of the basin with visible and near-IR spectroscopy of vestoids and with archived data over the same spectral range for HED meteorites. The vestoids are only slightly more akin to the Rheasilvia basin than to Vesta as a whole, suggesting that the crustal material ejected is a well-mixed collection of eucritic and diogenitic materials. The basin itself is more diogenitic, implying Vesta is differentiated and the impact that created Rheasilvia uncovered a mineralogically distinct layer. The Rheasilvia basin exhibits a larger range in pyroxene band strengths than Vesta as a whole, further implying that the basin offers a view into a complex, differentiated protoplanet. The discrepancy between the spectral properties of the HED meteorites and Vesta, in particular the meteorites' deeper pyroxene absorption band and the redder color of the vestoids, can be explained by the abundance of smaller particles on Vesta and by the addition of low-albedo exogenous particles to its surface, which in turn are due to its larger gravity and longer exposure time to impact processing. Solar phase effects are slight and do not explain the spectral discrepancies between the HEDs, Vesta, and the vestoids.

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons in the production of this material. The escape of Pluto's atmosphere provides a potential feedstock for a complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation for the dark coloration on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.

Cassini Visual Infrared Mapping Spectrometer (VIMS) observations of Mimas, Tethys, and Dione obtained during the nominal and extended missions at large solar phase angles were analyzed to search for plume activity. No forward scattered peaks in the solar phase curves of these satellites were detected. The upper limit on water vapor production for Mimas and Tethys is one order of magnitude less than the production for Enceladus. For Dione, the upper limit is two orders of magnitude less, suggesting this world is as inert as Rhea (Pitman, K.M., Buratti, B.J., Mosher, J.A., Bauer, J.M., Momary, T., Brown, R.H., Nicholson, P.D., Hedman, M.M. [2008]. Astrophys. J. Lett. 680, L65–L68). Although the plumes are best seen at ∼2.0 μm, Imaging Science Subsystem (ISS) Narrow Angle Camera images obtained at the same time as the VIMS data were also inspected for these features. None of the Cassini ISS images shows evidence for plumes. The absence of evidence for any Enceladus-like plumes on the medium-sized saturnian satellites cannot absolutely rule out current geologic activity. The activity may below our threshold of detection, or it may be occurring but not captured on the handful of observations at large solar phase angles obtained for each moon. Many VIMS and ISS images of Enceladus at large solar phase angles, for example, do not contain plumes, as the active “tiger stripes” in the south pole region are pointed away from the spacecraft at these times. The 7-year Cassini Solstice Mission is scheduled to gather additional measurements at large solar phase angles that are capable of revealing activity on the saturnian moons.

Triton, the large satellite of Neptune, was imaged by the Voyager 2 spacecraft in 1989 with dark plumes originating in its volatile-rich south polar region. Southern summer solstice, a time when seasonal volatile transport should be at a maximum, occurred in 2001. Ground-based observations of Triton’s rotational light curve obtained from Table Mountain Observatory in 2000–2004 reveal volatile transport on its surface. When compared with a static frost model constructed from Voyager images, the light curve shows an increase in total amplitude. An earlier light curve obtained in 1992 from Mauna Kea Observatory is consistent with the static frost model. This movement of volatiles on the surface agrees with recent imaging results from the Hubble Space Telescope (Bauer, J.M., Buratti, B.J., Li, J.-Y., Mosher, J.A., Hicks, M.D., Schmidt, B.E., Goguen, J.D. [2010]. Astrophys. J. 723, L49–L52). The changes in the light curve can be explained by the transport of nitrogen frost on the surface or by the uncovering of bedrock of less volatile methane. We also find that Triton exhibits a large opposition surge at solar phase angles less than 0.1°. This surge cannot be entirely explained by the effects of coherent backscatter.

We report the detailed analysis of the spectrophotometric properties of Saturn’s icy satellites as derived by full-disk observations obtained by visual and infrared mapping spectrometer (VIMS) experiment aboard Cassini. In this paper, we have extended the coverage until the end of the Cassini’s nominal mission (June 1st 2008), while a previous paper ( Filacchione, G., and 28 colleagues [2007]. Icarus 186, 259–290 , hereby referred to as Paper I) reported the preliminary results of this study. During the four years of nominal mission, VIMS has observed the entire population of Saturn’s icy satellites allowing us to make a comparative analysis of the VIS–NIR spectral properties of the major satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus) and irregular moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso, Phoebe). The results we discuss here are derived from the entire dataset available at June 2008 which consists of 1417 full-disk observations acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage. The most important spectrophotometric indicators (as defined in Paper I: I / F continua at 0.55 μm, 1.822 μm and 3.547 μm, visible spectral slopes, water and carbon dioxide bands depths and positions) are calculated for each observation in order to investigate the disk-integrated composition of the satellites, the distribution of water ice respect to “contaminants” abundances and typical regolith grain properties. These quantities vary from the almost pure water ice surfaces of Enceladus and Calypso to the organic and carbon dioxide rich Hyperion, Iapetus and Phoebe. Janus visible colors are intermediate between these two classes having a slightly positive spectral slope. These results could help to decipher the origins and evolutionary history of the minor moons of the Saturn’s system. We introduce a polar representation of the spectrophotometric parameters as function of the solar phase angle (along radial distance) and of the effective longitude interval illuminated by the Sun and covered by VIMS during the observation (in azimuth) to better investigate the spatial distribution of the spectrophotometric quantities across the regular satellites hemispheres. Finally, we report the observed spectral positions of the 4.26 μm band of the carbon dioxide present in the surface material of three outermost moons Hyperion, Iapetus and Phoebe.

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25–5.1 μm spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (∼5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the “shepherd” satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 μm that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262–267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO2 on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO2 on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion.

Saturn’s satellite Dione is becoming an increasingly important object in the outer Solar System, as evidence for its current activity accumulates. Infrared observations of the surface can provide clues to the history of the body and currently active processes. Using data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), we perform three sets of analyses that are sensitive to the ice state, temperature, thermal history, grain size and composition of surface ice. These are calculation of a “crystallinity factor”, spectral ratios and water ice band depths. In our analysis, we focus on the dichotomy between the wispy and dark terrain on Dione’s trailing hemisphere, to better understand the source of the different materials and their current properties. Our results suggest two different scenarios: (1) the ice from the wispy region has a higher crystallinity and water ice content than the dark region or (2) the wispy region contains larger grains. Both of these models imply recent geologic activity on Dione.

Using radiances acquired with Cassini's Visual and Infrared Mapping Spectrometer (VIMS), we construct high solar phase angle curves for Saturn's second largest moon, Rhea. Ground-based studies of Rhea and Saturnian icy satellites are focussing on low phase angles; to our knowledge, these are the first solar phase curve data for Rhea on phase angles >70°. We compare these data to similar phase curves for Enceladus at near-infrared wavelengths to estimate the amount of water vapor that could possibly be generated and thus set an upper limit on the amount of geologic activity that may be occurring on Rhea. We find that Enceladus's plume manifests itself in the VIMS solar phase curve at a phase angle near 160° and peaks most strongly for -->λ = 2.017810 μm. No such peak can be found in the Rhea VIMS phase curve. Absence of a forward scattering peak supports the recent determination that particles in Rhea's surrounding ring are not small. We calculate that the maximum water vapor column density that could be supplied from Rhea ranges from -->1.52 × 1014 to -->1.91 × 1015 cm−2, 2 orders of magnitude less than what is calculated by Cassini UVIS for Enceladus. This implies that for Rhea, the level of active internal (endogenic) processes is exceedingly small, if any.

Cassini-Huygens is a multidisciplinary, international planetary mission consisting of an orbiting spacecraft and a probe. The Huygens probe successfully landed on Titan's surface on January 14, 2005, while the orbiter has performed observations of Saturn, its rings, satellites, and magnetosphere since it entered orbit around Saturn on July 1, 2004. The Cassini mission has been prolific in its scientific discoveries about the Saturn system. In this special section, we present new mission results with a focus on the 'icy satellites,' which we define as all Saturn's moons with the exception of Titan. The results included in this section have come out of the Cassini SOST--Satellites Orbiter Science Team--a multi-instrument and multidiscipline group that works together to better understand the icy satellites and their interactions with Saturn and its rings. Other papers included in this issue present ground-based observations and interior modeling of these icy moons.

Photometric and spectral analysis of data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) has yielded significant results regarding the properties and composition of the surface of Saturn's satellite Enceladus. We have obtained spectral cubes of this satellite, containing both spatial and spectral information, with a wavelength distribution in the infrared far more extensive than from any previous observations and at much higher spatial resolution. Using a composite mosaic of the satellite, we map the distribution of crystalline and amorphous ices on the surface of Enceladus according to a “crystallinity factor” and also the depth of the temperature- and structure-dependent 1.65 micron water-ice band. These maps show the surface of Enceladus to be mostly crystalline, with a higher degree of crystallinity at the “tiger-stripe” cracks and a larger amorphous signature between these stripes. These results suggest recent geological activity at the “tiger stripe” cracks and an intriguing atmospheric environment over the south pole where amorphous ice is produced either through intense radiative bombardment, flash-freezing of cryovolcanic liquid, or rapid condensation of water vapor particles on icy microspherules or on the surface of Enceladus.

Scientists have recently discovered a dynamic, geologically active world in Saturn's icy moon, Enceladus. This small satellite has a south polar hot spot at least 70 K hotter than surrounding regions and a plume of water vapor and ice particles surging from the "tiger stripe" cracks that flank the pole. Spectral data of the surface of Enceladus can help us understand the processes occurring on and beneath the surface and in the regional south polar atmosphere. Using observations from Cassini's Visual and Infrared Mapping Spectrometer (VIMS), we have tentatively detected hydrogen peroxide on the surface of Enceladus, by identification of the 3.5 μm hydrogen peroxide band. Hydrogen peroxide is produced in the solar system by bombardment of water ice with energetic particles, such as protons, electrons, and magnetospheric ions. Its detection indicates an intense radiative environment at Enceladus and can help us understand plume and surface chemistry, as well as constrain the timescale of plume activity.

Saturn's icy satellites are among the main scientific objectives of the Cassini-VIMS (Visual and Infrared Mapping Spectrometer) experiment. This paper contains a first systematic and comparative analysis of the full-disk spectral properties of Dione, Enceladus, Epimetheus, Hyperion, Iapetus, Mimas, Phoebe, Rhea and Tethys as observed by VIMS from July 2004 to June 2005. The disk integrated properties (350–5100 nm reflectance spectra and phase curves at 550–2232 nm) and images of satellites are reported and discussed in detail together with the observed geometry. In general, the spectra in the visible spectral range are almost featureless and can be classified according to the spectral slopes: from the bluish Enceladus and Phoebe to the redder Iapetus, Hyperion and Epimetheus. In the 1000–1300 nm range the spectra of Enceladus, Tethys, Mimas and Rhea are characterized by a negative slope, consistent with a surface largely dominated by water ice, while the spectra of Iapetus, Hyperion and Phoebe show a considerable reddening pointing out the relevant role played by darkening materials present on the surface. In between these two classes are Dione and Epimetheus, which have a flat spectrum in this range. The main absorption bands identified in the infrared are the 1520, 2020, 3000 nm H2O/OH bands (for all satellites), although Iapetus dark terrains show mostly a deep 3000 nm band while the 1520 and 2020 nm bands are very faint. In this spectral range, the Iapetus spectrum is characterized by a strong reddening. The CO2 band at 4260 nm and the Fresnel ice peak around 3100 nm are evident only on Hyperion, Phoebe and Iapetus. The phase curves at 550 and at 2232 nm are reported for all the available observations in the 0°–144° range; Rhea shows an opposition surge at visible wavelengths in the 0.5°–1.17° interval. The improvement on the retrieval of the full-disk reflectance spectra can be appreciated by a direct comparison with ground-based telescopic data available from literature. Finally, data processing strategies and recent upgrades introduced in the VIMS-V calibration pipeline (flat-field and destriping–despiking algorithm) are discussed in appendices.

We examined the spectral properties of a selection of Titan's impact craters that represent a range of degradation states. The most degraded craters have rims and ejecta blankets with spectral characteristics that suggest that they are more enriched in water ice than the rims and ejecta blankets of the freshest craters on Titan. The progression is consistent with the chemical weathering of Titan's surface. We propose an evolutionary sequence such that Titan's craters expose an intimate mixture of water ice and organic materials, and chemical weathering by methane rainfall removes the soluble organic materials, leaving the insoluble organics and water ice behind. These observations support the idea that fluvial processes are active in Titan's equatorial regions., Key Points We examine the spectral properties of Titan's impact cratersDegraded crater rims are more enriched in water ice than fresh crater rimsObservations suggest chemical weathering of organics in water ice matrix

International audience; The Visual and Infrared Mapping Spectrometer observed Phoebe, Iapetus, Titan and Saturn's rings during Cassini's approach and orbital insertion. Phoebe's surface contains water ice, CO{2}, and ferrous iron. Iapetus contains CO{2} and organic materials. Titan's atmosphere shows methane fluorescence, and night-side atmospheric emission that may be CO{2} and CH{3}D. As determined from cloud motions, the winds at altitude 25-30 km in the south polar region of Titan appear to be moving in a prograde direction at velocity ˜ 1 m s-1. Circular albedo features on Titan's surface, seen at 2.02 mu m, may be palimpsests remaining from the rheological adjustment of ancient impact craters. As such, their long-term persistence is of special interest in view of the expected precipitation of liquids and solids from the atmosphere. Saturn's rings have changed little in their radial structure since the Voyager flybys in the early 1980s. Spectral absorption bands tentatively attributed to Fe2+ suggest that iron-bearing silicates are a source of contamination of the C ring and the Cassini Division.

We have obtained broadband spectrophotometric observations of four of the recently discovered small satellites of Saturn (Gladman et al., 2001, Nature 412, 163–166). The new data enable an understanding of the provenance, composition, and interrelationships among these satellites and the other satellites of Saturn, particularly Iapetus, Phoebe, and Hyperion. Temporal coverage of one satellite (S21 Tarvos) was sufficient to determine a partial rotational lightcurve. Our major findings include: (1) the satellites are red and similar in color, comparable to D-type asteroids, some KBOs, Iapetus, and Hyperion; (2) none of the satellites, including those from the “Phoebe Group” has any spectrophotometric relationship to Phoebe; and (3) S21 Tarvos exhibits a rotational lightcurve, although the data are not well-constrained and more observations are required to fit a definitive period. Dust created by meteoritic impacts and ejected from these satellites and additional undiscovered ones may be the source of the exogenous material deposited on the low-albedo side of Iapetus. Recent work which states that the small irregular satellites of Saturn have impacted Phoebe at least 6–7 times in the age of the Solar System (Nesvorny et al., 2003, Astron. J. 126, 398–429), suggests that such collisions may have propelled additional material from both Phoebe and the small irregular satellites toward Iapetus. The accretion of material from outer retrograde satellites may be a process that also occurs on Callisto and the uranian satellites.

The first near-infrared (0.9 μm) photometric observations of Saturn's five medium-sized satellites were obtained during the August 1995 ring plane crossing with Palomar Observatory's 60-in. telescope. The albedos of the satellites are very high, indicating that a very low fraction of dark opaque contaminants is present on their icy surfaces. The geometric albedos for the leading sides of the satellites are Enceladus: 1.02; Tethys: 0.90; Dione: 0.72; and Rhea: 0.76. The geometric albedo of the trailing side of Mimas is 0.72. The amplitudes of the satellites' rotational lightcurves are somewhat less than those measured at visible wavelengths. They range from 0.40 (Dione) to 0.10 (Rhea) to 0.05 (Tethys). Both Enceladus and Mimas are brighter on the trailing side, unlike the other three satellites. These results suggest that the optical properties of all five satellites are determined by interactions with Saturn's E-ring.

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51 degrees to 68 degrees north and all longitudes observed (10 degrees to 190 degrees west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.

The plume at the south pole of Enceladus is several times brighter when that moon is near the apocentre of its eccentric orbit around Saturn than when it is near its orbital pericentre, showing that more material appears to be escaping from beneath Enceladus’ surface at times when models predict its fissures should be under tension. Between 2005 and 2012 the Cassini spacecraft's Visual and Infrared Mapping Spectrometer (VIMS) obtained 252 images of the plume of water vapour and ice particles emitting from near the south pole of Saturn's moon Enceladus. These images have been analysed with a view to establishing the nature of the geological forces driving the plume. The authors show that as Enceladus moves along its elliptical orbit, the brightness of the plume peaks — and larger amounts of material join the plume — when the moon is furthest from Saturn. This is consistent with a model in which tidal forces have an important role in controlling plume activity, perhaps by changing the width of the conduits between the surface and various underground reservoirs supplying the fissures through which the plume emerges. Saturn’s moon Enceladus emits a plume of water vapour and micrometre-sized ice particles from a series of warm fissures located near its south pole1,2,3,4,5,6,7,8,9,10. This geological activity could be powered or controlled by variations in the tidal stresses experienced by Enceladus as it moves around its slightly eccentric orbit. The specific mechanisms by which these varying stresses are converted into heat, however, are still being debated11,12,13,14,15,16. Furthermore, it has proved difficult to find a clear correlation between the predicted tidal forces and measured temporal variations in the plume’s gas content17,18,19 or the particle flux from individual sources20,21. Here we report that the plume’s horizontally integrated brightness is several times greater when Enceladus is near the point in its eccentric orbit where it is furthest from Saturn (apocentre) than it is when near the point of closest approach to the planet (pericentre). More material therefore seems to be escaping from beneath Enceladus’ surface at times when geophysical models predict its fissures should be under tension12,15,16 and therefore may be wider open.

A New Dawn Since 17 July 2011, NASA's spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684 ) use Dawn's observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687 ) report on the asteroid's overall geometry and topography, based on global surface mapping. Vesta's surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690 ) report on Vesta's complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694 ) describe two giant impact basins located at the asteroid's south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697 ) present the mineralogical characterization of Vesta, based on data obtained by Dawn's visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700 ) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet.

We present an overview of fundamental photometric properties of icy surfaces throughout the Solar System and investigate the extent to which these properties reflect the evolution of the bodies on which they reside. We review photometric models and their parameters and discuss the physical interpretability of those parameters. We focus on those fundamental photometric properties, primarily albedo and the near-opposition phase function, which are independent of any interpretation from the application of a photometric model. Finally, we offer suggestions for future work, both observational and laboratory measurements, which will enhance the scientific return from continued photometric studies of icy bodies in the Solar System.

Ontario Lacus is the largest lake of the whole southern hemisphere of Titan, Saturn’s major moon. It has been imaged twice by each of the Cassini imaging systems (Imaging Science Subsystem (ISS) in 2004 and 2005, Visual and Infrared Mapping Spectrometer (VIMS) in 2007 and 2009 and RADAR in 2009 and 2010). We compile a geomorphological map and derive a “hydrogeological” interpretation of Ontario Lacus, based on a joint analysis of ISS, VIMS and RADAR SAR datasets, along with the T49 altimetric profile acquired in December 2008. The morphologies observed on Ontario Lacus are compared to landforms of a semi-arid terrestrial analog, which resembles Titan’s lakes: the Etosha Pan, located in the Owambo Basin (Namibia). The Etosha Pan is a flat-floored depression formed by dissolution, under semi-arid conditions, of a surface evaporitic layer (calcretes) controlled by groundwater vertical motions. We infer that Ontario Lacus is an extremely flat and shallow depression lying in an alluvial plain surrounded by small mountain ranges under climatic conditions similar to those of terrestrial semi-arid regions. Channels are seen in the southern part of Ontario Lacus in VIMS and RADAR data, acquired at a 2-years time interval. Their constancy in location with time implies that the southern portion of the depression is probably not fully covered by a liquid layer at the time of the observations, and that they most probably run on the floor of the depression. A shallow layer of surface liquids, corresponding to the darkest portions of the RADAR images, would thus cover about 53% of the surface area of the depression, of which almost 70% is located in its northern part. These liquid-covered parts of the depression, where liquid ethane was previously identified, are interpreted as topographic lows where the “alkanofer” raises above the depression floor. The rest of the depression, and mostly its southern part, is interpreted as a flat and smooth exposed floor, likely composed of a thick and liquid-saturated coating of photon-absorbing materials in the infrared. This hypothesis could explain its dark appearance both in the infrared and radar data and the persistence of channels seen on the depression floor over the time. Shorelines are observed on the border of Ontario Lacus suggesting past high-stand levels of the alkanofer table. The analogy with the Etosha Pan suggests that Ontario Lacus’ depression developed at the expense of a soluble layer covering the region. Dissolution of this layer would be controlled by vertical motions of the alkanofer table over the time. During flooding events, liquid hydrocarbons covering the depression floor would dissolve the surface layer, increasing progressively the diameter of the depression on geological timescales. During drought episodes, liquid hydrocarbons of the underground alkanofer would evaporate, leading to crystallization of “evaporites” in the pores and at the surface of the substratum, and to the formation of the regional soluble layer. The presence of specific landforms (lunette dunes or evaporites) is compatible with such evaporitic regional settings. Alternatively, but not exclusively, the surface soluble layer might have formed by accumulation on the ground of soluble compounds formed in the atmosphere.

In the last few years Cassini-VIMS, the Visible and Infared Mapping Spectrometer, returned to us a comprehensive view of the Saturn's icy satellites and rings. After having analyzed the satellites' spectral properties (Filacchione et al. (2007a)) and their distribution across the satellites' hemispheres (Filacchione et al. (2010)), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2,264 disk-integrated observations of the satellites and a 12x700 pixels-wide rings radial mosaic acquired with a spatial resolution of about 125 km/pixel. The comparative analysis of these data allows us to retrieve the amount of both water ice and red contaminant materials distributed across Saturn's system and the typical surface regolith grain sizes. These measurements highlight very striking differences in the population here analyzed, which vary from the almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and Phoebe. Rings spectra appear more red than the icy satellites in the visible range but show more intense 1.5-2.0 micron band depths. The correlations among spectral slopes, band depths, visual albedo and phase permit us to cluster the saturnian population in different spectral classes which are detected not only among the principal satellites and rings but among co-orbital minor moons as well. Finally, we have applied Hapke's theory to retrieve the best spectral fits to Saturn's inner regular satellites using the same methodology applied previously for Rhea data discussed in Ciarniello et al. (2011)., Comment: 44 pages, 27 figures, 7 tables. Submitted to Icarus